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  Rob Miles
  Edition 2.0 July 2010 2010 Department of Computer Science, The University of Hull.
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  All rights reserved.robmiles. Cottingham Road HULL HU6 7RX UK Department: Email: rob@robmiles. copy or transmission of this publication may be made without written permission.com If you find a mistake in the text please report the error to foundamistake@robmiles.hull. 28 July 2010 iii . No reproduction.ac. Edition 2.0 Wednesday.dcs.com Blog: and I will take a look. Robert Blackburn Building The University of Hull. The author can be contacted at: The Department of Computer Science.
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  the fastest.csharpcourse. don't get too persistent.e.dcs. just ones which are better in a particular context. These are based on real programming experience and are to be taken seriously. If you have not programmed before.ac. Programming is not rocket science it is. puns.com. They contain a number of Programming Points.Introduction Welcome Introduction Welcome Welcome to the Wonderful World of Rob Miles™.9. The keys to learning programming are: Practice – do a lot of programming and force yourself to think about things from a problem solving point of view Study – look at programs written by other people. the smallest. Staying up all night trying to sort out a problem is not a good plan. If you have any comments on how the notes can be made even better (although I of course consider this highly unlikely) then feel free to get in touch Above all. We will cover what to do when it all goes wrong later in section 5. If you have programmed before I'd be grateful if you'd still read the text.com C# Programming © Rob Miles 2010 5 . well. Font. i. There are also bits written in a Posh These are really important and should be learnt by heart. the easiest to use etc. enjoy programming. Rob Miles rob@robmiles. The bad news about learning to program is that you get hit with a lot of ideas and concepts at around the same time when you start.com www. However. programming. The principle reason why most folks don't make it as programmers is that they give up. The website for the book is at. and this can be confusing. And you have to work hard at it. and programming. Persistence – writing programs is hard work. Not because they are stupid. Reading the notes These notes are written to be read straight through.hull. If you haven't solved a programming problem in 30 minutes you should call time out and seek help. It is worth it just for the jokes and you may actually learn something. Or at least walk away from the problem and come back to it. And remember that in many cases there is no best solution. You can learn a lot from studying code which other folk have created. Figuring out how somebody else did the job is a great starting point for your solution.uk Getting a copy of the notes These notes are made freely available to Computer Science students at the University of Hull. and then referred to afterwards. do not worry. In this book I'm going to give you a smattering of the C# programming language. This is a world of bad jokes. It just makes you all irritable in the morning.
  C# Programming © Rob Miles 2010 6 . A better way is to describe it as: A device which processes information according to instructions it has been given. to be useful. while technically correct.1 An Introduction to Computers Qn: Why does a bee hum? Ans: Because it doesn't know the words! One way of describing a computer is as an electric box which hums. and further because there are people willing to pay you to do it. This is not what most people do with computers. A programmer has a masochistic desire to tinker with the innards of the machine. particularly amongst those who then try to program a fridge. Software uses the physical ability of the hardware. If a computer has a soul it keeps it in its software. The box.2 Hardware and Software If you ever buy a computer you are not just getting a box which hums. At this point we must draw a distinction between the software of a computer system and the hardware. and it stops working when immersed in a bucket of water. we are going to consider computers. to ensure that you achieve a ―happy ending‖ for you and your customer. A user has a job which he or she finds easier to do on a computer running the appropriate program. This. This general definition rules out fridges but is not exhaustive. Finally you will take a look at programming in general and the C# language in particular. Hardware is the physical side of the system. 1. 1. Essentially if you can kick it. We must therefore make a distinction between users and programmers. because it sets the context in which all the issues of programming itself are placed. You will discover what you should do when starting to write a program.e.1. Software is what makes the machine tick. i. The instructions you give to the computer are often called a program. One of the golden rules is that you never write your own program if there is already one available. The business of using a computer is often called programming. Before we can look at the fun packed business of programming though it is worth looking at some computer terminology: 1. because you will often want to do things with computers which have not been done before. it is hardware. which can run programs. However for our purposes it will do.1. we are going to learn how to program as well as use a computer. a keen desire to process words with a computer should not result in you writing a word processor! However. Hardware is the impressive pile of lights and switches in the corner that the salesman sold you. They use programs written by other people.Computers and Programs Computers 1 Computers and Programs In this chapter you are going to find out what a computer is and get an understanding of the way that a computer program tells the computer what to do. can lead to significant amounts of confusion. must also have sufficient built-in intelligence to understand simple commands to do things. Most people do not write programs.1 Computers Before we consider programming. This is an important thing to do.
  Put duff data into a computer and it will do equally useless things. as well as a useful thing to blame. So why am I being so pedantic? Because it is vital to remember that a computer does not "know" what the data it is processing actually means. It gives computer programs a platform on which they can execute. some processing is performed. You will have to learn to talk to an operating system so that you can create your C# programs and get them to go.388. A computer works on data in the same way that a sausage machine works on meat. Put a bicycle into a sausage machine and it will try to make sausages out of it. It looks after all the information held on the computer and provides lots of commands to allow you to manage things.Computers and Programs Computers do something useful. An example. It also lets you run programs. and something comes out of the other end: Data Computer Data This makes a computer a very good "mistake amplifier". I regard data and information as two different things: Data is the collection of ons and offs which computers store and manipulate. All computers are sold with some software. It is called software because it has no physical existence and it is comparatively easy to change. and then generate further information.. A computer program tells the computer what to do with the information coming in.. Without it they would just be a novel and highly expensive heating system..608 in your account! Data Processing Computers are data processors. 1.3 Data and Information People use the words data and information interchangeably. A program is unaware of the data it is processing in the same way that a sausage machine is unaware of what meat is. The software which comes with a computer is often called its Operating System. A computer program is just a sequence of instructions which tell a computer what to do with the data coming in and what form the data sent out will have. The Operating System makes the machine usable. it is the user who gives meaning to these patterns. As far as the computer is concerned data is just patterns of bits. something is put in one end. C# Programming © Rob Miles 2010 7 . As far as the computer is concerned data is just stuff coming in which has to be manipulated in some way. humans work on information. They seem to think that one means the other. Software is the voice which says "Computer Running" in a Star Trek film. Windows 7 is an operating system.1. Information is fed into them... Strictly speaking computers process data. It is only us people who actually give meaning to the data (see above). they do something with it. Information is the interpretation of the data by people to mean something. ones you have written and ones from other people. Remember this when you get a bank statement which says that you have £8.
  However the CD player and games console could not be made to work without built-in data processing ability. CD Player: A computer is taking a signal from the disk and converting it into the sound that you want to hear. Tell people that you program computers and you will get one of the following responses: 1. and ever will have. examples of typical data processing applications are: Digital Watch: A micro-computer in your watch is taking pulses from a crystal and requests from buttons. road speed. You will not press a switch to make something work. you will press a switch to tell a computer to make it work. Games Console: A computer is taking instructions from the controllers and using them to manage the artificial world that it is creating for the person playing the game. "That's interesting". Car: A micro-computer in the engine is taking information from sensors telling it the current engine speed. A look which indicates that you can't be a very good one as they all drive Ferraris and tap into the Bank of England at will. 3. is much more than that. timing of the spark etc. For example a doctor may use a spread sheet to calculate doses of drugs for patients. setting of the accelerator etc and producing voltages out which control the setting of the carburettor. which you might think is entirely reading and writing numbers. as software writers are moving. You must make sure that the code you write will actually fit in the target machine and operate at a reasonable speed. It is the kind of thing that you grudgingly admit to doing at night with the blinds drawn and nobody watching. 1. In this case a defect in the program could result in illness or even death (note that I don't think that doctors actually do this – but you never know. 2. to optimise the performance of the engine. Asked to solve every computer problem that they have ever had. reading in numbers and printing out results. followed by a long description of the double glazing that they have just had fitted. 4. processing this data and producing a display which tells you the time. I will mention them when appropriate. oxygen content of the air. As software engineers it is inevitable that a great deal of our time will be spent fitting data processing components into other devices to drive them. C# Programming © Rob Miles 2010 8 . The power and capacity of modern computers makes this less of an issue than in the past. Note that some of these data processing applications are merely applying technology to existing devices to improve the way they work. A blank stare. Most reasonably complex devices contain data processing components to optimise their performance and some exist only because we can build in intelligence. but you should still be aware of these aspects. and we will have to make sure that they are not even aware that there is a computer in there! You should also remember that seemingly innocuous programs can have life threatening possibilities. These embedded systems will make computer users of everybody.2 Programs and Programming I tell people I am a "Software Engineer"..) Programmer’s Point: At the bottom there is always hardware It is important that you remember your programs are actually executed by a piece of hardware which has physical limitations.. Programming is a black art. These are the traditional uses of computers. It is into this world that we. It is important to think of business of data processing as much more than working out the company payroll.
  so as soon as they are given a problem they immediately start thinking of ways to solve it. tut tutting. Having looked at it for a while. Solving the Wrong Problem Coming up with a perfect solution to a problem the customer has not got is something which happens surprisingly often in the real world. The computer has to be made to understand what you are trying to tell it to do. In the real world such a definition is sometimes called a Functional Design Specification or FDS. We shall assume that the customer knows even less about computers than we do! Initially we are not even going to talk about the programming language. The worst thing you can say to a customer is "I can do that". From Problem to Program Programming is not about mathematics. the right thing was being built. since the developers had stopped asking them questions. If you think that learning to program is simply a matter of learning a programming language you are very wrong. 1. we are simply going to make sure that we know what the customer wants. Programming is defined by me as deriving and expressing a solution to a given problem in a form which a computer system can understand and execute. I am going to start on the basis that you are writing your programs for a customer. he will open his bag and produce various tools and parts. The developers of the system quite simply did not find out what was required. Both you and the C# Programming © Rob Miles 2010 9 . fit them all together and solve your problem. In fact if you think that programming is simply a matter of coming up with a program which solves a problem you are equally wrong! There are many things you must consider when writing a program. Many software projects have failed because the problem that they solved was the wrong one. I like to think of a programmer as a bit like a plumber! A plumber will arrive at a job with a big bag of tools and spare parts. He or she has problem and would like you to write a program to solve it. You are given a problem to solve. The art of programming is knowing which bits you need to take out of your bag of tricks to solve each part of the problem. You look at the problem for a while and work out how to solve it and then fit the bits of the language together to solve the problem you have got.1 What is Programmer? And remember just how much plumbers earn…. which both you and the customer agree on. You have at your disposal a big bag of tricks.2. This tells you exactly what the customer wants. Instead you should think "Is that what the customer wants?" This is a kind of self-discipline. not all of them are directly related to the problem in hand. Programming is just like this. and only at the final handover was the awful truth revealed. this almost a reflex action. Programmers pride themselves on their ability to come up with solutions. What you should do is think "Do I really understand what the problem is?" Before you solve a problem you should make sure that you have a watertight definition of what the problem is.Computers and Programs Programs and Programming Programming is defined by most people as earning huge sums of money doing something which nobody can understand. Unfortunately it is also the most difficult part of programming as well. type of computer or anything like that. The customers assumed that. in this case a programming language. The art of taking a problem and breaking it down into a set of instructions you can give a computer is the interesting part of programming. it is about organization and structure. It is therefore very important that a programmer holds off making something until they know exactly what is required. One or two things fall out of this definition: You need to be able to solve the problem yourself before you can write a program to do it. but instead created what they thought was required.
  "This looks like a nice little earner" you think. This is true even (or perhaps especially) if I do a job for a friend. He knows you are a programmer of sorts and would like your help in solving a problem which he has: He has just started making his own window units and is looking for a program which will do the costing of the materials for him. then you can think about ways of solving the problem.2. Modern development techniques put the customer right at the heart of the development. Once you have got your design specification. you are sitting in your favourite chair in the pub contemplating the universe when you are interrupted in your reverie by a friend of yours who sells double glazing for a living. Programmer’s Point: The specification must always be there I have written many programs for money. He wants to just enter the dimensions of the window and then get a print out of the cost to make the window. The height of the window. 1. There are lots of ways of representing this information in the form of diagrams. This is called prototyping. Writing some form of specification forces you to think about your problem at a very detailed level. Information coming out The information that our customer wants to see is: the area of glass required for the window the length of wood required to build a frame. It also forces you to think about what your system is not going to do and sets the expectations of the customer right at the start.. as a developer don’t really know much about the customer’s business and they don’t know the limitations and possibilities of the technology. These work on the basis that it is very hard (and actually not that useful) to get a definitive specification at the start of a project. The first thing you need to do is find out exactly what the customer wants you to do. What the system does with the information. and the bottom line is that if you provide a system which behaves according to the design specification the customer must pay you. and once you have agreed to a price you start work. This is not true. What flows out of the system.2 A Simple Problem Consider the scenario.. C# Programming © Rob Miles 2010 10 . there is no customer to satisfy. and involve them in the design process. Specifying the Problem When considering how to write the specification of a system there are three important things: What information flows into the system. You might think that this is not necessary if you are writing a program for yourself.Computers and Programs Programs and Programming customer sign it. for now we will stick with written text when specifying each of the stages: Information going in In the case of our immortal double glazing problem we can describe the information as: The width of a window.
  0 metres inclusive. we now have to worry about how our program will decide when the information coming in is actually valid. in metres and being a value between 0. Remember that we are selling double glazing. in square metres. and two pieces of wood the height of the window. Note that we have also added units to our description.5 metres and 2.5 Metres and 3. In the case of the above we could therefore expand the definition of data coming in as: The width of the window. and work can commence. this is very important . Having written this all up in a form that both you and the customer can understand.perhaps our customer buys wood from a supplier who sells by the foot.25 feet per metre. he or she must understand that if information is given which fits within the range specified. so two panes will be required. In the case of our window program the metadata will tell us more about the values that are being used and produced. we must then both sign the completed specification. in which case our output description should read: Note that both you and the customer must understand the document! The area of glass required for the window. in metres and being a value between 0. Being sensible and far thinking people we do not stop here..Computers and Programs Programs and Programming You can see what we need if you take a look at the diagram below: Height of Window Width of Window The area of the glass is the width multiplied by the height. For any quantity that you represent in a program that you write you must have at least this level of metadata . The length of wood required for the frame. The height of the window. Programmer’s Point: metadata is important Information about information is called metadata. To make the frame we will need two pieces of wood the width of the window. specifically the units in which the information is expressed and the valid range of values that the information may have.5 metres inclusive. given in feet using the conversion factor of 3. The word meta in this situation implies a "stepping back" from the problem to allow it to be considered in a broader context. C# Programming © Rob Miles 2010 11 . This must be done in conjunction with the customer. your program will regard the data as valid and act accordingly.
  set up a phased payment system so that you get some money as the system is developed. we have come full circle here. if you are going to use prototypes it is a good thing to plan for this from the C# Programming © Rob Miles 2010 12 . What will happen is that you will come up with something which is about 60% right. You then go back into your back room. The customer will tell you which bits look OK and which bits need to be changed.to emerge later with the perfect solution to the problem. there is no ambiguity which can lead to the customer requesting changes to the work although of course this can still happen! The good news for the developer is that if changes are requested these can be viewed in the context of additional work. reminiscent of a posh tailor who produces the perfect fit after numerous alterations. At this point the supplier knows that if a system is created which will pass all the tests the customer will have no option but to pay for the work! Note also that because the design and test procedures have been frozen.. sometimes even down to the colour of the letters on the display! Remember that one of the most dangerous things that a programmer can think is "This is what he wants"! The precise interaction with the user . Testing is a very important part of program development. There is even one development technique where you write the tests before you write the actual program that does the job. Customer Involvement Note also in a "proper" system the customer will expect to be consulted as to how the program will interact with the user.. you could for example say: “If I give the above program the inputs 2 metres high and 1 metre wide the program should tell me I need 4 square metres of glass and 19. Again. you can expect to write as much code to test your solution as is in the solution itself. This is not going to happen. This is actually a good idea.. is something which the customer is guaranteed to have strong opinions about. In a large system the person writing the program may have to create a test harness which is fitted around the program and will allow it to be tested. Remember this when you are working out how much work is involved in a particular job.Computers and Programs Programs and Programming Proving it Works In a real world you would now create a test which will allow you to prove that the program works. suggests changes and then wait for the next version to find something wrong with. how the information is presented etc. and emerge with another system to be approved. Ideally all this information should be put into the specification. Fact: If you expect to derive the specification as the project goes on either you will fail to do the job. However. Actually. . Rob's law says that 60% of the duff 40% will now be OK. muttering under your breath. which should include layouts of the screens and details of which keys should be pressed at each stage. In terms of code production. Quite often prototypes will be used to get an idea of how the program should look and feel.5 feet of wood. including the all-important error conditions. Both the customer and the supplier should agree on the number and type of the tests to be performed and then sign a document describing these.. They will get a bit upset when the delivery deadline goes by without a finished product appearing but they can always cheer themselves up again by suing you. because I did mention earlier that prototyping is a good way to build a system when you are not clear on the initial specificaiton. All the customer does is look at something. for which they can be expect to be paid.” The test procedure which is designed for a proper project should test out all possible states within the program. so you accept changes for the last little bit and again retreat to your keyboard. Getting Paid Better yet. and one we will explore later.what the program does when an error is encountered.
  Please note that this does not imply that tape worms would make good programmers! Computers are too stupid to understand English. particularly when you might have to do things like trade with the customer on features or price. You do not work for your customers.25 and print it The programming portion of the job is now simply converting the above description into a language which can be used in a computer. The customer may well say "But I am paying you to be the computer expert. One very good reason for doing this kind of thing is that it gets most of the program written for you . why can we not use something like English?" There are two answers to this one: 1.3 Programming Languages Once we know what the program should do (specification). an art. This is no excuse. This is very important. It is very hard to express something in an unambiguous way using English.. and there are limits to the size of program that we can create and the speed at which it can talk to us. therefore we cannot make a computer which can understand English. Explain the benefits of "Right First Time" technology and if that doesn't work produce a revolver and force the issue! Again. At the moment. If you do not believe me. You might ask the question "Why do we need programming languages. We cannot make very clever computers at the moment. you work with them. Programmer’s Point: Good programmers are good communicators The art of talking to a customer and finding out what he/she wants is just that. if I could underline in red I would: All the above apply if you are writing the program for yourself. The best we can do is to get a computer to make sense of a very limited language which we use to tell it what to do. the kind of simple systems we are going to create as we learn to program are going to be so trivial that the above techniques are far too long winded. by using the most advanced software and hardware..often with the help of the customer.. and how we are going to determine whether it has worked or not (test) we now need to express our program in a form that the computer can work with. You are wrong. ask any lawyer! Time Flies like an Arrow. all to the better. Fruit Flies like a Banana! C# Programming © Rob Miles 2010 13 . If you want to call yourself a proper programmer you will have to learn how to do this. we can make computers which are about as clever as a tape worm. Computers are made clever by putting software into them. You are your own worst customer! You may think that I am labouring a point here. Tape worms do not speak very good English. Fact: More implementations fail because of inadequate specification than for any other reason! If your insistence on a cast iron specification forces the customer to think about exactly what the system is supposed to do and how it will work. When we start with our double glazing program we now know that we have to: read in the width verify the value read in the height verify the value calculate width times height times 2 and print it calculate ( width + height ) * 2 * 3. One of the first things you must do is break down the idea of "I am writing a program for you" and replace it with "We are creating a solution to a problem".. 1. English would make a lousy programming language. To take the second point. I know nothing about these machines". 2. English as a language is packed full of ambiguities.Computers and Programs Programming Languages start rather than ending up doing extra work because your initial understanding of the problem was wrong. To take the first point...
  C is famous as the language the UNIX operating system was written in. If I make a mistake with the professional tool I could quite easily lose my leg. They are simple enough to be made sense of by computer programs and they reduce ambiguity. This makes the language much more flexible. The managed code is fussed over by the system which runs it.1 Dangerous C I referred to C as a dangerous language. Programmer’s Point: Computers are always stupid I reckon that you should always work on the basis that any computer will tolerate no errors on your part and anything that you do which is stupid will always cause a disaster! This concentrates the mind wonderfully. If you ever make the mistake of calling the language C hash you will show your ignorance straight away! C# is a very flexible and powerful programming language with an interesting history. i. Rob Miles.4 C# There are literally hundreds of programming languages around. which is a highly dangerous and entertaining language which was invented in the early 1970s. An unmanaged program goes faster.4. In programming terms what this means is that C lacks some safety features provided by other programming languages. 1. but do not think that it is the only language you will ever learn. you will need to know at least 3! We are going to learn a language called C# (pronounced C sharp). This makes sure that it is hard (but probably not impossible) to crash your computer running managed code. and enable direct access to parts of the underlying computer system. To get the maximum possible performance. having borrowed (or improved) features provided by these languages. As I am not an experienced chain saw user I would expect it to come with lots of built in safety features such as guards and automatic cut outs. causing your programs to run more slowly.4. However. all this fussing comes at a price. So what do I mean by that? Consider the chain saw. but if it crashes it is capable of taking the computer C# Programming © Rob Miles 2010 14 . C# is a great language to start programming in.2 Safe C# The C# language attempts to get the best of both worlds in this respect. because of all the safety stuff I might not be able to cut down certain kinds of tree. If I. you can mark your programs as unmanaged. It was developed by Microsoft Corporation for a variety of reasons. If I was a real lumberjack I would go out and buy a professional chain saw which has no safety features whatsoever but can be used to cut down most anything. and was specially designed for this. The origins of both Java and C++ can be traced back to a language called C. want to use a chain saw I will hire one from a shop.Computers and Programs C# Programming languages get around both of these problems. during your career you will have to learn more than just one.e. some political and others marketing. something the amateur machine would not let happen. so I have a much greater chance of crashing the computer with a C program than I do with a safer language. However. Programmer’s Point: The language is not that important There are a great many programming languages around. C# bears a strong resemblance to the C++ and Java programming languages. These will make me much safer with the thing but will probably limit the usefulness of the tool. 1. if I do something stupid C will not stop me. 1. some technical. A C# program can contain managed or unmanaged parts.
  Later on we will look at how you can break a program of your own down into a number of different chunks (perhaps so several different programmers can work on it).5 Creating C# Programs Microsoft has made a tool called Visual Studio. each of which is in charge of part of the overall system. which is a great place to write programs. They are then located automatically when your program runs. along with an integrated editor. i. These extra features are available to your C# program but you must explicitly ask for them.4. 1. Only if no errors are found by the compiler will it produce any output.NET Framework.4. 1. C# is a compiled programming language. which can be used to compile and run C# programs. A compiler is a very large program which knows how to decide if your program is legal. It comprises the compiler. The use of objects is as much about design as programming. C# is a great language to start learning with as the managed parts will make it easier for you to understand what has happened when your programs go wrong. Another free resources is the Microsoft . for now the thing to remember is that you need to show your wonderful C# program to the compiler before you get to actually run it. This provides a bunch of command line tools.3 C# and Objects The C# language is object oriented. There is a free version. This is not because I don't know much about it (honest) but because I believe that there are some very fundamental programming issues which need to be addressed before we make use of objects in our programs. It is provided in a number of versions with different feature sets. We will look in more detail at this aspect of how C# programs work a little later. I am very keen on object oriented programming. which is a great place to get started. set up network connections and the like. The C# language is supplied with a whole bunch of other stuff (to use a technical term) which lets C# programs do things like read text from the keyboard. How you create and run your programs is up to you. Objects are an organisational mechanism which let you break your program down into sensible chunks. Switching to unmanaged mode is analogous to removing the guard from your new chainsaw because it gets in the way. but may indicate that you have made a mistake somewhere. but I am not going to tell you much about it just yet. called Visual Studio Express edition.which may be part of the compiling and running system. in case you had forgotten to add a bit of your program. and debugger. The computer cannot understand the language directly. so a program called a compiler converts the C# text into the low level instructions which are much simpler. 1. print on the screen. The compiler would tell you about this. test and extend.Computers and Programs C# with it. An example of a warning situation is where you create something but don't use it for anything.e. It also lets you create programs which can have a high degree of reliability and stability.4. things that you type to the command prompt. The compiler will also flag up warnings which occur when it notices that you have done something which is not technically illegal. and we have to know how to program before we can design larger systems.4 Making C# Run You actually write the program using some form of text editor . The first thing it does is check for errors in the way that you have used the language itself. C# Programming © Rob Miles 2010 15 . Object Oriented Design makes large projects much easier to design. These low level instructions are in turn converted into the actual commands to drive the hardware which runs your program.
  Some parts of your program will simply provide this information to tell the compiler what to do. You must also decide on sensible names that you will use to identify these items. If you had sat down with a pencil and worked out the solution first you would probably get to a working system in around half the time. It also can be told about any options for the construction of your program which are important.NET framework. I am going to assume that you are using a computer which has a text editor (usually Notepad) and the . i.Computers and Programs C# I'm not going to go into details of how to download and install the .e. To take these in turn: Controlling the Compiler . which you will then spend hours fiddling with to get it going. This is not good technique. The C# compiler needs to know certain things about your program. I reckon that you write programs best when you are not sitting at the computer.4. for example a single structure could hold all the information about a particular bank customer. As part of the program design process you will need to decide what items of data need to be stored. A variable is simply a named location in which a value is held whilst the program runs. The recipe would be a list of ingredients followed by a sequence of actions to perform on them. not a cook. This is what the compiler acts on. All computer languages support variables of one form or another. Rather than writing the program down on a piece of paper you instead put it into a file on the computer. The Human Computer Of course initially it is best if we just work through your programs on paper. A program can be regarded as a recipe.NET framework installed. C# also lets you build up structures which can hold more than one item. the best approach is to write (or at least map out) your solution on paper a long way away from the machine.6 What Comprises a C# Program? If your mum wanted to tell you how to make your favourite fruitcake she’d write the recipe down on a piece of paper. often called a source file. that is for other documents. You will almost certainly end up with something which almost works. The ingredients will be values (called variables) that you want your program to work with. but written for a computer to follow. A source file contains three things: instructions to the compiler information about the structures which will hold the data to be stored and manipulated. The program itself will be a sequence of actions (called statements) that are to be followed by the computer. Programmer’s Point: Great Programmers debug less I am not impressed by hacking programmers who spend whole days at terminals fighting with enormous programs and debugging them into shape. Once you are sitting in front of the keyboard there is a great temptation to start pressing keys and typing something in which might work. instructions which manipulate the data. C# Programming © Rob Miles 2010 16 . Storing the Data Programs work by processing data. It needs to know which external resources your program is going to use. The data has to be stored within the computer whilst the program processes it. types in the program and makes it work first time! 1. I am impressed by someone who turns up.
  We will look at methods in detail later in these notes. Your mum might add the following instruction to her cake recipe: Now write the words “Happy Christmas” on top of the cake in pink icing. They are added automatically by the editor as you write your program. The colours just serve to make the programs easier to understand. and you try to make the name of the function fit what it does. These kinds of messages are coloured red in this text. Identifiers and Keywords You give a name to each method that you create. In fact. Seasoned programmers break down a problem into a number of smaller ones and make a method for each. and C# works in exactly the same way. The C# language actually runs your program by looking for a method with a special name. it is what needs to be written. and when Main finishes. for example add two numbers together and store the result. The really gripping thing about programs is that some statements can change which statement is performed next. which are used during the cooking process. One method may refer to others. for example ShowMenu or SaveToFile.Computers and Programs C# Describing the Solution The actual instructions which describe your solution to the problem must also be part of your program. Main. instruction to do something is in a C# program is called a statement. C# Programming © Rob Miles 2010 17 . Text in a Computer Program There are two kinds of text in your program. These save you from "re-inventing the wheel" each time you write a program. A single. To continue our cooking analogy. In a recipe a keyword would be something like "mix" or "heat" or "until". The names of objects will be given in a different shade of blue in some of the listings in this text. This method is called when your program starts running. A statement is an instruction to perform one particular operation. Keywords will appear blue in some of the listings in this text. these are things like mixing bowls and ovens. or very large. They would let you say things like "heat sugar until molten" or "mix until smooth". The names that you invent to identify things are called identifiers. She is using double quote characters to mark the text that is to be drawn on the cake. you'll find that programs look a lot like recipes. You also create identifiers when you make things to hold values. Later on we will look at the rules and conventions which you must observe when you create identifiers. It can have any name you like. woodLength might a good choice when we want to hold the length of wood required. It can return a value which may or may not be of interest. Colours and Conventions The colours that I use in this text are intended to line up with the colours you will see when you edit your programs using a professional program editor such as the one supplied as part of Visual Studio. so that your program can look at things and decide what to do. Such a lump is called a method. There are the instructions that you want the computer to perform and there are the messages that you want the program to actually display in front of the user. your program ends. The words which are part of the C# language itself are called keywords. and your program can contain as many methods as you see fit. A method can be very small. Objects Some of the things in the programs that we write are objects that are part of the framework we are using. In the case of C# you can lump statements together to form a lump of program which does one particular task. simple. ―Happy Christmas‖ is not part of the instructions. and do not have any special meaning. The C# language also has a huge number of libraries available which you can use.
  woodLength.ReadLine(). C# Programming © Rob Miles 2010 18 .1 The Program Example Perhaps the best way to start looking at C# is to jump straight in with our first ever C# program. height. We can now go through each line in turn and try to see how it fits into our program. 2.Parse(widthString). Here it is: using System. } } Code Sample 1 . height = double.1. The stuff after the two lines is concerned with displaying the answer to the user. Then we will use additional features of the C# language to improve the quality of the solution we are producing.WriteLine( "The area of the glass is " + glassArea + " square metres" ) .Simple Data Processing A First C# Program 2 Simple Data Processing In this chapter we are going to create a genuinely useful program (particularly if you are in the double glazing business). widthString = Console. class GlazerCalc { static void Main() { double width. woodLength = 2 * ( width + height ) * 3. This is the problem we set out to solve as described in section0. string widthString. Console.25 . We will start by creating a very simple solution and investigating the C# statements that perform basic data processing.1 A First C# Program The first program that we are going to look at will read in the width and height of a window and then print out the amount of wood and glass required to make a window that will fit in a hole of that size. Broadly speaking the stuff before these two lines is concerned with setting things up and getting the values in to be processed. glassArea. Console.WriteLine ( "The length of the wood is " + woodLength + " feet" ) . heightString.Parse(heightString). If you gave it to a C# compiler it would compile.ReadLine(). width = double. 2. glassArea = 2 * ( width * height ) . The actual work is done by the two lines that I have highlighted. heightString = Console. and you could run it.GlazerCalc Program This is a valid program.
  If you miss out the Main method the system quite literally does not know where to start. However. For now. i. Main You choose the names of your methods to reflect what they are going to do for you. this must be a mistake. One of these useful things provided with C# is the Console object which will let me write things which will appear on the screen in front of the user.Simple Data Processing A First C# Program using System. This is an instruction to the C# compiler to tell it that we want to use things from the System namespace. This method (and there must be one. Oh. In the case of our double glazing calculator the class just contains a single method which will work out our wood lengths and glass area. in other words the program above should be held in a file called GlazerCalc. static This keyword makes sure that the method which follows is always present. void A void is nothing. This means that if I refer to something by a particular name the compiler will look in System to see if there is anything matching that name. A big part of learning to program is learning how to use all the additional features of the system which support your programs.e. and one other thing. I've called ours GlazerCalc since this reflects what it does. In programming terms the void keyword means that the method we are about to describe does not return anything of interest to us. The method will just do a job and then finish. We will use other namespaces later on. You need to invent an identifier for every class that you create. A C# program is made up of one or more classes. When we get to consider objects we will find that this little keyword has all kinds of interesting ramifications. In some cases we write methods which return a result (in fact we will use such a method later in the program). C# Programming © Rob Miles 2010 19 . When your program is loaded and run the first method given control is the one called Main. just make sure that you pick sensible names for the classes that you create. in order to stop someone else accidentally making use of the value returned by our Main method. We have namespaces in our conversations too. This makes our programs safer. A namespace is a place where particular names have meaning. if I am using the "Football" namespace and I say “That team is really on fire” I'm saying something good. don't worry too much about classes. the word static in this context means "is part of the enclosing class and is always here". If I am using the "Firefighter" namespace I'm saying something less good. There is a convention that the name of the file which contains a particular class should match the class itself. If I want to just refer to this as Console I have to tell the compiler I'm using the System namespace. and only one such method) is where your program starts running. But for now I'd be grateful if you'd just make sure that you put it here in order to make your programs work properly. as we shall see later. class GlazerCalc Classes are the basis of object oriented programming. Except for Main. we are explicitly stating that it returns nothing. In the case of C# the System namespace is where lots of useful things are described.cs. but a class can contain much more than that if it needs to. in that the compiler now knows that if someone tries to use the value returned by this method. A class is a container which holds data and program code to do a particular job. you 2010 2010 2010
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  for two panes of glass). Note that I use a factor of 3.25 . However. so I multiply the result in meters by this factor.ReadLine(). Console. so I did what you would do in mathematics. I've put one multiplication in brackets to allow me to indicate that I am working out two times the area (i. The calculation is an expression much like above.25 feet in a meter.25 to allow for the fact that the customer wants the length of wood in feet. The + and * characters in the expression are called operators in that they cause an operation to take place. There is no need to do this particularly. i. These are what the operators work on. The other items in the expression are called operands.Simple Data Processing A First C# Program heightString = Console. This line repeats the calculation for the area of the glass. It is a string of text which is literally just in the program. not instructions to the compiler itself. all multiplication and division will be done first. followed by addition and subtraction.Parse(heightString). glassArea = 2 * ( width * height ) . 2 Another TV show reference C# Programming © Rob Miles 2010 23 . except that this one takes what it is given and then prints it out on the console. This is the bit that does the work. this time it is important that you notice the use of parenthesis to modify the order in which values are calculated in the expression. Note that the area is given in square meters. + Plus is an addition operator. Normally C# will work out expressions in the way you would expect.e. In this case it means "add two strings together". so no conversion is required. The string of text is enclosed in double quote characters to tell the compiler that this is part of a value in the program. We have seen these used in the calls of Parse and also ReadLine "The length of the wood is " This is a string literal.e. It takes the height and width values and uses them to calculate the length of wood required. I put brackets around the parts to be done first. woodLength = 2*(width + height)*3. When I write programs I use brackets even when the compiler does not need them. height = double. This is the actual nub of the program itself. the plus here means something completely different 2.WriteLine This is a call of a method. ( This is the start of the parameters for this method call. These two statements simply repeat the process of reading in the text of the height value and then converting it into a double precision value holding the height of the window. There are around 3. In the above expression I wanted to do some parts first. This makes the program clearer. We have seen it applied to add two integers together. but I think it makes it slightly clearer. just like the ReadLine method.
  This means that it is going to perform string concatenation rather than mathematical addition. This difference in behaviour is all because of the context of the operation that is being performed. We are adding the word feet on the end. The variable woodLength is tagged with metadata which says "this is a double precision floating point value. in this program the length of the wood required. giving the output: 5 But the line of code: Console. Here it is with operators.0 + 3. When the method is called the program first assembles a completed string out of all the components.0 ). Previously we have used woodLength as a numeric representation of a value. The C# compiler must therefore ask the woodLength data item to convert itself into a string so it can be used correctly in this position. adding (or concatenating) them to produce a single result. C# Programming © Rob Miles 2010 24 .03 The string "2.0 added on the end. Here it has a string on the left hand side. However.0 + 3. This would perform a numeric calculation (2. and so the program works as you would expect. use a plus with this and you concatenate".WriteLine ( "2.0" + 3. use a plus with this and you perform arithmetic". Whenever I print a value out I always put the units on the end.0 ). in the context it is being used at the moment (added to the end of a string) it cannot work like that. Consider: Console. Fortunately it can do this. It would then produce the output: 2.Simple Data Processing A First C# Program You will have to get used to the idea of context in your programs. The C# system uses the context of an operation to decide what to do.0" has the text of the value 3. It would ask the value 3 to convert itself into a string (sounds strange – but this is what happens. woodLength This is another example of context at work. between two double precision floating point numbers it means "do a sum". It is very important that you understand precisely what is going on here. This result value would then be asked to provide a string version of itself to be printed. It then passes the resulting string value into the method which will print it out on the console. This makes it much easier to ensure that the value makes sense.0) and produce a double precision floating point value. + " feet" Another concatenation here. Would regard the + as concatenating two strings. The variable heightString is tagged with information that says "this is a string. We have seen this with namespaces. ) The bracket marks the end of the parameter being constructed for the WriteLine method call. In the case of the previous +.WriteLine ( 2. You can think of all of the variables in our program being tagged with metadata (there is that word again) which the compiler uses to decide what to do with them.
  glassArea. It marks the end of the class GlazerCalc.Simple Data Processing A First C# Program . Items inside braces are indented so that it is very clear where they belong.width = double.WriteLine( "The area of the glass is " + glassArea + " square metres" ) . Punctuation That marks the end of our program. woodLength.widthString = Console.heightString = Console. I do it because otherwise I am just about unable to read the program and make sense of what it does. A block of code starts with a { and ends with a }. heightString.ReadLine(). otherwise you will get what is called a compilation error. The program is essentially complete. This first close brace marks the end of the block of code which is the body of the Main method. We have added all the behaviours that we need.} } . If we want to (and we will do this later) we may put a number of methods into a class. } The second closing brace has an equally important job to the first.Console. For now however. note that just because the compiler reckons your program is OK is no guarantee of it doing what you want! Another thing to remember is that the layout of the program does not bother the compiler. However. And so we use the second closing brace to mark the end of the class itself.height = double.. A class is a container for a whole bunch of things. consider the effect of missing out a "(" character. I do not do this because I find such listings artistically pleasing. the following is just as valid: using System. one of the things you will find is that the compiler does not always detect the error where it takes place. The semi-colon marks the end of this statement. One of the things that you will have noticed is that there is an awful lot of punctuation in there.WriteLine ( "The length of the wood is " + woodLength + " feet" ) .ReadLine().string widthString. including methods.Parse(widthString). C# Programming © Rob Miles 2010 25 . When the compiler sees this it says to itself "that is the end of the Main method".Parse(heightString).Console. However.25 . This simply indicates that the compiler is too stupid to make sense of what you have given it! You will quickly get used to hunting for and spotting compilation errors. height.woodLength = 2 * ( width + height ) * 3. This is vital and must be supplied exactly as C# wants it. we only want the one method in the class.glassArea = 2 * ( width * height ) . } Now for some really important stuff. In C# everything exists inside a class. JustlikeIfindithardtoreadenglishwithouttheproperspacing I find it hard to read a program if it has not been laid out correctly.class GlazerCalc{static void Main(){double width.
  1 Variables and Data In the glazing program above we decided to hold the width and the height of the windows that we are working on in variables that we described as double. Think of this as C# creating a box of a particular size. To handle real values the computer actually stores them to a limited accuracy. Too much accuracy may slow the machine down .Simple Data Processing Manipulating Data 2. you always have an exact number of these items.2 Manipulating Data In this section we are going to take a look at how we can write programs that manipulate data. how values can be stored. A variable is a named location where you can store something. We also need to consider the range of possible values that we need to hold so that we can choose the appropriate type to store the data. Before we can go much further in our programming career we need to consider just what this means. Because we know that it works in terms of ons and offs it has problems holding real values.e. specifically designed to hold items of the given type. These are real. which we hope is adequate (and usually is). The declaration also identifies the type of the thing we want to store.too little may result in the wrong values being used. i. for example the number of sheep in a field. 2. When you are writing a specification you should worry about the precision to which values are to be held. otherwise it is useless. apples in a basket. teeth on a cog. You can think of it as a box of a particular size with a name painted on the box. A computer is digital. You chose the name to reflect what is going to be stored there (we used sensible names like woodLength in the above program). This provides us with the ability to perform the data processing part of programs. retrieved and generally fiddled with. Programs also contain literal values. This means that when we want to store something we have to tell the computer whether it is an integer or a real. In the second case we can never hold what we are looking at exactly. Even if you measure a piece of string to 100 decimal places it is still not going to give you its exact length . You tell C# about a variable you want to create by declaring it. The type of the variable is part of the metadata about that variable. What the data actually means is something that you as programmer decide (see the above digression on data). These are referred to as reals. the speed of a car. Nasty real world type things. Programs operate on data. These are referred to as integers.2 Storing Numbers When considering numeric values there are two kinds of data: Nice chunky individual values. and what other types of data we can store in programs that we write. For each type of variable the C# language has a way in which literal values of that type are expressed. A programming language must give you a way of storing the data you are processing.you could always get the value more accurately. 2. A literal value is just a value in your program which you use for some purpose.2. for example the current temperature. the length of a piece of string. You also need to choose the type of the variable (particular size and shape of box) from the range of storage types which C# provides.2. they are integral. it operates entirely on patterns of bits which can be regarded as numbers. The box is tagged with some metadata (there is that word again) so that the system knows what can be put into it and how that box can be used. In the first case we can hold the value exactly. C# Programming © Rob Miles 2010 26 .
  I could use it in my program as follows: numberOfSheep = 23 . Remember that the language itself is unaware of any such considerations. Which could cause my program big problems. This means that if I do something stupid: sbyte tinyVal = 128. An example of an integer variable would be something which kept track of the number of sheep in a field: int numberOfSheep. When you edit your program source using Visual Studio (or another code editor that supports syntax highlighting) you will find that the names of types that are built into the C# language (such as int and float) are displayed in blue. Programmer’s Point: Check your own maths Something else which you should bear in mind is that a program will not always detect when you exceed the range of a variable.000 sheep and the number of sheep never goes negative you must add this behaviour yourself. The bigger the value the larger the number of bits that you need to represent it. int. C# provides a variety of integer types. in the range -2.147. integer literal values An integer literal is expressed as a sequence of digits with no decimal Point: 23 This is the integer value 23. If I put the value 255 into a variable of type byte this is OK. The only issue is one of range. C# Programming © Rob Miles 2010 27 .647 If you want to hold even larger integers than this (although I've no idea why you'd want this) there is a long version. depending on the range of values you would like to store: sbyte byte short ushort int uint long ulong char Note that we can go one further negative than positive. This creates a variable with could keep track of over two thousand million sheep! It also lets a program manipulate "negative sheep" which is probably not meaningful (unless you run a sheep bank of course and let people borrow them). In fact this may cause the value to "wrap round" to 0. This is because the numbers are stored using "2's complement" notation. can hold frighteningly large numbers in C#.648 to 2.483.Simple Data Processing Manipulating Data Storing integer values Integers are the easiest type of value for the computer to store. because 255 is the biggest possible value of the type can hold. If you want to make sure that we never have more than 1.147. not an integer. as shown above. Each value will map onto a particular pattern of bits. if I add one to the value in this variable the system may not detect this as an error.. If I am using one of the "shorter" types the literal value is regarded by the compiler as being of that type: sbyte tinyVal = 127. However. In this statement the 127 is regarded as an sbyte literal.483.
  Finally. An example of a float variable could be something which held the average price of ice cream: float averageIceCreamPriceInPence. C# provides a type of box which can hold a real number. If you want more precision (although of course your programs will be use up more computer memory and run more slowly) you can use a double box instead (double is an abbreviation for double precision). as float or as double.e.7E308 and a precision of 15 digits. They have a decimal point and a fractional part. hence the name float. If you put an f on the end it becomes a floating point literal value. not as good as most pocket calculators). decimal robsOverdraft. with real numbers the compiler is quite fussy about how they can and can't be combined. This means that you have to take special steps to make sure that you as programmer make clear that you want this to happen and that you can live with the consequences. C# Programming © Rob Miles 2010 28 . A standard float value has a range of 1. This uses twice the storage space of a double and holds values to a precision of 28-29 digits. An example of a double variable could be something which held the width of the universe in inches: double univWidthInInches. Depending on the value the decimal point floats around in the number. real literal values There are two ways in which you can store floating point numbers. A float literal can be expressed as a real number with an f after it: 2.0E-324 to 1. This is because when you move a value from a double precision variable into an ordinary floating point one some of the precision is lost. When it comes to putting literal values into the program itself the compiler likes to know if you are writing a floating point value (smaller sized box) or double precision (larger sized box).Simple Data Processing Manipulating Data (the maximum value that an sbyte can hold is 127) the compiler will detect that I have made a mistake and the program will not compile. It is used in financial calculations where the numbers are not so large but they need to be held to very high accuracy. if you want the ultimate in precision but require a slightly smaller range you can use the decimal type.4E48 with a precision of only 7 digits (i. This is takes up more computer memory but it has a range of 5.5f A double literal is expressed as a real number without the f: 3. Unlike the way that integers work.5E-45 to 3.4605284E15 This is a double precision literal value which is actually the number of meters in a light year. Storing real values "Real" is a generic term for numbers which are not integers.5 You can also use exponents to express double and float values: 9. This process is known as casting and we will consider it in detail a bit later.
  tend to use just integers (int) and floating point (float) variable types. If you are editing your program using an editor that supports syntax highlighting a character literal is shown in red. 2. This is a sequence of characters which starts with a special escape character. You can use them as follows: char beep = '\a' . Some clear the screen when you send the Form feed character.000 different character designs including a wide range of foreign characters. This is achieved by the use of an escape sequence. Some systems will make a beep when you send the Alert character to them. C# uses a character set called UNICODE which can handle over 65. This may seem wasteful (it is most unlikely I'll ever need to keep track of two thousand million sheep) but it makes the programs easier to understand.2. C# provides variables for looking after both of these types of information: char variables A char. A character is what you get when you press a key on a keyboard or display a single character on the screen. The escape character is the \ (backslash) character. Escape in this context means "escape from the normal hum-drum conventions of just meaning what you are and let's do something special". C# Programming © Rob Miles 2010 29 . It is what your program would get if you asked it to read a character off the keyboard and the user held down shift and pressed A.Simple Data Processing Manipulating Data Programmer’s Point: Simple variables are probably best You will find that I. at other times it will be a string. char literal values You express a character by enclosing it in single quotes: 'A' This means "the character A". An example of a character variable could be something which held the command key that the user has just pressed: char commandKey. Character Escape Sequences This leads to the question "How do we express the ' (single quote) character". and most programmers.3 Storing Text Sometimes the information we want to store is text. This can be in the form of a single character.
  C# Programming © Rob Miles 2010 30 . Note that I have to put leading zeroes in front of the two hex digits. you can express a character literal as a value from the Unicode character set. An example of a string variable could be something which holds the line that the user has just typed in: string commandLine. However. I do this by putting an @ in front of the literal: @"\x0041BCDE\a" If I print this string I get: \x0041BCDE\a This can be useful when you are expressing things like file paths. The bad news is that you must express this value in hexadecimal which is a little bit harder to use than decimal.Simple Data Processing Manipulating Data Note that the a must be in lower case. As an example however. C# uses the Unicode standard to map characters onto numbers that represent them. 41). string literal values A string literal value is expressed enclosed in double quotes: "this is a string" The string can contain the escape sequences above: "\x0041BCDE\a" If we print this string it would print out: ABCDE . if at all. The best news of all is that you probably don't need to do this kind of thing very often. A string variable can hold a line of text. string variables A type of box which can hold a string of text. or it can be very long. If you wish. Character code values We have already established that the computer actually manipulates numbers rather than actual letters. which is that you can use it to get string literals to extend over several lines: @"The quick brown fox jumps over the lazy dog" This expresses a string which extends over three lines. because there are special characters which mean "take a new line" (see above) it is perfectly possible for a single string to hold a large number of lines of text. In C# a string can be very short.and try to ring the bell. I can therefore put this in my program as: char capitalA = '\x0041' .e. for example "War and Peace" (that is the book not the three words). I happen to know that the Unicode value for capital a (A) is 65. If I am just expressing text with no escape characters or anything strange I can tell the compiler that this is a verbatim string. for example "Rob". The line breaks in the string are preserved when it is stored. This is represented in hexadecimal (base 16) as four sixteens and a single one (i. The good news is that this gives you access to a huge range of characters (as long as you know the codes for them). The verbatim character has another trick.
  2.4 Storing State using Booleans A bool (short for boolean) variable is a type of box which can hold whether or not something is true. They introduce a lack of precision that I am not too keen on. As an example. you might think that being asked to work with the speed of a car you would have to store a floating point value. bool literal values These are easily expressed as either true or false: networkOK = true . when you are given the prospect of storing floating point information. char 29yesitsme . when you find out that the speed sensor only gives answers to an accuracy of 1 mile per hour. For example. Fred and fred are different identifiers. I find that with a little bit of ingenuity I can work in integers quite comfortably. one of which are not valid (see if you can guess which one and why): int fred . However. along with "always use the keyboard with the keys uppermost" is: Always give your variables meaningful names.e. One of the golden rules of programming. the best relationships are meaningful ones.5 Identifiers In C# an identifier is a name that the programmer chooses for something in the program. Also when considering how to store data it is important to remember where it comes from. This illustrates another metadata consideration. If you are storing whether or not a subscription has been paid or not there is no need to waste space by using a type which can hold a large number of possible values. Instead you just need to hold the states true or false. An example of a bool variable could be one which holds the state of a network connection: bool networkOK. C# has some rules about what constitutes a valid identifier: All identifiers names must start with a letter. This is really stupid. find out how it is being produced before you decide how it will be stored. Here are a few example declarations.5 pounds (needing the use of float) I will store the price as 150 pence.Simple Data Processing Manipulating Data 2. I tend to use floating point storage only as a last resort. float jim . C# Programming © Rob Miles 2010 31 . rather than store the price of an item as 1. this makes the job much simpler. Programmer’s Point: Think about the type of your variables Choosing the right type for a variable is something of a skill. According to the Mills and Boon romances that I have read. In the example above I've used the double type to hold the width and height of a window. Sometimes that is all you want. 2. The name of a variable is more properly called an identifier. We will see other places where we create identifiers. I would suspect that my glazing salesman will not be able to measure to accuracy greater than 1 mm and so it would make sense to only store the data to that precision. i.2. These are the only two values which the bool type allows. Upper and lower case letters are different. It does not take into account the precision required or indeed how accurately the window can be measured. After the letter you can have either letters or numbers or the underscore "_" character.
  Perhaps the name averageIceCreamPriceInPence is a bit over the top in this respect. An assignment gives a value to a specified variable. Gozzintas take the result on the right hand side of the assignment and drop it into the box on the left. The equals in the middle is there mainly to confuse us. I like to think of it as a gozzinta (see above). it does not mean equals in the numeric sense. These are assignment statements.2. first = 1 .Simple Data Processing Manipulating Data The convention in C# for the kind of variables that we are creating at the moment is to mix upper and lower case letters so that each word in the identifier starts with a capital: float averageIceCreamPriceInPence. Expressions can be as simple as a single value and as complex as a large calculation. 2.is a piece of programming naughtiness which would cause all manner of nasty errors to appear. We can then use the result as we like in our program. There are two parts to an assignment. C# does this by means of an assignment statement. second = 2 . second and third. . Remember that you can always do sensible things with your layout to make the program look OK: averageIceCreamPriceInPence = computedTotalPriceInPence / numberOfIceCreams. C# Programming © Rob Miles 2010 32 . does not know or care what you are doing). presumably because of the "humps" in the identifier which are caused by the capitals. operators and operands. These are each of integer type. which must be of a sensible type (note that you must be sensible about this because the compiler. and get the value out. The value which is assigned is an expression. Programmer’s Point: Think about the names of your variables Choosing variable names is another skill you should work at. Expressions An expression is something which can be evaluated to produce a result. for example consider the following: class Assignment { static void Main () { int first. the thing you want to assign and the place you want to put it. They are made up of two things. Within the Main function we have declared three variables. third . This is sometimes called camel case. which means that: 2 = second + 1. But I could live with it. first. as we already know. second. second = second + first .6 Giving Values to Variables Once we have got ourselves a variable we now need to know how to put something into it. The last three statements are the ones which actually do the work. They should be long enough to be expressive but not so long that your program lines get too complicated. } } The first part of the program should be pretty familiar by now.
  because of the difficulty of drawing one number above another on a screen we use this character instead Addition.g. generally speaking things tend to be worked out how you would expect them. just as in traditional maths all the multiplication and division is performed first in an expression. multiplication. They are usually literal values or the identifiers of variables. provided you make sure that you have as many open ones as close ones. This is not a complete list of all the operators available. Here are a few example expressions: 2 + 3 * 4 -1 + 3 (2 + 3) * 4 These expressions are worked out (evaluated) by C# moving from left to right.2. This can cause problems as we shall see now. Again. -1. but it will do for now. C# does this by giving each operator a priority. e. You can put brackets inside brackets if you want. third are identifiers and 2 is a literal value. Because these operators work on numbers they are often called the numeric operators. Being a simple soul I tend to make things very clear by putting brackets around everything. Note that we use exactly the same character as for unary minus. * / + unary minus. Note that this means that the first expression above will therefore return 14 and not 20. It is probably not worth getting too worked up about this expression evaluation as posh people call it. Most operators work on two operands. It worries about whether or not the operation I'm about C# Programming © Rob Miles 2010 33 . division.Simple Data Processing Manipulating Data Operands Operands are things the operators work on. 2. If you want to force the order in which things are worked out you can put brackets around the things you want done first. It is also possible to use operators in a way which causes values to be moved from one type to another. subtraction. It then looks for the next ones down and so on until the final result is obtained. In the program above first. as in the final example. the minus that C# finds in negative numbers. When C# works out an expression it looks along it for all the operators with the highest priority and does them first. A literal value is something which is literally there in the code.7 Changing the Type of Data Whenever I move a value from one type to another the C# compiler gets very interested in what I am doing. note the use of the * rather than the more mathematically correct but confusing x. I am listing the operators with the highest priority first. A literal value has a type associated with it by the compiler. second. one each side. For completeness here is a list of all operators. Operators Operators are the things which do the work: They specify the operation to be performed on the operands. just as you would yourself. But of course you should remember that some of them (for example +) can be used between other types of data as well. Unary means applying to only one item. followed by the addition and subtraction. what they do and their precedence (priority). In the program above + is the only operator.
  If I decide to switch to a smaller case I will have to take everything out of the large case and put it into the smaller one. This means that if you do things like this: int i . . . You cast a value by putting the type you want to see there in brackets before it. I. In the above code the message to the compiler is "I don't care that this assignment could cause the loss of information. I do not think of this as a failing in C#.would cause the compiler to complain (even though at the moment the variable x only holds an integer value). Casting We can force C# to regard a value as being of a certain type by the use of casting. This means that if I write: int i = 1 . In C# terms the "size" of a type is the range of values (the biggest and smallest) and the precision (the number of decimal places).5.. int i = x . You can regard casting as the compiler's way of washing its hands of the problem. The bigger case will take everything that was in the smaller case and have room for more. A cast takes the form of an additional instruction to the compiler to force it to regard a value in a particular way.5. and the range of floating point values is much greater than that for integers. Widening and Narrowing The general principle which C# uses is that if you are "narrowing" a value it will always ask you to explicitly tell it that this is what you want to do. If a program fails because data is lost it is not because the compiler did something silly. as the writer of the program. each type of variable has a particular range of possible values. Nothing in C# checks for mistakes like this. However: float x = 1. since the compiler knows that a double is wider than a float.. If you are widening there is no problem.999 . float f = (float) d . It considers every operation in terms of "widening and narrowing" values.. will take the responsibility of making sure that the program works correctly". for example: double d = 1.the cast is doomed to fail.the program will not notice but the user certainly will! C# Programming © Rob Miles 2010 34 . . It is up to you when you write your program to make sure that you never exceed the range of the data types you are using . However. float x = i. To understand what we mean by these terms we could consider suitcases. The value which gets placed in i will be invalid. As we saw above.Simple Data Processing Manipulating Data to perform will cause data to be lost from the program. i = (int) 123456781234567890. It gives you great flexibility.would cause an error as well. Note that this applies within a floating point values as well. For example: double d = 1. If I am packing for a trip I will take a case. at the cost of assuming you know what you are doing. float f = d . This works fine because the floating point type can hold all the values supported by the integer type. But it might not have room. if I change to a bigger case there is no problem. The compiler is concerned that you may be discarding information by such an assignment and treats it as an error. so I have to leave behind one of my shirts. This is "narrowing".
  because it involves a floating point value would however be evaluated to give a double precision floating point result. This process discards the fractional part.4f . It therefore would calculate this to be the integer value 0 (the fractional part is always truncated).999 (which would be compiled as a value of type double) and casts it to int. .2.0 by the string "stupid". and this includes how it allows literal values to be used.4 . it is not.4 is a double precision value when expressed as a literal. x = 3. This code looks perfectly legal. should give an integer result. C# Programming © Rob Miles 2010 35 . and the variable x has been declared as a floating point. However. These are just values you want to use in your calculations. consider the following: 1/2 1/2.999 . The code above takes 1.25 to convert the length value from meters to feet (there are about 3. Essentially. consider this: float x . i = 3. The C# compiler knows that it is daft to divide the value 3. even though the original number was much closer to 2. C# keeps careful track of the things that it is combining. This casts the double precision literal into a floating point value.8 Types of Data in Expressions When C# uses an operator. the more accurate answer of 0. . The second expression.will be treated with the contempt they richly deserve.25 feet in a meter). This means that: float x . decimal) into one without.0 You might think that these would give the same result. x = 3. This can lead to problems. if the two operands are integer it says that the result should be integer.4 . This is because the literal value 3. it makes a decision as to the type of the result that is to be produced. in our double glazing program we had to multiply the wood length in meters by 3.Simple Data Processing Manipulating Data A cast can also lose information in other ways: int i . x = (float) 3. For example.4 / "stupid" . i = (int) 1. This is so that statements like: int i . If the two are floating point it says that the result should be floating point. so that the assignment works. If I want to put a floating point literal value into a floating point variable I can use casting: float x . To make life easier the creators of C# have added a different way we can express a floating point literal value in a program. which involves only integers.would compile correctly. double. Not so. However. The compiler thinks that the first expression. You should remember that this truncation takes place whenever you cast from a value with a fractional part (float.5. 2. If you put an f after the value this is regarded as a floating point value. which means that the variable I will end up with the value 1 in it. Casting and Literal Values We have seen that you can put "literal" values into your program.
  this can make the program clearer. Console.2. so that we get 1. who runs a chemist shop. fraction = (float) i / (float) j .WriteLine ( "The length of the wood is " + woodLength + " feet" ) .25 .ReadLine().Simple Data Processing Manipulating Data The way that an operator behaves depends on the context of its use. "Ro" + "b" would give the result "Rob".Parse(heightString). and the number of bottles that he needs. store it in an appropriate type of location and then uses the stored values to calculate the result that the user requires: string widthString = Console. string heightString = Console. He enters the cost of the tablets and the number he wants. the only hard part is figuring out how many bottles that are needed for a particular number of tablets. glassArea = 2 * ( width * height ) . j = 2 .Parse(widthString). Console. If you want complete control over the particular kind of operator the compiler will generate for you the program must contain explicit casts to set the correct context for the operator. 2. class CastDemo { static void Main () { int i = 3. You can very easily modify your program to do this job. If you just divide the number of tablets by 100 an integer division will give you the wrong answer (for any number of tablets less than 100 your program will tell you that 0 bottles are needed). woodLength = 2 * ( width + height ) * 3. One way to solve this is to add 99 to the number of tablets before you C# Programming © Rob Miles 2010 36 . consider another friend of yours.e. i. height = double. Programmer’s Point: Casts can add clarity I tend to put the casts in even if they are not needed.ReadLine(). double width = double. He wants a program that will work out the total cost of tablets that he buys.WriteLine ( "fraction : " + fraction ) . The tablets are always sold in bottles of 100. Console.9 Programs and Patterns At this point we can revisit our double glazing program and look at the way that it works.5 printed out rather than 1. Later on we will see that the + operator. The interesting thing about this is that it is a pattern of behaviour which can be reused time and time again. can be used between strings to concatenate them together. The code that actually does the work boils down to just a few lines which read in the data. As an example. which normally performs a numeric calculation.WriteLine("The area of the glass is " + glassArea + " square metres" ) . It may not affect the result of the calculation but it will inform the reader of what I am trying to do. using System. float fraction . } } The (float) cast in the above tells the compiler to regard the values in the integer variables as floating point ones.
  print it out) which is common to many applications.ReadLine().Parse(pricePerBottleString). The different blocks should be indented and the statements spread over the page in a well formed manner. It should have good punctuation and grammar. A good program is well laid out. The interesting thing here is that the program for the chemist is actually just a variation on the program for the double glazing salesman.WriteLine( "The total price is " + salePrice ) . I think that while it is not a story as such. All the names in the text should impart meaning and be distinct from each other. They may have to make a decision based on the data which they are given. This makes the active part of the program as follows: int bottleCount = ((tabletCount + 99) / 100) . do something with it and then print out the result. In this section we are going to consider how we can give our programs that extra level of complexity. We can then put the rest of the code around this to make a finished solution: Remember that if you divide two integers the result is always rounded down and the fractional part discarded. int salePrice = bottleCount * pricePerBottle .ReadLine(). Part of the skill of a programmer is identifying the nature of a problem in terms of the best pattern to be used to solve it. I'm not completely convinced that this is true. works out some answers and then prints out the result you can make use of this pattern. However. string tabletCountString = Console. Console. 2. we will need to create programs which do things which are more complex that this.Simple Data Processing Writing a Program perform the division. I have found that some computer manuals are works of fiction. int bottleCount = ((tabletCount + 99) / 100) . int pricePerBottle = int. int tabletCount = int. int salePrice = bottleCount * pricePerBottle . process it. At no point should the hapless reader be forced to backtrack or brush up on knowledge that the writer assumes is there. They may need to repeat things until something is true. string pricePerBottleString = Console.3 Writing a Program The programs that we have created up until now have been very simple. Both conform to a pattern of behaviour (read data in. Any time that you are asked to write a program that reads in some data. They just read data in.Parse(tabletCountString). but programs are something else. C# Programming © Rob Miles 2010 37 . and look at the general business of writing programs. Console. a good program text does have some of the characteristics of good literature: It should be easy to read.3. It should look good on the page. forcing the number of bottles required to "round up" any number of tablets greater than 0.WriteLine ( "The number of bottles is " + bottleCount ) .1 Software as a story Some people say that writing a program is a bit like writing a story. The various components should be organised in a clear and consistent way. 2. They may need to read in a large amount of data and then process that in a number of different ways.
  If you chose sensible identifiers you should find that your program will express most of what it does directly from the code itself. Block Comments When the C# compiler sees the "/*" sequence which means the start of a comment it says to itself: “Aha! Here is a piece of information for greater minds than mine to ponder. A program without comments is a bit like an aeroplane which has an autopilot but no windows. Basically there are three types of program flow: 1. when it was last modified and why.2 Controlling Program Flow Our first double glazing program is very simple. If you change what you wrote you should add information about the changes that you made and why. Line Comments Another form of comment makes use of the // sequence. They help to make your program much easier to understand. This marks the start of a comment which extends to the end of that particular line of code. It is useful for putting a quick note on the end of a statement: position = position + 1 . 2. But don't go mad. // move on to the next customer I've annotated the statement to give the reader extra information about what it actually does. If you write something good you should put your name on it. it runs straight through from the first statement to the last. You will be very surprised to find that you quickly forget how you got your program to work. There is a chance that it might take you to the right place. 3. A big part of a well written program is the comments that the programmer puts there. I will ignore everything following until I see a */ which closes the comment. Programmer’s Point: Don't add too much detail Writing comments is a very sensible thing to do. and when it was last changed. and the name of the programmer who wrote it – even if it was you. 2.” As an example: /* This program works out glass and wood required for a double glazing salesman. straight line chosen depending on a given condition repeated according to a given condition C# Programming © Rob Miles 2010 38 . but it will be very hard to tell where it is going from the inside. Often you will come across situations where your program must change what it does according to the data which is given to it. // add one to goatCount This is plain insulting to the reader I reckon. and then stops. */ Be generous with your comments. You can also use comments to keep people informed of the particular version of the program .Simple Data Processing Writing a Program It should be clear who wrote it. to in logical expressions. Relational operators work on operands, just like numeric ones. However any expression involving them can only produce one of two values, true or false. Relational operators available are as follows:
  C# Programming © Rob Miles 2010 2010 2010
  41
  class GlazerCalc { static void Main() { double width. widthString = Console. There is a convention that you always give constant variables names which are expressed in CAPITAL LETTERS.Parse(heightString). Console. heightString = Console. This both more meaningful (we are using PI not some anonymous value) and makes the program quicker to write. We can do this by making a variable which is constant.WriteLine ( "Width is too large. MIN_WIDTH = 0.75 .0. for some reason.Parse(widthString).WriteLine ( "Using minimum" ) . it can never be changed. This means that you can do things like: circ = rad * 2 * PI . const const const const double double double double MAX_WIDTH = 5.75 and 3. for example: const double MAX_WIDTH 5.0 . my maximum glass size becomes 4.0 . These come from the metadata I was so careful to gather when I wrote the specification for the program. MIN_HEIGHT = 0.Write ( "Give the width of the window : " ). i. This is so that when you read your program you can tell which things have been defined. If.WriteLine ( "Using maximum" ) . glassArea.e.Simple Data Processing Writing a Program values for heights and widths. what I would like to do is replace each number with something a bit more meaningful. const double PI=3.Write ( "Give the height of the window : " ).0 .but these are not packed with meaning and make the program hard to change.0 . } Console. height = double. 0.5 metres I have to look all through the program and change only the appropriate values.5 . Console. woodLength. Now I could just use the values 0. heightString. Anywhere you use a magic number you should use a constant of this form. I do not like the idea of "magic numbers" in programs.ReadLine(). height. C# Programming © Rob Miles 2010 42 . width = MAX_WIDTH .5. Console. if (width < MIN_WIDTH) { Console. and also much easier to change.141592654. width = double.ReadLine(). width = MIN_WIDTH . This makes your programs much easier to read.\n\n" ) . } if (width > MAX_WIDTH) { Console. 5. MAX_HEIGHT = 3. string widthString.WriteLine ( "Width is too small" ) . We can therefore modify our double glazing program as follows: using System.
  C# allows us to do this by providing looping constructions. do -.while loop In the case of our little C# program we use the do -. depending on precisely what you are trying to do. giving a proper number should cause our loop to stop.while construction which looks like this: C# Programming © Rob Miles 2010 43 . 2.\n\n" ) . glassArea = 2 * ( width * height ) .25 . This means that if we get the number correctly first time the loop will execute just once. In the case of our program we want to repeatedly get numbers in until while we are getting duff ones.WriteLine ( "Using height = MAX_HEIGHT . (the rest is finding out why the tool didn't do what you expected it to!).\n\n" ) . Console. maximum" ) .WriteLine ( "Using height = MIN_HEIGHT .3. However I would still not call it perfect.WriteLine( "Height Console. minimum" ) .Simple Data Processing Writing a Program if (height < MIN_HEIGHT) { Console.. i. C# has three ways of doing this. or a given number of times. However often you want to repeat something while a particular condition is true. You might think that I have pulled a fast one here. } } This program fulfils our requirements. What we would really like is a way that we can repeatedly fetch values for the width and height until we get one which fits. is too large. Most of the skill of programming involves picking the right tool or attachment to do the job in hand.WriteLine( "The area of the glass is " + glassArea + " square metres" ) . Console. with the height having to be entered again. If our salesman gives a bad height the program simply limits the value and needs to be re-run. } if (height > MAX_HEIGHT) { Console. woodLength = 2 * ( width + height ) * 3. Note that we get three methods not because we need three but because they make life easier when you write the program (a bit like an attachment to our chainsaw to allow it to perform a particular task more easily).e. } is too small.WriteLine( "Height Console.WriteLine ( "The length of the wood is " + woodLength + " feet" ) .3 Loops Conditional statements allow you to do something if a given condition is true.
  e. energy futures and cosmology. i. Wet Your Hair Add Shampoo and Rub vigorously. A condition in this context is exactly the same as the condition in an if construction. class Forever { public static void Main () { do Console. for loop Often you will want to repeat something a given number of times. Note that the test is performed after the statement or block. i. Your electricity runs out. Rinse with warm water. This allows us to repeat a chunk of code until the condition at the end becomes false. The loop constructions we have given can be used to do this quite easily: C# Programming © Rob Miles 2010 44 . Just as it is possible with any old chainsaw to cut off your leg if you try really hard so it is possible to use any programming language to write a program which will never stop. (if you put the do in the compiler will take great delight in giving you an error message ..Simple Data Processing Writing a Program do statement or block while (condition) . raising the intriguing possibility of programs like: using System. } } This is a perfectly legal C# program. This is a chainsaw situation. You get bored with it. It reminds me of my favourite shampoo instructions: 1. I wonder how many people are still washing their hair at the moment? while loop Sometimes you want to decide whether or not to repeat the loop before you perform it.WriteLine ( "Hello mum" ) . 2. 4. How long it will run for is an interesting question. For our program this is exactly what we want. it will run until: 1. even if the test is bound to fail the statement is performed once. If you think about how the loop above works the test is done after the code to be repeated has been performed once. while ( true ). 2. Repeat.but you had already guessed that of course!). The universe implodes.e. the answer contains elements of human psychology. not a powerful chainsaw situation. 3. 3. we need to ask for a value before we can decide whether or not it is valid.
  i = 1 . i = i + 1 . } } } The setup puts a value into the control variable which it will start with. The update is the statement which is performed to update the control variable at the end of each loop. for ( i = 1 . C# provides a construction to allow you to set up a loop of this form all in one: for ( setup .loop to continue. } } } The variable which controls things is often called the control variable. class WhileLoopInsteadOfFor { public static void Main () { int i . And serve you right. If you are so stupid as to mess around with the value of the control variable in the loop you can expect your program to do stupid things. finish test . 4. Perform the update. and is usually given the name i. class ForLoop { public static void Main () { int i . Writing a loop in this way is quicker and simpler than using a form of while because it keeps all the elements of the loop in one place instead of leaving them spread about the program. Eventually it will reach 11. Repeat from step 2. 3. if you put i back to 0 within the loop it will run forever. The test is a condition which must be true for the for -. i = i + 1 ) { Console. This useless program prints out hello mum 10 times. at which point the loop terminates and our program stops. i < 11 . 5. It does this by using a variable to control the loop. or update it. Test to see if we have finished the loop yet and exit to the statement after the for loop if we have. Perform the statements to be repeated. update ) { things we want to do a given number of times } We could use this to re-write the above program as: using System. This means that you are less likely forget to do something like give the control variable an initial value. 2.e. The variable is given an initial value (1) and then tested each time we go around the loop. Note that the three elements are separated by semicolons.Simple Data Processing Writing a Program using System.WriteLine ( "Hello mum" ) . while ( i < 11 ) { Console. i. Put the setup value into the control variable. The precise sequence of events is as follows: 1. The control variable is then increased for each pass through the statements.WriteLine ( "Hello mum" ) . C# Programming © Rob Miles 2010 45 .
  . This is a standard programming trick that you will find very useful. For this reason the goto is condemned as a potentially dangerous and confusing device. In every case the program continues running at the statement after the last statement of the loop. bit we get to if aborted becomes true . more complex stuff . There is rarely need for such convoluted code. Note that we are using two variables as switches.. but can still lead to problems.. This happens when you have gone as far down the statements as you need to. This means that if my program is at the statement immediately following the loop. normally false becomes true when the loop has to be abandoned and the variable runningOK. increment and test.. You can do this with the break statement. The goto is a special one that lets execution jump from one part of the program to another. if (aborted) { break . C# provides the continue keyword which says something along the lines of: C# Programming © Rob Miles 2010 46 .Simple Data Processing Writing a Program Programmer’s Point: Don't be clever/stupid Some people like to show how clever they are by doing cunning things with the setup. Going back to the top of a loop Every now and then you will want to go back to the top of a loop and do it all again... they are actually used to represent states within the program as it runs. which can do things other than simple assignment. Your program would usually make some form of decision to quit in this way.. Programmer’s Point: Be careful with your breaks The break keyword smells a little like the dread goto statement. for example in the following program snippet the variable aborted. condition and update statements.. they do not hold values as such. Some programmers think they are very clever if they can do all the work "inside" the for part at the top and have an empty statement after it. The break construction is less confusing than the goto. there are a number of ways it could have got there. one for every break in the loop above. becomes false when it is time to finish normally. your program may decide that there is no need or point to go on and wishes to leap out of the loop and continue the program from the statement after it.. Breaking Out of Loops Sometimes you may want to escape from a loop whilst you are in the middle of it. In this respect we advise you to exercise caution when using it. normally true. This is a command to leave the loop immediately. I find it most useful so that I can provide a "get the heck out of here" option in the middle of something. i.. When you are writing programs the two things which you should be worrying about are "How do I prove this works?" and "How easy is this code to understand?" Complicated code does not help you do either of these things. } . I call these people "the stupid people". } ... while (runningOK) { complex stuff .. The break statement lets you jump from any point in a loop to the statement just outside the loop.. You can break out of any of the three kinds of loop.e.. This can make the code harder to understand. which programmers are often scared of.
  e... for ( item = 1 . To do this we have to combine two tests to see if the value is OK. Programmer’s Point: Get used to flipping conditions One of the things that you will have to come to terms with is the way that you often have to reverse the way you look at things. if you get a value which is larger than the maximum or smaller than the minimum ask for another.. do all the updating and stuff and go around if you are supposed to. if (Done_All_We_Need_This_Time) continue . C# Programming © Rob Miles 2010 47 .. item processing stuff . } The continue causes the program to re-run the loop with the next value of item if it is OK to do so. i.Simple Data Processing Writing a Program Please do not go any further down this time round the loop. item < Total_Items . Essentially we want to keep asking the user for a value until we get one which is OK. In the following program the bool variable Done_All_We_Need_This_Time is set true when we have gone as far down the loop as we need to.. .. Complete Glazing Program This is a complete solution to the problem that uses all the tricks discussed above.. Rather than saying "Read me a valid number" you will have to say "Read numbers while they are not valid". Go back to the top of the loop. You can regard it as a move to step 2 in the list above.. Our loop should continue to run if: width > MAX_WIDTH or width < MIN_WIDTH To perform this test we use one of the logical operators described above to write a condition which will be true if the width is invalid: if ( width < MIN_WIDTH || width > MAX_WIDTH ) .... Remember the notes about reversing conditions above when you write the code. additional item processing stuff ... This means that you will often be checking to find the thing that you don't want. item=item+1 ) { . rather than the thing that you do. More Complicated Decisions We can now think about using a loop to test for a valid width or height..
  The ++ is called a unary operator.0 .Parse(widthString).0 . C# allows us to be terser if we wish. However.Write ( "Give the height of the window between " + MIN_HEIGHT + " and " + MAX_HEIGHT + " :" ). do { Console. both in terms of what we have to type and what the computer will actually do when it runs the program. height = double.Write ( "Give the width of the window between " + MIN_WIDTH + " and " + MAX_WIDTH + " :" ). } while ( width < MIN_WIDTH || width > MAX_WIDTH ) . It causes the value in that operand to be increased by one. The purpose of the above statement is to add 1 to the variable window_count. woodLength = 2 * ( width + height ) * 3.g.would do the same thing. window_count = window_count + 1 In this case the operator is + and is operating on the variable window_count and the value 1. class GlazerCalc { static void Main() { double width.WriteLine( "The area of the glass is " + glassArea + " square metres" ) .75 .5 . You can see examples of this construction in the for loop definition in the example above. glassArea = 2 * ( width * height ) .WriteLine ( "The length of the wood is " + woodLength + " feet" ) . Console. const double MAX_WIDTH = 5.ReadLine(). woodLength. heightString. do { Console. e.3. Console. string widthString.Parse(heightString). heightString = Console. width = double. const double MIN_WIDTH = 0. because it works on just one operand. the line: window_count++ .operator which can be used to decrease (decrement) variables. } while ( height < MIN_HEIGHT || height > MAX_HEIGHT ).ReadLine().Simple Data Processing Writing a Program using System. widthString = Console.25 . glassArea. it is a rather long winded way of expressing this. const double MAX_HEIGHT = 3. There is a corresponding -.4 Operator Shorthand So far we have looked at operators which appear in expressions and work on two operands. C# Programming © Rob Miles 2010 48 . height. const double MIN_HEIGHT = 0. } } 2. We can express ourselves more succinctly and the compiler can generate more efficient code because it now knows that what we are doing is adding one to a particular variable.
  In order to show how this is done. You determine whether you want to see the value before or after the sum by the position of the ++ : i++ ++i Means “Give me the value before the increment” Means “Give me the value after the increment” As an example: int i = 2. C# provides a way of getting either value. The fact that you can produce code like: height = width = speed = count = size = 0 . j = ++i . all return the value after the operator has been performed. Programmer’s Point: Always strive for simplicity Don't get carried away with this. Nowadays when I am writing a program my first consideration is whether or not the program is easy to understand. This is perfectly legal (and perhaps even sensible) C#. I don't think that the statement above is very easy to follow – irrespective of how so much more efficient it is. I still don't do it. If you do this you are advised to put brackets around the statement which is being used as a value. particularly when we get around to deciding things (see later).would make j equal to 3. in that you do not know if you get the value before or after the increment. C# has some additional operators which allow us to shorten this to: house_cost += window_cost. Most of the time you will ignore this value. I will leave you to find them! Statements and Values One of the really funky things about C# is that all statements return a value. An assignment statement always returns the value which is being assigned (i. but sometimes it can be very useful. which you can use in another statement if you like. the bit on the right of the gozzinta). so that the value in house_cost is increased by window_cost. j . consider the following: i = (j=0). but again is rather long winded. C# Programming © Rob Miles 2010 49 .Simple Data Processing Writing a Program The other shorthand which we use is when we add a particular value to a variable. which is OK. It has the effect of setting both i and j to 0. this makes the whole thing much clearer for both you and the compiler! When you consider operators like ++ there is possible ambiguity. depending on which effect you want. . .e. The += operator combines addition and the assignment. We could put: house_cost = house_cost + window_cost. The other special operators.does not mean that you should. This is perfectly OK. += etc.. This value can then be used as a value or operand.
  Console. This would print out: i: 150 f: 1234. Note that doing this means that if the number is an integer it is printed out as 12. the WriteLine method will fail with an error. f ) . double f = 1234. double f = 1234. Specifying the number of printed digits I can specify a particular number of digits by putting in a given number of zeroes: int i = 150 . f.5 Neater Printing Note that the way that a number is printed does not affect how it is stored in the program.WriteLine ( "i: {0:0000} f: {1:00000. This would print out: i: 0150 f: 01234. Adjusting real number precision Placeholders can have formatting information added to them: int i = 150 . i. If you have run any of the above programs you will by now have discovered that the way in which numbers are printed leaves much to be desired. Console.56789 i: 150 f: 1234.3.WriteLine ( "i: {0:#.##0. f ). Of course if I do something mad. This would print out: i: 150 f: 1234.00}".##0} f: {1:##. i. and is also somewhat easier to use if you are printing a large number of values.Simple Data Processing Writing a Program 2.57 The 0 characters stand for one or more digits.56789 The {n} part of the string says ―parameter number n. for example {99}. Console. When placed after a decimal point they can be used to control the number of decimal places which are used to express a value.56789 . i. counting from 0‖. f ). This provides more flexibility. Integers seem to come out OK.57 Note that if I do this I get leading zeroes printed out.WriteLine ( "i: {1} f: {0}". f ) .00.WriteLine ( "i: {0:0} f: {1:0. which is useful if you are printing things like cheques.56789 . I have used the # character to get my thousands printed out with commas: C# Programming © Rob Miles 2010 50 .56789 .00}". but floating point numbers seem to have a mind of their own.00}". Console. double f = 1234. it just tells the printing method how it is supposed to be printed. Using Placeholders in Print Strings A placeholder just marks the place where the value is to be printed. Console. To get around this C# provides a slightly different way in which numbers can be printed. i. double f = 1234. In the second write statement I have swapped the order of the numbers. but since I've swapped the order of the parameters too the output is the same.56789 .WriteLine ( "i: {0} f: {1}". i ) . Really Fancy Formatting If you want really fancy levels of control you can use the # character. Consider: int i = 150 . A # in the format string means ―put a digit here if you have one‖: int i = 150 .
  even a piece of text. if I want the numbers left justified I make the width negative: int i = 150 .00}".-15:0. f ) . i.00 The integer value is printed in a column 10 characters wide.56789 . Printing in columns Finally I can add a width value to the print layout information.15:0.00}". Note also though that I have included a 0 as the smallest digit.10:0} f: {1.234. double f = 1234.-15:0.WriteLine ( "i: {0. 0.10:0} f: {1.56789 .-10:0} f: {1.Simple Data Processing Writing a Program i: 150 f: 1. The value 150 does not have a thousands digit so it and the comma are left out.WriteLine ( "i: {0.WriteLine ( "i: {0.00 Note that this justification would work even if you were printing a string rather than a number. Console. 0 ) .00}". i.00}". C# Programming © Rob Miles 2010 51 . This is so that when I print the value 0 I actually get a value printed. otherwise when I print zero I get nothing on the page.-10:0} f: {1. which makes printing in columns very easy. so if you want to print columns of words you can use this technique to do it. Console. You can specify the print width of any item. 0. f ) .15:0. Console.57 0. This is very useful if you want to print material in columns: int i = 150 . 0 ) . double f = 1234.57 f: 0.WriteLine ( "i: {0. This would produce the output: i: i: 150 f: 0 f: 1234.57 Note that the formatter only uses the # characters and commas that it needs. Console. This would produce the output: i: 150 i: 0 f: 1234. and the double is printed in a 15 character wide column. At the moment the output is right justified.
  Essentially you take a block of code and give it a name. WriteLine and ReadLine. As a silly example: C# Programming © Rob Miles 2010 52 . Again. Main is the method we write which is where our program starts. as with lots of features of the C# language. In this section we are going to consider why methods are useful and how you can create your own. C# lets us create other methods which are used when our program runs. 3. but they do help with the organisation of our programs. We have exactly the same piece of code to check widths and heights. We will need both of these when we start to write larger programs. Methods give us two new weapons: We can use methods to let us re-use a piece of code which we have written. To do this you need to define a method to do the work for you. Method and Laziness We have already established that a good programmer is creatively lazy. However. 3. If we added a third thing to read. Then you can refer to this block of code to do something for you. We can also use methods to break down a large task into a number of smaller ones. Your programs will contain methods that you create to solve parts of the problem and they will also use methods that have been provided by other people. methods don't actually make things possible. One of the tenets of this is that a programmer will try to do a given job once and once only. we would have to copy the code a third time. WriteLine and ReadLine were provided by the creators of C# to give us a way of displaying text and reading information from the user. This is not very efficient. Up until now all our programs have been in a single method. for example frame thickness. What we would like to do is write the checking code once and then use it at each point in the program. The method is the block of code which follows the main part in our program. This is what methods are all about. it makes the program bigger and harder to write.1.1 Methods We have already come across the methods Main.
  When the method starts the value supplied for the parameter is copied into it. } } The method silly has a single integer parameter.WriteLine ( "i is : " + i ) . we can use methods to save us from writing the same code twice. 3.2 Parameters At this point methods are useful because they let us use the same block of statements at many points in the program. The method is given the data to work on. As an example. } public static void Main () { silly ( 101 ) . This means that when the program runs we get output like this: i is : 101 i is : 500 3. } } In the main method I make two calls of doit. silly ( 500 ) . The result of running the above program would be: Hello Hello So. You have already used this feature. the methods ReadLine and Parse both return results that we have used in our programs: C# Programming © Rob Miles 2010 53 . they become more useful if we allow them to have parameters. In this case it contains a single statement which prints "Hello" on the console. class MethodDemo { static void silly ( int i) { Console. Within the block of code which is the body of this method we can use the parameter i as an integer variable. doit(). consider the code below: using System .1. Each time I call the method the code in the block which is the body of the method is executed.1. class MethodDemo { static void doit () { Console. However.3 Return values A method can also return a value to the caller.Creating Programs Methods using System .WriteLine ("Hello"). We simply put the code inside a method body and then call it when we need it. } public static void Main () { doit(). A parameter is a means of passing a value into a method call.
  double age = readValue ( "Enter your age: ". return i.Creating Programs Methods using System . // prompt for the user double low. but this is actually a rather pointless thing to do: sillyReturnPlus (5).ReadLine (). return result .4 A Useful Method Now we can start to write genuinely useful methods: static double readValue ( string prompt. } while ( (result < low) || (result > high) ). Console.WriteLine (prompt + " between " + low + " and " + high ). // will compile but do nothing useful 3.WriteLine ( "res is : " + res ) . We can use this method to read anything and make sure that value supplied is within a particular range. // lowest allowed value double high // highest allowed value ) { double result = 0.1. MIN_WIDTH. string resultString = Console. Console. result = double.WriteLine ( "i is : " + i ) . do { Console. It is actually OK to ignore the value returned by a method. in other words a call of sillyReturnPlus can take the place of an integer. } } The method sillyReturnPlus takes the value of the parameter and returns it plus one. class ReturnDemo { static int sillyReturnPlus ( int i) { i = i + 1. The value that a method returns can be used anywhere in a program where a variable of that type could be used. It can then be used to read values and make sure that they are in range. The second reads an age between 0 and 70.Parse(resultString). The first call of readValue gets the width of a window. 0.WriteLine ( "res is : " + res ) . MAX_WIDTH) . } The readValue method is told the prompt to use and the lowest and the highest allowed values. double windowWidth = readValue ( "Enter width of window: ". C# Programming © Rob Miles 2010 54 . res = sillyReturnPlus (5). See if you can work out what this code would display: res = sillyReturnPlus (5) + sillyReturnPlus (7) + 1. Console. 70) . } public static void Main () { int res.
  WriteLine ( "i is : " + i ) . Method Limitations A method is very good for getting work done. Consider: static void addOneToParam ( int i) { i = i + 1. This means that you can write calls like: test = 20 . Console. However. From what we have seen of methods. prints the result out and then returns: int test = 20 .WriteLine ( "test is : " + test ) . Moving code around and creating methods is called refactoring. but it can be a bit limited because of the way that it works. Console. C# Programming © Rob Miles 2010 55 .Creating Programs Methods Programmer’s Point: Design with methods Methods are a very useful part of the programmer's toolkit. Once you have worked out what the customer wants and gathered your metadata you can start thinking about how you are going to break the program down into methods. There are two reasons why this is a good idea: 1: 2: It saves you writing the same code twice. 3. If you do this you should consider taking that action and moving it into a method. It then passes this value into the call. The value of test is being used in the call of addOneToParam.1. This would print out: i is : 120 Pass by value is very safe. only the value of a parameter is passed into a call to a method. Often you find that as you write the code you are repeating a particular action. because nothing the method does can affect variables in the code which calls it. addOneToParam(test + 99). or write one which returns an age. If a fault is found in the code you only have to fix it in one place. This will be an important part of the Software Engineering we do later. The program works out the result of the expression to be passed into the method call as a parameter. addOneToParam(test). it is a limitation when we want to create a method which returns more than one value. The piece of C# above calls the method with the variable test as the parameter. For example. This is because. what do I mean by "passing parameters by value". unless you specify otherwise. As you will find out when you try. they can only return one value.5 Parameter Passing By Value So. When it runs it prints out the following: i is : 21 test is : 20 It is very important that you understand what is happening here. changing the value of a parameter does not change the value of the thing passed in. So I could write a method which returns the name of a person. if I want to write a method which reads in the name and the age of a person I have a problem. } The method addOneToParam adds one to the parameter. But I can’t write a method that returns both values at the same time as a method can only return one value. They form an important part of the development process.
  rather than passing in "20" in our above call the compiler will generate code which passes in "memory location 5023" instead (assuming that the variable test is actually stored at 5023). This memory location is used by the method. test = 20 .6 Parameter Passing By Reference It is very important that you understand how references work. Generally speaking you have to be careful with side effects. Programmer’s Point: Document your side-effects A change by a method to something around it is called a side effect of the method. Note that C# is careful about when a parameter is a reference. changes to the parameter change the variable whose reference you passed” If you find references confusing you are not alone. In this case the output is as follows: i is : 21 test is : 21 In this case the method call has made changes to the content of the variable. 3. The program will say ―Get the value from location 5023‖ rather than ―The value is 1‖. } Note that the keyword ref has been added to the information about the parameter. So. You have to put the word ref in the method heading and also in the call of the method. it is going to have the “side effect” of changing something outside the method itself (namely the value of the parameter passed by reference).1. This is the case when we want to read in the name and age of a user. we use them in real life all the time with no problems.WriteLine ( "i is : " + i ) . If you say ―Deliver the carpet to 23 High Street. Instead you want to just allow the method to change the variable. Console. Instead it just wants to deliver results to them. The code above makes a call to the new method. rather than the content of the variable. Using a reference in a program is just the same. Sometimes you don't want this. In other words: “If you pass by reference. However.WriteLine ( "test is : " + test ) . Inside the method. when someone calls our addOneToRefParam method. In this case I can replace the ref with the keyword out: C# Programming © Rob Miles 2010 56 . rather than using the value of the variable the reference is used to get the actual variable itself. addOneToRefParam(ref test). If you don't understand these you can't call yourself a proper programmer! Fortunately C# provides a way that. instead a reference to that variable is supplied instead.7 Passing Parameters as "out" references When you pass a parameter as a reference you are giving the method complete control of it. and also has the word ref in front of the parameter. The original value of the parameters is of no interest to the method.Creating Programs Methods 3. instead of the value. Effectively the thing that is passed into the method is the position or address of the variable in memory. Console. as someone reading your program has to know that your method has made changes in this way.‖ you are giving the delivery man a reference. Consider the code: static void addOneToRefParam ( ref int i) { i = i + 1.1. rather than sending the value of a variable into a method. In other words.
  age = readInt ( "Enter your age : ". It means that if I mark the parameters as out I must have given them a value for the program to compile. } static int readInt ( string prompt. do { string intString = readString (prompt) . Note that I must use the out keyword in the call of the method as well.ReadLine (). so that the user can't enter an empty string when a number is required. readPerson ( out name. In the code above I have written a couple of library methods which I can use to read values of different types: static string readString ( string prompt ) { string result . 3.8 Method Libraries The first thing that a good programmer will do when they start writing code is to create a set of libraries which can be used to make their job easier. Note how I have rather cleverly used my readString method in my readInt one. It makes sure that a programmer can't use the value of the parameter in the method. This is very useful.1. out age ) . The readInt method reads a number within a particular range. It also allows the compiler to make sure that somewhere in the method the output parameters are assigned values. 100 ) . out int age ) { name = readString ( "Enter your name : " ) . readString and readInt. I can call readPerson as follows: string name .Creating Programs Methods static void readPerson ( out string name. do { Console. Note that it uses two more methods that I have created. return result. as I am protected against forgetting to do that part of the job. } while ( result == "" ) . } The method readPerson reads the name and the age of a person. The readPerson method will read the person and deliver the information into the two variables.Write ( prompt ) . int high ) { int result . return result . This makes it harder for me to get the program wrong. result = int. result = Console. } while ( ( result < low ) || ( result > high ) ). int low. int age . 0. } The readString method will read text and make sure that the user does not enter empty text. We can use the methods as follows: C# Programming © Rob Miles 2010 57 .Parse(intString). Programmer’s Point: Languages can help programmers The out keyword is a nice example of how the design of a programming language can make programs safer and easier to write.
  Creating Programs Variables and Scope string name. The scope of a local variable is the block within which the variable is declared. Any block can contain any number of local variables. If the return value is 0 this means that the method returned correctly.2 Nested Blocks We have seen that in C# the programmer can create blocks inside blocks. This means that only statements in the inner block can use this variable. 100). When the execution of the program moves outside a block any local variables which are declared in the block are automatically discarded. 0. The methods that we have created have often contained local variables.2. This adds another dimension to program design. the variable result in the readInt method is local to the method block. but you must declare it before you use it. You need to consider whether or not the method should deal with the problem itself or pass the error onto the system which tried to use it.2. i. The C# compiler also looks after the part of a program within which a variable has an existence. In fact one thing I tend to do when working on a project is create little library of useful methods like these which I can use.1 Scope and blocks We have already seen that a block is a number of statements which are enclosed in curly brackets. age = readInt ( "Enter your age : ". 3. In other words the code: C# Programming © Rob Miles 2010 58 . name = readString( "Enter your name : " ). If the method involves talking to the user it is possible that the user may wish to abandon the method. or that the user does something that may cause it to fail. Each of these nested blocks can have its own set of local variables: { int i . If the method passes the error on to the code which called it you have to have a method by which an error condition can be delivered to the caller. int age. } } The variable j has the scope of the inner block.2 Variables and Scope We have seen that when we want to store a quantity in our program we can create a variable to hold this information. 3. As far as the C# language is concerned you can declare a variable at any point in the block. I could add methods to read floating point values as well. variables which are local to that block. If the method deals with the error itself this may lead to problems because the user may have no way of cancelling a command. I often solve this problem by having my methods return a code value.e. as well as making sure that it does the required job! We are going to discuss error management later 3. in that you also have to consider how the code that you write can fail. If the return value is non-zero this means that the method did not work and the value being returned is an error which identifies what went wrong. Programmer’s Point: Always consider the failure behaviours Whenever you write a method you should give some thought to the ways that it could fail. { int j . This is called the scope of a variable. The C# compiler makes sure that the correctly sized chunk of memory is used to hold the value and it also makes sure that we only ever use that value correctly.
  { int j . C# has an additional rule about the variables in the inner blocks: { int i . Variables Local to a Method Consider the following code: C# Programming © Rob Miles 2010 59 . } The variable i is declared and initialized at the start of the for loop and only exists for the duration of the block itself.3 Data Member in classes Local variables are all very well. In order to remove this possibility the compiler refuses to allow this.WriteLine ( "Hello" ) . { int i . { int j . This allows you to declare a control variable which exists for the duration of the loop itself: for ( int i = 0 . } . for example C++. For loop local variables A special kind of variable can be used when you create a for loop construction. It is however perfectly acceptable to reuse a variable name in successive blocks because in this situation there is no way that one variable can be confused with another. This is because inside the inner block there is the possibility that you may use the "inner" version of i when you intend to use the outer one. { int i . where this behaviour is allowed. } j = 99 . In order to keep you from confusing yourself by creating two versions of a variable with the same name. i = i + 1 ) { Console. i < 10 . as the variable j does not exist at this point in the program. } { int i .2.Creating Programs Variables and Scope { int i . but they disappear when the program execution leaves the block where they are declared. so this code is OK.would cause an error. } } This is not a valid program because C# does not let a variable in an inner block have the same name as one in an outer block. } } The first incarnation of i has been destroyed before the second. 3. We often need to have variables that exist outside the methods in a class. Note that this is in contrast to the situation in other languages.
  For now you just have to remember to put in the static keyword. } } The variable member is now part of the class MemberExample. if you were creating a program to play chess it would be sensible to make the variable that stores the board into a member of the class. For example. Console. otherwise your program will not compile. If a statement in OtherMethod tries to use local the program will fail to compile.WriteLine ("local is :" + local). } } The variable local is declared and used within the Main method.WriteLine ("member is now : " + member). display the board. and can’t be used anywhere else.WriteLine ("member is : " + member). This means declaring it outside the methods in the class: class MemberExample { // the variable member is part of the class static int member = 0 . } static void Main () { Console. Then the methods that read the player move. we will investigate the precise meaning of static later on.Creating Programs Variables and Scope class LocalExample { static void OtherMethod () { local = 99. static void OtherMethod () { member = 99. This is not something to worry about just now. Console. and so the Main method and OtherMethod can both use this variable. Static class members Note that I have made the data member of the class static. and calculate the computer move could all use the same board information. // this will not compile } static void Main () { int local = 0. OtherMethod(). Class variables are very useful if you want to have a number of methods ―sharing‖ a set of data. Variables which are Data Members of a Class If I want to allow two methods in a class to share a variable I will have to make the variable a member of the class. so that it is part of the class and not an instance of the class. The program above would print out: member is : 0 member is now : 99 This is because the call of OtherMethod would change the value of member to 99 when it runs. C# Programming © Rob Miles 2010 60 .
  C# Programming © Rob Miles 2010 61 . You decide how much money you are going to be paid.1000). ". You discuss sensible ranges (no player can score less than 0 or more than 1000 runs in a game). ". and what information it will print out. 0.1000). It turns out that you now know about nearly all the language features that are required to implement every program that has ever been written. like the background static noise on your radio when you tune it off station.1000).1 Why We Need Arrays Your fame as a programmer is now beginning to spread far and wide.3. ". Programmer’s Point: Plan your variable use You should plan your use of variables in your programs. ". given a set of player scores from a game he wants a list of those scores in ascending order. The next person to come and see you is the chap in charge of the local cricket team. If it helps you can think of static as something which is always with us. Our programs can also make decisions based on the values supplied by the user and also repeat actions a given number of times. You should decide which variables are only required for use in local blocks and which should be members of the class. to make this the perfect project. 0. ".1000). ". you agree on when the software must be finished and how you are going to demonstrate it to your new customer and you write all this down and get it signed. 0. 0. 0. 3. Finally.Creating Programs Arrays One common programming mistake is to confuse static with const. With all this sorted. 0. He would like to you write a program for him to help him analyse the performance of his players. score11 . The next thing you do is refine the specification and add some metadata.1000). and therefore not going anywhere. score6. score5. Only one thing is missing. ". Alternatively you can think of it as stationary. : ". What the customer wants is quite simple. score10.1000). all you have to do now is write the actual program itself. I am very careful to keep the number of member variables in a class to the minimum possible and use local variables if I only need to store the value for a small part of the code. ―This is easy‖ you think.1000). 0. Marking a variable with static means ―the variable is part of the class and is always present‖. ". calculate results and print them.1000). You draw out a rough version of what the program will accept. 0. ".3 Arrays We now know how to create programs that can read values in. score3. 0. 0. score7 score8.1000). Marking a variable as const means ―the value cannot be changed‖. : ". Arrays are one way to do this. When a game of cricket is played each member of the team will score a particular number of runs.1000). Now you can start putting the data into each variable. 3. and that is the ability to create programs which store large amounts of data. score9. The first thing to do is define how the data is to be stored: int score1. 0. score4. and we are going to find out about them next. and when. Bear in mind that if you make a variable a member of the class it can be used by any method in that class (which increases the chances of something bad happening to it). score2.1000).
  each large enough to hold a single integer. It then gets a piece of rope and ties the tag scores to this box. If you follow the rope from the scores tag you reach the array box.1000). Visual Basic numbers array elements starting at 1. This part is called the subscript. You can think of this as a tag which can be made to refer to a given array. Hmmmm. class ArrayDemo { public static void Main () { int [] scores = new int [11] .3. When an array is created all the elements in the array are set to 0. Consider the following: using System. This is very important. The bit which makes the array itself is the new int [11]. In the program you identify which element you mean by putting its number in square brackets [ ] after the array name.. i=i+1) { scores [i] = readInt ( "Score : ". i<11. we know that computers are very good at sorting this kind of thing. It then gets some pieces of wood and makes a long thin box with 11 compartments in it. If you find this confusing. 0. } } } The int [] scores part tells the compiler that we want to create an array variable. it probably doesn't use wood or rope. (as long as you don't fall off the end of the array). If you look at the part of the program which reads the values into the array you will see that we only count from 0 to 10.Creating Programs Arrays All we have to do next is sort them. C# provides us with a thing called an array. One thing adding to this confusion is the fact that this numbering scheme is not the same in other languages. Just deciding whether score1 is the largest value would take an if construction with 10 comparisons! Clearly there has to be a better way of doing this. It then paints the whole box red . An array allows us to declare a whole row of a particular kind of box. scores [i+1] . C# Programming © Rob Miles 2010 62 .2 Array Elements Each compartment in the box is called an element. after all. When C# sees this it says ―Aha! What we need here is an array‖. 3. Actually. This means that you specify the element at the start of the array by giving the subscript 0. In fact you can use any expression which returns an integer result as a subscript. ―The first element has the subscript 0‖ then the best way to regard the subscript is as the distance down the array you have to travel to get to the element that you need. but you should get a picture of what is going on here.because boxes which can hold integers are red. Array Element Numbering C# numbers the boxes starting at 0. This is awful! There seems to be no way of doing it. I’m sorry about this... Note that the thing which makes arrays so wonderful is the fact that you can specify an element by using a variable.. for ( int i=0. i..is quite OK. An attempt to go outside the array bounds of scores cause your program to fail as it runs. We can then use things called subscripts to indicate which box in the row that we want to use.... There is consequently no element scores [11].e. it just goes to show that not everything about programming is consistent. 2010 2010 2010
  65
  and the program will use the catch which matches the level of the code in the try block. Note that once the exception has been thrown there is no return to the code in the try block. which is managing the execution of your program inside the computer. try { age = int.Message).4. 3.4 Exception Nesting When an exception is thrown the run time system.WriteLine("Thank you"). Console. The code in this catch clause will display the exception details and then stop the program. which contains the message: Input string was not in a correct format. An exception is a type of object that contains details of a problem that has occurred. The Exception type has a property called Message which contains a string describing the error.WriteLine("Invalid age value"). If the call of Parse throws an exception the code in the catch block runs and will display a message to the user.Parse(ageString). try { age = int. The Exception object also contains other properties that can be useful. Console. i. the parse action takes place inside the try block. since it means that the message reflects what has actually happened. If your program does not contain a catch for the exception it will be caught by a catch which is part of the run time system.Creating Programs Exceptions and Errors int age.Parse(ageString). This is useful if the code inside the try block could throw several different kinds of exception. This is exactly how it works. } The catch now looks like a method call. The program above ignores the exception object and just registers to the exception event but we can improve the diagnostics of our program by catching the exception if we wish: int age.e. Programs can have multiple levels of try – catch.4. with the Exception e being a parameter to the method. } catch (Exception e) { // Get the error message out of the exception Console. However.WriteLine("Thank you"). When Parse fails it creates an exception object that describes the bad thing that has just happened (in this case input string not in the correct format). This message is obtained from the exception that was thrown. 3.WriteLine(e. Within the catch clause the value of e is set to the exception that was thrown by Parse. C# Programming © Rob Miles 2010 66 . } The code above uses Parse to decode the age string. Exceptions work in the same way. } catch { Console. if the parse fails the program will not display the message "Thank you". If a user types in an invalid string the program above will write the text in the exception.3 The Exception Object When I pass a problem up to my boss I will hand over a note that describes it. will look for the enclosing catch clause.
  once execution leaves that inner block the outer catch is the one which will be run in the event of an exception being thrown. 3. The faster you can get a fault to manifest itself the easier it is to identify the cause. and the program never returns to the try part of the program. The code in the catch clause could return from the method it is running within or even throw an exception itself. not try and handle things by returning a null reference or empty item. Programmer’s Point: Don’t Catch Everything Ben calls this Pokemon® syndrome: “Gotta catch’em all”. Statements inside the finally clause will run irrespective of whether or not the program in the try block throws an exception. Don’t feel obliged to catch every exception that your code might throw. Returning a placeholder will just cause bigger problems later when something tries to use the empty item that was returned. However. Fortunately C# provides a solution to this problem by allowing you to add a finally clause to your try-catch construction. releasing resources and generally tidying up. In these situations any code following your try – catch construction would not get the chance to run. we know that when an exception is thrown the statements in the catch clause are called. These actions include things like closing files. If someone asks your Load method to fetch a file that doesn’t exist the best thing it can do is throw a “file not found” exception. However.5 Adding a Finally Clause Sometimes there are things that your program must do irrespective of whether or not as exception is thrown. If code in the innermost block throws an exception the run time system will find the inner catch clause. try { // Code that } catch (Exception { // Code that } finally { // Code that // is thrown } might throw an exception outer) catches the exception is obeyed whether an exception or not C# Programming © Rob Miles 2010 67 .4.
  3.6 Throwing an Exception Now that we know how to catch exceptions. 3. Throwing an exception might cause your program to end if the code does not run inside a try – catch construction.7 Exception Etiquette Exceptions are best reserved for situations when your program really cannot go any further.Creating Programs The Switch Construction In the above code the statements in the finally part are guaranteed to run.1 Making Multiple Decisions Suppose you are refining your double glazing program to allow your customer to select from a pre-defined range of windows. the next thing we need to consider is how to throw them. When you make a new exception you can give it a string that contains the message the exception will deliver. Programmer’s Point: Plan your Exception Handling When you design a program you should consider what events count as “showstoppers” and how you are going to deal with them. but there is really very little left to know about programming itself.5. either when the statements in the try part have finished.4. 3. For example if a user of the double glazing program enters a window height which is too large the program can simply print the message ―Height too large‖ and ask for another value. This means that errors at this level are not worthy of exception throwing. The statement above makes a new exception and then throws it. or just before execution leaves the catch part of your program. I reserve my exceptions for catastrophic events that my program really cannot deal with and must pass on to something else. When you come to write the program you will probably end up with something like: C# Programming © Rob Miles 2010 68 .4.. and you should too.5 The Switch Construction We now know nearly everything you need to know about constructing a program in the C# language. Each method asks the relevant questions and works out the price of that kind of item. 3. This turns out to be very easy: throw new Exception("Boom"). Most of the rest of C is concerned with making the business of programming simpler. You can even create your own custom exception types based on the one provided by the System use these in your error handling. A good example of this is the switch construction. You may find it rather surprising. If you are going to throw an exception this should be in a situation where your program really could not do anything else.
  2 Selecting using the if construction When you come to perform the actual selection you end up with code which looks a bit like this: int selection . Our program can use this method to get the selection value and then pick the method that needs to be used. This is called the switch construction. if ( selection == 1 ) { handleCasement(). but is rather clumsy. } static void handleStandard () { Console.5. We already have a method that we can use to get a number from the user (it is called readInt and is given a prompt string and high and low limits).5. } static void handlePatio () { Console.WriteLine("Handle Casement"). 1. 3. If you write the above using it your program would look like this. } else { if ( selection == 2 ) { handleStandard(). } else { if ( selection == 3 ) { handlePatio() .WriteLine("Handle Standard").3 The switch construction Because you have to do this a lot C# contains a special construction to allow you to select one option from a number of them based on a particular value. At the moment they just print out that they have been called. } } } This would work OK. 3. the next thing you need to do is write the code that will call the one that the user has selected.WriteLine("Handle patio").Creating Programs The Switch Construction static void handleCasement () { Console. } else { Console. C# Programming © Rob Miles 2010 69 . Once you have the methods in place. You have to write a large number of if constructions to activate each option. 3 ) . } These methods are the ones which will eventually deal with each type of window.WriteLine ( "Invalid number" ). Later you will go on and fill the code in (this is actually quite a good way to construct your programs). selection = readInt ( "Window Type : ".
  when the break is reached the switch is finished and the program continues running at the statement after the switch. your users would not thank you for doing this. The break statement after the call of the relevant method is to stop the program running on and performing the code which follows. break . break . You can use the switch construction with types other than numbers if you wish: switch (command) { case "casement" : handleCasement (). default : Console. This gives the switch somewhere to go if the switch value doesn't match any of the cases available.WriteLine ( "Invalid number" ) . case 3 : handlePatio () . the compiler will give you an error if you make a mistake. in our case (sorry!) we put out an appropriate message. break . case "patio" : handlePatio () . Multiple cases You can use multiple case items so that your program can execute a particular switch element if the command matches one of several options: C# Programming © Rob Miles 2010 70 . since it means that they have to type in the complete name of the option. break . } The switch construction takes a value which it uses to decide which option to perform. It executes the case which matches the value of the switch variable. Of course this means that the type of the cases that you use must match the switch selection value although. break . However. case 2 : handleStandard () . Another other useful feature is the default option. in true C# tradition. break . break . case "standard" : handleStandard () . break . default : Console. and of course if they type a character wrong the command is not recognised.WriteLine ( "Invalid command" ) .Creating Programs The Switch Construction switch (selection) { case 1 : handleCasement (). In the same way as you break out of a loop. } This switch uses a string to control the selection of the cases.
  However.6 Using Files If you want your program to be properly useful you have to give it a way of storing data when it is not running.WriteLine ( "Invalid command" ) . We can write a C# program which creates a file on a Windows PC and then use the same program to create a file on a UNIX system with no problems. Instead you should put a call to a method as I have above. case "standard" : case "s" : handleStandard () .ToUpper()) { case "CASEMENT" : case "C" : . which can make the testing of the commands much easier: switch (command. case "patio" : case "P" : handlePatio () . so that streams can be used to read and write to files... What we want to do is use C# to tell the operating system to create files and let us access them. 3. If you want to perform selection based on strings of text like this I’d advise you to take a look at the ToUpper and ToLower methods provided by the string type.6. Data can flow up or down your stream. The operating system actually does the work. } The above switch will select a particular option if the user types the full part of the name or just the initial letter. in files. break .. break . 3.Creating Programs Using Files switch (command) { case "casement" : case "c" : handleCasement (). Programmer’s Point: switches are a good idea Switches make a program easier to understand as well as quicker to write. It is also easier to add extra commands if you use a switch since it are just a matter of putting in another case. break . break . default : Console. We know that you can store data in this way. These can be used to obtain a version of a string which is all in upper or lower case. and the C# library you are using will convert your request to use streams into instructions for the operating system you are using at the time: C# Programming © Rob Miles 2010 71 . that is how we have kept all the programs we have created so far. The good news is that although different operating systems use different ways to look after their files. the way in which you manipulate files in C# is the same for any computer. The stream is the thing that links your program with the operating system of the computer you are using.1 Streams and Files C# makes use of a thing called a stream to allow programs to work with files. I'd advise against putting large amounts of program code into a switch case. Files are looked after by the operating system of the computer. A stream is a link between your program and a data resource.
  C# has a range of different stream types which you use depending on what you want to do. Each time you write a line to the file it is added onto the end of the lines that have already been written. which would be bad. It means that you could use the two statements above to completely destroy the contents of an existing file. The above statement calls the WriteLine method on the stream to make it write the text ―hello world‖ into the file test. writer. If your program got stuck writing in an infinite loop it is possible that it might fill up the storage device. All of the streams are used in exactly the same way. file is created in place of what was there. empty.6. The variable writer will be made to refer to the stream that you want to write into. these are the StreamWriter and StreamReader types. This is exactly the same technique that is used to write information to the console for the user to read. C# Programming © Rob Miles 2010 72 . then the action will fail with an appropriate exception. by using new. A properly written program should probably make sure that any exceptions like this (they can also be thrown when you open a file) are caught and handled correctly. Most useful programs ask the user whether or not an existing file should be overwritten. In fact you can use all the writing features including those we explored in the neater printing section to format your output. since the Console class.Creating Programs Using Files C# program stream File in Windows A C# program can contain an object representing a particular stream that a programmer has created and connected to a file.txt for output and connect the stream to it. This is potentially dangerous. If this happens. The program performs operations on the file by calling methods on the stream object to tell it what to do. is implemented as a stream. which connects a C# program to the user.WriteLine("hello world"). and the write cannot be performed successfully.3 Writing to a Stream Once the stream has been created it can be written to by calling the write methods it provides. you will find out later how to do this.txt. the call of WriteLine will throw an exception. StreamWriter writer . All that happens is that a brand new. Note however that this code does not have a problem if the file test. If this process fails for any reason.6. perhaps your operating system is not able/allowed to write to the file or the name is invalid.txt already exists. 3.txt"). In fact you are already familiar with how streams are used. 3. The ReadLine and WriteLine methods are commands you can give any stream that will ask it to read and write data. When the new StreamWriter is created the program will open a file called test. writer = new StreamWriter("test.2 Creating an Output Stream You create a stream object just like you would create any other one. We are going to consider two stream types which let programs use files. When the stream is created it can be passed the name of the file that is to be opened.
  If you were in charge of cataloguing a huge number of items you would find it very helpful to lump items into groups.5 Streams and Namespaces If you rush out and try the above bits of code you fill find that they don’t work. There is something else that you need to know before you can use the StreamWriter type. The using keyword allows us to tell the compiler where to look for resources. close the file or suffer the consequences. The designers of the C# language created a namespace facility where programmers can do the same kind of thing with their resources. So.Console. i.Console. Any further attempts to write to the stream will fail with an exception.Close().e. using System. when we reflected on the need to have the statement using System. but on top of this there are a whole lot of extra resources supplied with a C# installation. However. part of operating resource. This statement tells the compiler to look in the System namespace for resources. It will also be impossible to move or rename the file. Once a file has been closed it can then be accessed by other programs on the computer.6. That is.4 Closing a Stream When your program has finished writing to a stream it is very important that the stream is explicitly closed using the Close method: writer. An open stream consumes a small.IO namespace. A C# installation actually contains many thousands of resources. We have touched on namespaces before. a ―space where names have meaning‖. In this situation some of the data that you wrote into the file will not be there. the Console class in the System namespace. this object is defined in the System. Once we have specified a namespace in a program file we no longer need to use the fully qualified name for resources from that namespace. Museum curators do this all the time. Like lots of the objects that deal with input and output. The full name of the Console class that you have been using to write text to the user is System. each of which must be uniquely identified. A namespace is. 3. If your program has a stream connected to a file other programs may not be able to use that file. The C# language provides the keywords and constructions that allow us to write programs. Whenever the compiler finds an C# Programming © Rob Miles 2010 73 . but significant. Namespaces are all to do with finding resources. Forgetting to close a file is bad for a number of reasons: It is possible that the program may finish without the file being properly closed. at the start of our C# programs.txt and take a look at what is inside it. and the Greek ones in another.Creating Programs Using Files 3.6. When the Close method is called the stream will write out any text to the file that is waiting to be written and disconnect the program from the file. Sorry about that. These resources are things like the Console object that lets us read and write text to the user. Now we need to find out more about them.WriteLine("Hello World"). In fact it is quite OK to use this full form in your programs: System. If your program creates lots of streams but does not close them this might lead to problems opening other files later on. The above uses the fully qualified name of the console resource and calls the method WriteLine provided by that resource. once the close has been performed you can use the Notepad program to open test. quite literally. They put all the Roman artefacts in one room. we’ve not had to use this form because at the start of our programs we have told the compiler to use the System namespace to find any names it hasn’t seen before.
  Console. This is the same error that you will get if you try to use the StreamWriter class without telling the compiler to look in the System. Fortunately the StreamReader object provides a property called EndOfStream that a program can use to determine when the end of the file has been reached. in that the program will create a stream to do the actual work. The above program connects a stream to the file Test. In this case the stream that is used is a StreamReader. reads the first line from the file.Vases) and so the IO namespace is actually held within the System namespace. Detecting the End of an Input File Repeated calls of ReadLine will return successive lines of a file.txt and display every line in the file on the console. this does not imply that you use all the namespaces defined within it.txt. If the file can’t be found then the attempt to open it will fail and the program will throw an exception. However.Close(). when the compiler sees the statement: Console. C# Programming © Rob Miles 2010 74 .IO. However.txt").6.WriteLine("Hello World"). display it on the screen and then close the stream.the compiler will look in the System namespace. fail to find an object called Console and generate a compilation error. In other words. The while loop will stop the program when the end of the file is reached. } reader. 3.it knows to look in the System namespace for that object so that it can use the WriteLine method on it. .IO namespace. Namespaces are a great way to make sure that names of items that you create don’t clash with those from other programmers. and so you must include the above line for file handling objects to be available. Console.WriteLine (line).ReadLine(). In other words.WriteLine("Hello World").Creating Programs Using Files item it hasn’t seen before it will automatically look in the namespaces it has been told to use.Close(). just because you use a namespace. reader = new StreamReader("Test.WriteLine(line). reader. while (reader. TextReader reader = new StreamReader("Test. If the programmer miss-types the class name: Consle. The above program will open up the file test. .ReadLine(). StreamReader reader.6 Reading from a File Reading from a file is very similar to writing. When the property becomes true the end of the file has been reached. to use the file handing classes you will need to add the following statement at the very top of your program: using System. It is possible to put one namespace inside another (just like a librarian would put a cabinet of Vases in the Roman room which he could refer to as Roman.EndOfStream == false) { string line = reader. We will see how you can create your own namespaces later on.txt"). if your program reaches the end of the file the ReadLine method will return an empty string each time it is called. string line = reader.
  txt is in the MyProgs folder too.txt) then the system assumes the file that is being used is stored in the same folder as the program which is running. You can create your own folders inside these (for example Documents\Stories). another for Pictures and another one for Music.txt". If you use Windows you will find that there several folders created for you automatically. Each file you create is placed in a particular folder. which is held in the folder data which is on drive C. The location of a file on a computer is often called the path to the file. if you are running the program FileRead.7 File Paths in C# If you have used a computer for a while you will be familiar with the idea of folders (sometimes called directories).6. If you don’t give a folder location when you open a file (as we have been doing with the file Test. the location of the folder and the name of the file itself. which is in turn held in the folder 2009. path = @"c:\data\2009\November\sales. One can be used for Documents. I’d advise you to make sure that your path separators are not getting used as control characters.txt. C# Programming © Rob Miles 2010 75 . If you want to use a file in a different folder (which is a good idea. The backslash (\) characters in the string serve to separate the folders along the path to the file. as data files are hardly ever held in the same place as programs run from) you can add path information to a filename: string path. If you have problems where your program is not finding files that you know are there. In other words. These are used to organise information we store on the computer. The above statements create a string variable which contains the path to a file called sales. This file is held in the folder November. Note that I have specified a string literal that doesn’t contain control characters (that is what the @ at the beginning of the literal means) as otherwise the \ characters in the string will get interpreted by C# as the start of a control sequence.Creating Programs Using Files 3.exe in the folder MyProgs then the above programs will assume that the file Test. The path a file can be broken into two parts.
  implement transactions that change the content of one or more items in the system) are common to many other types of programs. The program we are making is for a bank. We will be creating the entire bank application using C# and will be exploring the features of C# that make this easy. There are many thousands of customers and the manager has also told us that there are also a number of different types of accounts (and that new types of account are invented from time to time). search for information for a particular person. the "United Friendly and Really Nice Bank of Lovely People ™". Programmer’s Point: Look for Patterns The number of different programs in the world is actually quite small. It is unlikely that you will get to actually implement an entire banking system during your professional career as a programmer (although it might be quite fun – and probably rather lucrative). Of course if C# Programming © Rob Miles 2010 76 . The bank manager has told us that the bank stores information about each customer. This information includes their name. Other data items might be added later. However.1 Our Case Study: Friendly Bank The bulk of this section is based on a case study which will allow you to see the features of C# in a strong context. The system must also generate warning letters and statements as required. balance and overdraft value. account number. address. At the moment we are simply concerned with managing the account information in the bank. from video games to robots. 4. A lot of the things that our bank is going to do (store a large amount of information about a large number of individuals. from a programming point of view it is an interesting problem and as we approach it we will uncover lots of techniques which will be useful in other programs that we might write. This equally as important. 4. By setting out the scope at the beginning you can make sure that there are no unpleasant surprises later on. a statement of what the system will not do. These notes should put the feature into a useful context.2 Enumerated Types These sound really posh.Creating Solutions Our Case Study: Friendly Bank 4 Creating Solutions 4. otherwise known as the Friendly Bank. If anyone asks you what you learnt today you can say "I learnt how to use enumerated types" and they will be really impressed. by implication. Bank Notes At the end of some sections there will be a description of how this new piece of C# will affect how we create our bank system.1. as a customer will not usually have clear idea of what you are doing and may well expect you to deliver things that you have no intention of providing. You are taking the role of a programmer who will be using the language to create a solution for a customer. This is also.1 Bank System Scope The scope of a system is a description of the things that the system is going to do.
  but how these are managed is not a problem for me. Sample states Enumerated types are very useful when storing state information. I could do something with numbers if I like: Empty sea = 1 Attacked = 2 Battleship = 3 Cruiser = 4 Submarine = 5 Rowing boat = 6 However. States are not quite the same as other items such as the name of a customer or the balance of their account. I am sort of assembling more metadata here. For example. openSea = SeaState.Creating Solutions Enumerated Types they know about programming they'll just say "Oh. sometimes we want to hold a range of particular values or states. in that I have decided that I need to keep track of the sea and then I have worked out exactly what I can put in it. If we want to hold something which is either true or false we can use a bool. C# Programming © Rob Miles 2010. For example I must write: SeaState openSea .2. and must be managed solely in terms of these named enumerations.EmptySea. We know that if we want to hold an integer value we can use an int type. 4. Of course C# itself will actually represent these states as particular numeric values. Cruiser. this would mean that I have to keep track of the values myself and remember that if we get the value 7 in a sea location this is clearly wrong. It can only have the given values above. But if you think of "enumerated" as just meaning "numbered" things get a bit easier. However. Attacked. you mean you’ve numbered some states" and not be that taken with it. C# has a way in which we can create a type which has just a particular set of possible values. My variable openSea is only able to hold values which represent the state of the sea contents. Submarine. I have created a type called SeaState which can be used to hold the state of a particular part of the sea. These types are called "enumerated types": enum SeaState { EmptySea. To understand what we are doing here we need to consider the problem which these types are intended to solve. RowingBoat } .1 Enumeration and states Enumerated sounds posh. Battleship.
  OffTheMarket etc) then you should think in terms of using enumerated types to hold the values. Green. RedAmber. like keywords are.2 Creating an enum type The new enum type can be created outside any class and creates a new type for use in any of my programs: using System. Amber } . "New". This shows that these items are extra types I have created which can be used to create variables. "Active". Sold. or states (for example OnSale. It is important that you understand what is happening here. Frozen.Red. C# Programming © Rob Miles 2010 78 .2. UnderOffer. Every account will contain a variable of type AccountState which represents the state of that account. "Closed" and "Under Audit" as states for our bank account.Creating Solutions Enumerated Types Note that types I create (like SeaState) will be highlighted by the editor in a blue colour which is not quite the same as keywords. Previous we have used types that are part of C#. Active. 4. The program becomes simpler to write. If this is the case it is sensible to create an enumerated type which can hold these values and no others enum AccountState { New. enum TrafficLight { Red. Closed } . We now have a variable which can hold state information about an account in our bank. For example. Programmer’s Point: Use enumerated types Enumerated types are another occasion where everyone benefits if you use them. Now we have reached the point where we are actually creating our own data types which can be used to hold data values that are required by our application. You should therefore use them a lot. for example int and double. easier to understand and safer. UnderAudit. light = TrafficLight. we could have the states "Frozen". class EnumDemonstration { public static void Main () { TrafficLight light . For the bank you want to hold the state of an item as well as other information about the customer. } } Every time that you have to hold something which can take a limited number of possible values. but they are not actually part of the C# language.
  get in written form exactly what they expect your system to do. 4. This is all very well.Creating Solutions Structures 4. Establish precisely the specification.integer value The Friendly Bank have told you that they will only be putting up to 50 people into your bank storage so. If we were talking about a database (which is actually what we are writing). would be called a field.3.string customer address . string [] addresses = new string [MAX_CUST] . (Remember that array subscript values start at 0).3 Structures Structures let us organise a set of individual values into a cohesive lump which we can map onto one of the items in the problem that we are working on. A structure is a collection of C# variables which you want to treat as a single entity.. In our program we are working on the basis that balance[0] holds the balance of the first customer in our database. Consider how you will go about storing the data.e. 2. the lump of data for each customer would be called a record and an individual part of that lump.integer value account balance .1 What is a Structure? Often when you are dealing with information you will want to hold a collection of different things about a particular item.integer value overdraft limit .2 Creating a Structure C# lets you create data structures. However it would me much nicer to be able to lump your record together in a more definite way.3. 4. In C# a lump of data would be called a structure and each part of it would be called a field. after a while you come up with the following: const int MAX_CUST = 50. int [] accountNos = new int [MAX_CUST] . Negotiate an extortionate fee. What we have is an array for each single piece of data we want to store about a particular customer. Like any good programmer who has been on my course you would start by doing the following: 1. int [] overdraft = new int [MAX_CUST] . A sample structure From your specification you know that the program must hold the following: customer name . string [] names = new string [MAX_CUST] . and so on. overdraft [0] holds the overdraft of the first customer. for example the overdraft value. int [] balances = new int [MAX_CUST] . and you could get a database system working with this data structure. 3. i.string account number . AccountState [] states = new AccountState [MAX_CUST] .
  would refer to the integer field AccountNumber in the structured variable RobsAccount. public string Address . public int Overdraft .Creating Solutions Structures struct Account { public AccountState State. called Bank which can hold all the customers. 4. so that: Bank [25].3. (i. We can assign one structure variable to another. This defines a structure.3 Using a Structure A program which creates and sets up a structure looks like this: using System.AccountNumber . This would copy the information from the RobsAccount structure into the element at the start of the Bank array. public string Name .Name . for example: RobsAccount. When the assignment is performed all the values in the source structure are copied into the destination: Bank[0] = RobsAccount. public int Balance . UnderAudit. The second declaration sets up an entire array of customers. Active. Account [] Bank = new Account [MAX_CUST].would be the string containing the name of the customer in the element with subscript 25. just as we would any other variable type. which can hold the information for a single customer. which contains all the required customer information. } . (full stop) separating them. We refer to individual members of a structure by putting their name after the struct variable we are using with a . C# Programming © Rob Miles 2010 80 . Frozen. the AccountNumber value in RobsAccount) You can do this with elements of an array of structures too. // enum enum AccountState { New. Closed } . The first declaration sets up a variable called RobsAccount. Having done this we can now define some variables: Account RobsAccount . public int AccountNumber . called Account.e. 2010 2010 2010
  83
  in that it prints out the name "Rob".Creating Solutions Objects. Console. But when we create a reference we don't actually get one of the things that it refers to.Name = "Rob". class StructsAndObjectsDemo { public static void Main () { Account RobsAccount . We solve the problem by creating an instance of the class and then connecting our tag to it. RobsAccount.cs(12. } . Structures and References It then sets the name property of the variable to the string "Rob". .is an attempt to find the thing that is tied to this tag and set the name property to "Rob". } } The account information is now being held in a class. rather than a structure. "you are trying to follow a reference which does not refer to anything. If you have the tag you can then follow the rope to the object it is tied to. Creating and Using an Instance of a Class We can make a tiny change to the program and convert the bank account to a class: class Account { public string Name .Name = "Rob". and I could use them in just the same way.WriteLine (RobsAccount. Account. If we run this program it does exactly what you would expect. The account class is called. in effect. If the structure contained other items about the bank account these would be stored in the structure as well. This looks like a declaration of a variable called RobsAccount. Such references are allowed to refer to instances of the Account. The compiler therefore says.3): error CS0165: Use of unassigned local variable ' RobsAccount' So.Name ). and so it gives me an error because the line: RobsAccount. this is not what it seems. Since the tag is presently not tied to anything our program would fail at this point. The compiler knows this. The problem is that when we compile the program we get this: ObjectDemo. therefore I am going to give you a 'variable undefined' error". RobsAccount What you actually get when the program obeys that line is the creation of a reference called RobsAccount. what is going on? To understand what is happening you need to know what is performed by the line: Account RobsAccount. in that they can be tied to something with a piece of rope. quite simply. But in the case of objects. You can think of them as a bit like a luggage tag. This is achieved by adding a line to our program: C# Programming © Rob Miles 2010 84 .
  Consider the following code: C# Programming © Rob Miles 2010 85 . This is because an array is actually implemented as an object. you hold a tag which is tied onto an instance… Multiple References to an Instance Perhaps another example of references would help at this point.Name = "Rob". This is because the object instance does not have the identifier RobsAccount. Console. The two come hand in hand and are inseparable. not RobsAccount. RobsAccount. We use it to create arrays.Creating Solutions Objects. RobsAccount = new Account(). I'll repeat that in a posh font: “An object is an instance of a class” I have repeated this because it is very important that you understand this.WriteLine (RobsAccount. } . and that means that we must manage our access to a particular object by making use of references to it.2 References We now have to get used to the idea that if we want to use objects. we have to use references. Structures are kind of useful. 4. The new keyword causes C# to use the class information to actually make an instance. An object is an instance of a class. it is simply the one which RobsAccount is connected to at the moment. but you must remember that when you hold a reference you do not hold an instance. The thing that new creates is an object. Account RobsAccount Name: Rob We have seen this keyword new before. and what it can do. class StructsAndObjectsDemo { public static void Main () { Account RobsAccount . but for real object oriented satisfaction you have to have an object. Structures and References class Account { public string Name . Actually this is not that painful in reality. Note that in the above diagram I have called the object an Account. in that you can treat a reference as if it really was the object just about all of the time. } } The line I have added creates a new Account object and sets the reference RobsAccount to refer to it.Name ). A class provides the instructions to C# as to what is to be made.4. and so we use new to create it.
  Name ). Console. so you need to remember that changing the object that a reference refers to may well change that instance from the point of view of other objects.WriteLine (RobsAccount. The reference RobsAccount is made to refer to the new item. RobsAccount.Name = "Jim". RobsAccount = new Account(). sets the name property of it to Rob and then makes another account instance. Structures and References Account RobsAccount . Console. Account Temp . The question is.Name = "Jim".Name ). because they are the same object.Name ). this can be made clearer with a diagram: C# Programming © Rob Miles 2010 86 . The question is: What happens to the first instance? Again. Console. Temp = RobsAccount. This code makes an account instance. since that is the name in the object that RobsAccount is referring to.Name ).WriteLine (RobsAccount. what would the second call of WriteLine print out? If we draw a diagram the answer becomes clearer: Account RobsAccount Name: Jim Temp Both of the tags refer to the same instance of Account. This means that any changes which are made to the object that Temp refers to will also be reflected in the one that RobsAccount refers to. RobsAccount. RobsAccount = new Account(). which has the name set to Jim. Console. Temp.Creating Solutions Objects.WriteLine (RobsAccount. This indicates a trickiness with objects and references.Name = "Rob". RobsAccount = new Account(). RobsAccount. No References to an Instance Just to complete the confusion we need to consider what happens if an object has no references to it: Account RobsAccount .Name = "Rob".WriteLine (RobsAccount. There is no limit to the number of references that can be attached to a single instance. This means that the program would print out Jim.
  When you pay someone with one of these coins you don’t actually pick it up and give it to them. meaning another job for the garbage collector.Creating Solutions Objects. When I work with objects I worry about how much creating and destroying I am doing. They seem to make it harder to create and use objects and may be the source of much confusion. You should also remember that you can get a similar effect when a reference to an instance goes out of scope: { Account localVar . That is why we use references in our programs. The value of a ―coin‖ in the Yap currency is directly related to the number of men who died in the boat bringing the rock to the island. So why do we bother with them? To answer this we can consider the Pacific Island of Yap.4. Structures and References Account RobsAccount Name: Rob Account Name: Jim The first instance is shown ―hanging‖ in space. C# Programming © Rob Miles 2010 87 . Note that the compiler will not stop us from ―letting go‖ of items like this. } The variable localVar is local to the block. you must remember that creating and disposing of objects will take up computing power. Instead you just say ―The coin in the road on top of the hill is now yours‖. Consider a bank which contains many accounts. localVar = new Account(). This means that when the program execution leaves the block the local variable is discarded. called the ―Garbage Collector‖ which is given the job of finding such useless items and disposing of them. with nothing referring to it. The currency in use on this island is based around 12 feet tall stones which weigh several hundred pounds each. Programmer’s Point: Try to avoid the Garbage Collector While it is sometimes reasonable to release items you have no further use for. Just because the objects are disposed of automatically doesn’t mean that you should abuse the facility. In other words they use references to manage objects that they don’t want to have to move around. 4. Indeed the C# language implementation has a special process. As far as making use of data in the instance is concerned. it might as well not be there.3 Why Bother with References? References don’t sound much fun at the moment. If we wanted to sort them into alphabetical order of customer name we have to move them all around. This means that the only reference to the account is also removed.
  We can get over this. for example they might want to order it on both customer surname and also on account number. Sorting by use of a tree In the tree above each node has two references. each of which is ordered in a particular way: Sorting by using references If we just sort the references we don’t have to move the large data items at all.Creating Solutions Objects. the other to a node which is ―darker‖. Structures and References Sorting by moving objects around If we held the accounts as an array of structure items we would have to do a lot of work just to keep the list in order. References and Data Structures Our list of sorted references is all very good. one can refer to a node which is ―lighter‖. by structuring our data into a tree form. The bank may well want to order the information in more than one way too. and also speed up searching. Then C# Programming © Rob Miles 2010 88 . With references we just need to keep a number of arrays of references. New objects can be added without having to move any objects. If I want a sorted list of the items I just have to go as far down the ―lighter‖ side as I can and I will end up at the lightest. instead the references can be moved around. Without references this would be impossible. but if we want to add something to our sorted list we still have to move the references around.
  The thing that we are trying to do here is best expressed as: ―Put off all the hard work for as long as we can. for now we just need to remember that the reference and the object are distinct and separate. We will consider this aspect of object use later.Creating Solutions Designing With Objects I go up to the one above that (which must be the next lightest). Reference Importance The key to this way of working is that an object can contain references to other objects. it will not be possible to move them around memory if we want to sort them.5 Designing With Objects We are now going to start thinking in terms of objects. This means that the only way to manipulate them is to leave them in the same place and have lists of references to them. it is a programming document. Programmer’s Point: Data Structures are Important This is not a data structures document. Then I go down the dark side (Luke) and repeat the process. and if possible get someone else to do it. And if the manager comes along with a need for a new structure or view we can create that in terms of references as well.. But sometime in the future you are going to have to get your head around how to build structures using these things. This all comes back to the ―creative laziness‖ that programmers are so famous for.‖ Objects let us do this. don’t worry. The references are very small "tags" which can be used to locate the actual item in memory. Bank Notes: References and Accounts For a bank with many thousands of customers the use of references is crucial to the management of the data that they hold. The reason that we do this is that we would like a way of making the design of our systems as easy as possible. in which case the item is not in the structure. a bit like President Kennedy did all those years ago: C# Programming © Rob Miles 2010 89 . object based design turns this on its head. The accounts will be held in the memory of the computer and. Searching is also very quick. I just find the place on the tree that they need to be hung on and attach the reference there. If you don’t get all the stuff about trees just yet. The neat thing about this approach is also that adding new items is very easy. and it would also be possible to have several such lists. because of the size of each account and the number of accounts being stored. Just remember that references are an important mechanism for building up structures of data and leave it at that. This means that we can offer the manager a view of his bank sorted by customer name and another view sorted in order of balance. 4. as well as the data payload. Sorting a list of references is very easy. in that I can look at each node and decide which way to look next until I either find what I am looking for or I find there is no reference in the required direction.
  And once we have decided on the actions that the account must perform.Balance = 0. This will let me describe all the techniques that are required without getting bogged down too much. RobsAccount. Programmer’s Point: Not Everything Should Be Possible Note that there are also some things that we should not be able to do with our bank account objects. Instead we ask it to do these things for us.Balance = 99.5. In this section we are going to implement an object which has some of the behaviours of a proper bank account. along with what should be done. metadata and testing. For the sake of simplicity. Each time I create an instance of the class I get all the members as well. The account number of an account is something which is unique to that account and should never change. for now I’m just going to consider how I keep track of the balance of the accounts. since this is specially designed to hold financial values. we don’t have to worry precisely how they made it work – we just have to sit back and take the credit for a job well done. What a bank account object should be able to do is part of the metadata for this object. We can get this behaviour by simply not providing a means by which it can be changed. The first thing is to identify all the data items that we want to store in it. } The Account class above holds the member that we need to store about the balance of our bank accounts. C# Programming © Rob Miles 2010 90 . 4. The design of our banking application can be thought of in terms of identifying the objects that we are going to use to represent the information and then specifying what things they should be able to do. my fellow Americans: ask not what your country can do for you—ask what you can do for your country‖ (huge cheers) We don’t do things to the bank account. That way we can easily find out if bad things are being done. We might even identify some things as being audited. It is important at design time that we identify what should not be possible. we can consider our bank account in terms of what we want it to do for us. I’ve used the decimal type for the account balance. RobsAccount = new Account(). This means that any programmer writing the application can do things like: RobsAccount. We have already seen that it is very easy to create an instance of a class and set the value of a member: Account RobsAccount . we then might be able to get somebody else to make it do these things. Members of a class which hold a value which describes some data which the class is holding are often called properties.Creating Solutions Designing With Objects ―And so. the next thing we need to do is devise a way in which each of the actions can be tested. The really clever bit is that once we have decided what the bank account should do. If our specification is correct and they implement it properly. in that an object will keep track of what has been done to it. The reason that this works is that the members of the object are all public and this means that anybody has direct access to them. This brings us back to a couple of recurring themes in this document. We have seen that each of the data items in a class is a member of it and stored as part of it.1 Data in Objects So. class Account { public decimal Balance.
  The posh word for this is encapsulation. C# Programming © Rob Miles 2010 91 . whatever else happens.I will get an error when I try to compile the program: PrivateDemo.2 Member Protection inside objects If objects are going to be useful we have to have a way of protecting the data within them.3): error CS0122: 'PrivateMembers. thanks for the vote of confidence folks. For example.balance = 0. The first thing we need to do is stop the outside world from playing with our balance value: class Account { private decimal balance. This means that the outside world no longer has direct access to it. now you can’t change it at all‖. 4. Ideally I want to get control when someone tries to change a value in my objects. in our bank program we want to make sure that the balance is never changed in a manner that we can't control. } balance = balance .and take away all my money.cs(13.5. public bool WithdrawFunds ( decimal amount ) { if ( balance < amount ) { return false . Consider the program: class Account { private decimal balance = 0. but only using code actually running in the class. return true. Changing private members I can tell what you are thinking at this point. Instead it is now private. It turns out that I can change the value. If I write the code: RobsAccount. my part of the program does not go wrong. If we are going to provide a way of stopping this from happening we need to protect the data inside our objects. Well.Account.amount .balance' is inaccessible due to its protection level The balance value is now held inside the object and is not visible to the outside world. } } . You are thinking ―What is the point of making it private. and stop the change from being made if I don’t like it.Creating Solutions Designing With Objects . I want all the important data hidden inside my object so that I have complete control over what is done with it. } The property is no longer marked as public. . This technology is the key to my defensive programming approach which is geared to making sure that.
  since we want people to interact with our objects by calling methods in them. The method WithdrawFunds is a member of the Account class and can therefore access private members of the class.WriteLine ( "Cash Withdrawn" ) . In fact.). If you don’t care about possible corruption of the member and you want your program to run as quickly as possible you can make a data member public. In the code above the way that I am protecting the balance value is a reflection of how the customer wants me to make sure that this value is managed properly. These (for example the ubiquitous i) always start with a lower case letter. So perhaps now is a good time. public Methods You may have noticed that I made the WithdrawFunds method public. C# Programming © Rob Miles 2010 92 .Creating Solutions Designing With Objects class Bank { public static void Main () { Account RobsAccount. task you can make it private. They would call their balance value m_balance so that class members are easy to spot. and expect developers to adhere to these. If you want to write a method which is only used inside a class and performs some special. because I reckon that the name of the member is usually enough. make it private if it is a method member (i. This makes it easy for someone reading my code. The metadata that I gather about my bank system will drive how I provide access to the members of my classes. if ( RobsAccount. But I make the first letter of private members lower case (as in the case of the balance data member of our bank account). } } } This creates an account and then tries to draw five pounds out of it.WriteLine ( "Insufficient Funds" ) . the rules can be broken on special occasions. since the initial balance on my account is zero. The convention also extends to variables which are local to a block. because they can see from the name of a class member whether or not it is public or private. most development companies have documents that set out the coding conventions they use. RobsAccount = new Account(). Programmer’s Point: Metadata makes Members and Methods I haven’t mentioned metadata for at least five minutes. but opinions differ on this one.e. it holds data) of the class. This means that code running outside the class can make calls to that method. secret. This will of course fail.e. Some people go further and do things like put the characters m_ in front of variables which are members of a class. In general the rules are: if it is a data member (i. This has got to be the case. The most important thing in this situation is that the whole programming team adopts the same conventions on matters like these. but it shows how I go about providing access to members in an account. it does something) make it public Of course.WithdrawFunds (5) ) { Console. } else { Console. I don’t usually go that far.
  Creating Solutions Designing With Objects 4. public bool WithdrawFunds ( decimal amount ) { if ( balance < amount ) { return false . If it does not my program is faulty.3 A Complete Account Class We can now create a bank account class which controls access to the balance value: public class Account { private decimal balance = 0. for example: Account test = new Account(). C# Programming © Rob Miles 2010 93 . } public void PayInFunds ( decimal amount ) { balance = balance + amount .5.PayInFunds(50). I have created three methods which I can use to interact with an account object. At the end of this set of statements the test account should have 50 pounds in it.WriteLine ( "Pay In test failed" ). } balance = balance . I can pay money in.PayInFunds(50). They then search out the programmer who caused the failure and make him or her pay for coffee for the next week. The method GetBalance is called an accessor since it allows access to data in my business object. test. find out how much is there and withdraw cash. I could write a little bit of code to test these methods: Account test = new Account(). Of course I must still read the output from all the tests.amount . which is tedious. } } The bank account class that I have created above is quite well behaved. if ( test. Later we will consider the use of unit tests which make this much easier.GetBalance() != 50 ) { Console. My tests count the number of errors that they have found. and if you don’t notice it then you might think your code is OK. test. } My program now tests itself. Programmer’s Point: Make a Siren go off when your tests fail The above test is good but not great. in that it does something and then makes sure that the effect of that action is correct. so that it is impossible to ignore a failing test. return true. It just produces a little message if the test fails. If the error count is greater than zero they print out a huge message in flashing red text to indicate that something bad has happened. Some development teams actually connect sirens and flashing red lights to their test systems. } public decimal GetBalance () { return balance. You have to read that message.
  It is very important that you learn what static means in the context of C# programs. The other kind of program that is very hard to test in this way is any kind of game. Programmer’s Point: Some things are hard to test Test development is a good way to travel.4 Test Driven Development I love test driven development. When you fix bugs in your program you need to be able to convince yourself that the fixes have not broken some other part (about the most common way of introducing new faults into a program is to mend a bug). We have used it lots in just about every program that we have ever written: C# Programming © Rob Miles 2010 94 . I can be fairly confident that the user interface will be OK too. 4. The only solution in this situation is to set out very clearly what your tests are trying to prove (so that the human testers know what to look for) and make it very easy to change the values that will affect the gameplay. For example it should be easy to adjust how much damage a hit from an alien causes to your spacecraft. You don't do the testing at the end of the project. However. Some types of programs are really hard to test in this way. This is mainly because the quality of gameplay is not something you can design tests for. place develop using tests.e. Far better to test the code as you write it. when you have the best possible understanding of what it is supposed to do.6. Many projects are doomed because people start programming before they have a proper understanding of the problem. 3.Creating Solutions Static Items 4. If the bugs are in an old piece of code you have to go through the effort of remembering how it works. If you have a set of automatic tests that run after every bug fix you have a way of stopping this from happening.5. because although it is easy to send things into a program it is often much harder to see what the program does in response. If I ever write anything new you can bet your boots that I will write it using a test driven approach. You can write code early in the project which will probably be useful later on. As long as my object tests are passed.1 Static class members The static keyword lets us create members which are not held in an instance. since you might be using code that you wrote some time back. And there is a good chance that the tests that you write will be useful at some point too. Anything with a front end where users type in commands and get responses is very hard to test like this. Only when someone comes back and says “The game is too hard because you can’t kill the end of level boss without dying” can you actually do something about it. and the speed of all the game objects. So. the code that provides the user interface where the customer enters how much they want to withdraw will be very simple and connect directly to the methods provided by my objects. In the case of our bank account above. we can also create members which are held as part of the class. 2. i. 4. This is usually the worst time to test. My approach in these situations is to make the user interface part a very thin layer which sits on top of requests to objects to do the work. but it does not solve all your problems. You will thank me later. but in the class itself. This solves three problems that I can see: 1. they exist outside of any particular instance. Writing the tests first is actually a really good way of refining your understanding.6 Static Items At the moment all the members that we have created in our class have been part of an instance of the class. This means that whenever we create an instance of the Account class we get a balance member.
  It is part of the class AccountTest. 4. This is how my program actually gets to work. I solve the problem by making the interest rate member static: C# Programming © Rob Miles 2010 95 . and so this method must be there already. RobsAccount. Console. I think this is time for more posh font stuff: Static does not mean “cannot be changed”. } } The AccountTest class has a static member method called Main. and if I missed one account. This would be tedious. Members of a class which have been made static can be used just like any other member of a class. We know that this is the method which is called to run the program.GetBalance()). The customer has told us that one of the members of the account class will need to be the interest rate on accounts. (of course if I was doing this properly I'd make this stuff private and provide methods etc. In the program we can implement this by adding another member to the class which holds the current interest rate: public class Account { public decimal Balance . This means that to implement the change I'd have to go through all the accounts and update the rate. If the interest rate changes it must change for all accounts. for it is important: “A static member is a member of the class. RobsAccount. The snag is. Static does not mean "cannot be changed".WriteLine ("Balance:" + test. public decimal InterestRateCharged .PayInFunds (50). not part of an instance of the class.6.InterestRateChanged = 10. } Now I can create accounts and set balances and interest rates on them.Balance = 100. test. possibly expensive. Consider the interest rates of our bank accounts. If I made fifty AccountTest instances. but I'm keeping things simple just now).Creating Solutions Static Items class AccountTest { public static void Main () { Account test = new Account(). I've been told that the interest rate is held for all the accounts. not a member of an instance of the class” I don't have to make an instance of the AccountTest class to be able to use the Main method. Account RobsAccount = new Account(). In terms of C# the keyword static flags a member as being part of the class. otherwise it cannot run. Either a data member or a method can be made static. in that when it starts it has not made any instances of anything. I will write that down again in a posh font.2 Using a static data member of a class Perhaps an example of static data would help at this point. they would all share the same Main method.
  There might be a time where the age limit changes. and you don't want to have to update all the objects in your program. "With great power comes great responsibility". we can't call the method until we have an Account instance. For example.Balance = 100.Creating Solutions Static Items public class Account { public decimal balance . as Spiderman's uncle said. But you can also use them when designing your system.InterestRateChanged = 10. We have been doing this for ages with the Main method. } else { return false. } The interest rate is now part of the class. The snag is that. It would then return true or false depending on whether these are acceptable or not: public bool AccountAllowed ( decimal income. I can now call the method by using the class name: C# Programming © Rob Miles 2010 96 . int age ) { if ( ( income >= 10000 ) && ( age >= 18 ) ) { return true. Account. 4. } } This checks the age and income. You should be careful about how you provide access to static data items. So they should always be made private and updated by means of method calls. public static decimal interestRateCharged . It would take in their age and income. } } Now the method is part of the class. We can solve this by making the method static: public static bool AccountAllowed ( decimal income. RobsAccount.. you must be over 17 and have at least 1000 pounds income to be allowed an account. This means that I have to change the way that I get hold of it: Account RobsAccount = new Account(). we might have a method which decides whether or not someone is allowed to have a bank account. But of course. not part of any instance.3 Using a static method in a class We can make methods static too. at the moment. not an instance of the class. A change to a single static value will affect your entire system.6. int age ) { if ( ( income >= 10000 ) && ( age >= 18 ) ) { return true. Things like the limits of values (the largest age that you are going to permit a person to have) can be made static. } else { return false.
  21 ) ) { Console.21): error CS0120: An object reference is required for the nonstatic field.AccountAllowed ( 25000.WriteLine ( "Allowed Account" ). Using member data in static methods The Allowed method is OK. The compiler is unhappy because in this situation the method would not have any members to play with. how about this: The members minIncome and minAge are held within instances of the Account class.43): error CS0120: An object reference is required for the nonstatic field. method. in that I now have members of the class which set out the upper limits of the age and income. a static method can run without an instance (since it is part of the class).cs(19.minIncome' AccountManagement. We can fix this (and get our program completely correct) by making the income and age limits static as well: C# Programming © Rob Miles 2010 97 .minAge' As usual. However it is a bad program. } } } This is a better design. the compiler is telling us exactly what is wrong. but of course I have fixed the age and income methods into it. or property 'Account. } else { return false.Creating Solutions Static Items if ( Account. What the compiler really means is that "a static method is using a member of the class which is not static". since the class above will not compile: AccountManagement. int age) { if ( ( income >= minIncome) && ( age >= minAge) ) { return true. However. private int minAge = 18. If that doesn’t help. } This is nice because I have not had to make an instance of the account to find out if one is allowed. I might decide to make the method more flexible: public class Account { private decimal minIncome = 10000. public static bool AccountAllowed(decimal income. method.cs(19. or property 'Account. using language which makes our heads spin.
  The constructor method is a member of the class and it is there to let the programmer get control and set up the contents of the shiny new object. The limit values should not be held as members of a class. public static bool AccountAllowed(decimal income. so that it can execute without an instance. One of the rules of the C# game is that every single class must have a constructor method to be called when a new instance is created. for a change. when you are building your system you should think about how you are going to make such methods available for your own use. accounts for five year olds have a different interest rate from normal ones". being friendly here. not an instance of a class. therefore. Bank Notes: Static Bank Information The kind of problems that we can use static to solve in our bank are: static member variable: the manager would like us to be able to set the interest rate for all the customer accounts at once. Programmer’s Point: Static Method Members can be used to make Libraries Sometimes in a development you need to provide a library of methods to do stuff. This is because the C# compiler is. ―We’ve been making objects for a while and I’ve never had to provide a constructor method‖. since we want them to be the same for all instances of the class. A single static member of the Account class will provide a variable which can be used inside all instances of the class. int age) { if ( ( income > minIncome) && ( age > minAge) ) { return true. If you look closely at what is happening you might decide that what is happening looks quite a bit like a method call. But because there is only a single copy of this value this can be changed and thereby adjust the interest rate for all the accounts. This is actually exactly what is happening. for example sin and cos. you say. At this point we know we can’t use static because we need to hold different values for some of the instances. I must make it static. making them static is what we should have done in the first place.Creating Solutions The Construction of Objects public class Account { private static decimal minIncome . the compiler instead quietly creates a default one for you and uses that. Again.7 The Construction of Objects We have seen that our objects are created when we use new to bring one into being: test = new Account(). 4. ―But wait a minute‖. I can't make this part of any Account instance because at the time it runs an account has not been generated. When an instance of a class is created the C# system makes a call to a constructor method in that class. in that in this situation all we want is the method itself. this makes perfect sense. Any value which is held once for all classes (limits on values are another example of this) is best managed as a static value. C# Programming © Rob Miles 2010 98 . It makes sense to make these methods static. private static int minAge . Rather than shout at you for not providing a constructor method. In the C# system itself there are a huge number of methods to perform maths functions. } } } If you think about it. static member method: the manager tells us that we need a method to determine whether or not a given person is allowed to have an account. } else { return false. The time it becomes impossible to use static is when the manager says "Oh.
  In other words I want to do: robsAccount = new Account( "Rob Miles". "Hull".7. but it does show how the process works. This means that when my program executes the line: robsAccount = new Account().WriteLine ( "We just made an account" ). but it does not return anything..the program will print out the message: We just made an account Note that this is not very sensible. It is called when we perform new. It is public so that it can be accessed from external classes who might want to make instances of the class. } } This constructor is not very constructive (ho ho) but it does let us know when it has been called. 0 ). Feeding the Constructor Information It is useful to be able to get control when an Account is created. .2 Our Own Constructor For fun. in that it will result in a lot of printing out which the user of the program might not appreciate. If you don’t supply a constructor (and we haven’t so far) the compiler creates one for us. all I have to do is make the constructor method to accept these parameters and use them to set up the members of the class: C# Programming © Rob Miles 2010 99 . which we will discuss later. we could make a constructor which just prints out that it has been called: public class Account { public Account () { Console. address. As an example. but in this case it is simply solving the problem without telling you. 4. If I create my own constructor the compiler assumes that I know what I’m doing and stops providing the default one.7. It accepts no parameters. It turns out that I can do this very easily. This can cause problems.Creating Solutions The Construction of Objects You might think this is strange. in that normally the compiler loses no time in telling you off when you don’t do something. and initial balance of an account holder when the account is created. but it would be even nicer to be able to feed information into the Account when I create it.1 The Default Constructor A constructor method has the same name as the class. I might want to set the name. the address to Hull and the initial balance to zero. 4. public class Account { public Account () { } } This is what the default constructor looks like. This could create a new account and set the name property to Rob Miles.
  since I want to use the "default" one of zero. address = inAddress.7. In the context of the constructor of a class. Something a bit like this: C# Programming © Rob Miles 2010 100 . In that situation you have to either find all the calls to the default one and update them. This can cause confusion if we have made use of the default constructor in our program and we then add one of our own. } } The constructor takes the values supplied in the parameters and uses them to set up the members of the Account instance that is being created. balance = inBalance.27): error CS1501: No overload for method 'Account' takes '0' arguments What the compiler is telling me is that there is no constructor in the class which does not have any parameters. If I try to do this: robsAccount = new Account(). what this means is that you can provide several different ways of constructing an instance of a class. because it can tell from the parameters given at the call of the method which one to use. private decimal balance. In this respect it behaves exactly as any other method call. nothing in the account.3 Overloading Constructors Overload is an interesting word. // constructor public Account (string inName. "Hull").Creating Solutions The Construction of Objects class Account { // private member data private string name. the only way I can now make an Account object is by supplying a name. Just like me. The default constructor is no longer supplied by the compiler and our program now fails to compile correctly. but has a different set of parameters" The compiler is quite happy for you to overload methods.e. In the context of a C# program it means: "A method has the same name as another. For example. private string address.the compiler will stop being nice to me and produce the error: AccountTest. decimal inBalance) { name = inName. In other words. hem hem. address and starting balance.e. If your code does this the compiler simply looks for a constructor method which has two strings as parameters. I've missed off the balance value. Of course you don't have to do this because your design of the program was so good that you never have this problem. . the compiler only provides a default constructor if the programmer doesn't provide a constructor. many (but not all) of your accounts will be created with a balance value of zero. i. In the context of the "Star Trek" science fiction series it is what they did to the warp engines in every other episode. 4. i. and nothing else.cs(9. string inAddress. or create a default constructor of your own for these calls to use. Note that adding a constructor like this has one very powerful ramification: You must use the new constructor to make an instance of a class. This means that we would like to be able to write robsAccount = new Account("Rob Miles".
  string inAddress) { name = inName. even if you don't put a bug in your code. balance = 0. 7. } I've made three constructors for an Account instance. To do this I have had to duplicate code. . string inAddress) { name = inName. The first is supplied with all the information. and sets the address to "Not Supplied". balance = 0. Because it is bad. address = inAddress. C# provides a way in which you can call one constructor from another. int julianDate ) SetDate ( string dateInMMDDYY ) A call of: SetDate ( 23. } Overloading a method name In fact. for example you could provide several ways of setting the date of a transaction: SetDate ( int year. balance = inBalance. you can overload any method name in your classes. decimal inBalance) { name = inName. just use the block copy command in the text editor and you can take the same piece of program and use it all over the place. If you need to change this piece of code you have to find every copy of the code and change it. 2005 ). The scary thing is that it is quite easy to do. It is regarded as "dangerous extra work".Creating Solutions The Construction of Objects public Account (string inName. } public Account (string inName. balance = 0.would be matched up with the method which accepts three integer parameters and that code would be executed. address = inAddress. the second is not given a balance and sets the value to 0. But you should not do this. 4. Consider: C# Programming © Rob Miles 2010 101 . The third is not given the address either. This can be useful if you have a particular action which can be driven by a number of different items of data. string inAddress. So. } public Account (string inName) { name = inName.7. int month. int day ) SetDate ( int year. address = inAddress. you still might find yourself having to change it because the specification changes. address = "Not Supplied". This happens more often than you'd think. Good programmers hate duplicating code.4 Constructor Management If the Account class is going to have lots of constructor methods this can get very confusing for the programmer: public Account (string inName.
  0 ) { } The keyword this means "another constructor in this class". The syntax of these calls is rather interesting. Failure is not an option. This is sensible. because it reflects exactly what is happening. attempts to withdraw negative amounts of money from a bank account should be rejected. address = inAddress.PayInFunds (1234567890). on to the "proper" constructor to deal with. "Not Supplied". This means that the actual transfer of the values from the constructor into the object itself only happens in one method. In fact. For example.Creating Solutions The Construction of Objects public Account (string inName. 4. For example. the highlighted bits of the code are calls to the first constructor. The "this" constructor runs before the body of the other constructor is entered. So we know that when we create a method which changes the data in an object we have to make sure that the change is always valid. But what is to stop the following: RobsAccount = new Account ("Rob". Constructors cannot fail. You should create one "master" constructor which handles the most comprehensive method of constructing the object. In fact it is outside the block completely. decimal inBalance) { name = inName. Then you should make all the other constructor methods use this to get hold of that method. And this is a problem: Whenever we have written methods in the past we have made sure that their behaviour is error checked so that the method cannot upset the state of our object. They simply pass the parameters which are supplied. Programmer’s Point: Object Construction Should Be Planned The way in which objects are constructed is something that you should plan carefully when you write your program. 0 ) { } public Account ( string inName ) : this (inName. since the call of this does all the work. Constructors are a bit like this.7. "Hull". so the PayInFunds method will refuse to pay the money in. inAddress. There will be an upper limit to the amount of cash you can pay in at once. The whole basis of the way that we have allowed our objects to be manipulated is to make sure that they cannot be broken by the people using them. If you try to do something stupid with a method call it should refuse to perform the action and return something which indicates that it could not do the job. in the code above. C# Programming © Rob Miles 2010 102 .5 A constructor cannot fail If you watch a James Bond movie there is usually a point at which agent 007 is told that the fate of the world is in his hands. 1234567890). balance = inBalance. and the other constructor methods just make calls to it. in that the call to the constructor takes place before the body of the constructor method. we would not let the following call succeed: RobsAccount. As you can see in the code sample above. string inAddress. along with any default values that we have created. string inAddress ) : this (inName. } public Account ( string inName. the body of the constructor can be empty.
  The really clever way to do this is to make the constructor call the set methods for each of the properties that it has been given. Writing code which will handle all the possible failure conditions in a useful way is much trickier. This can be done. } if ( SetAddress ( inAddress) == false ) { throw new Exception ( "Bad address" + inAddress) . I type in my name wrong and it complains about that. at the expense of a little bit of complication: public Account (string inName. C# Programming © Rob Miles 2010 103 . } } This version of the constructor assembles an error message which describes everything which is wrong with the account. This poses a problem. Writing code to do a job is usually very easy. Whatever happens during the constructor call.e. and if any of them returns with an error the constructor should throw the exception at that Point: public Account (string inName. The only problem here is that if the address is wrong too. Each new thing which is wrong is added to the message and then the whole thing is put into an exception and thrown back to the caller. } if ( SetAddress ( inAddress) == false ) { errorMessage = errorMessage + " Bad addr " + inAddress. } } If we try to create an account with a bad name it will throw an exception. It looks as if we can veto stupid values at every point except the one which is most important. What I want is a way in which I can get a report of all the invalid parts of the item at once. string inAddress) { string errorMessage = "". constructors are not allowed to fail. This means that the user of the constructor must make sure that they catch exceptions when creating objects. i. } if ( errorMessage != "" ) { throw new Exception ( "Bad account" + errorMessage) .Creating Solutions The Construction of Objects Like James Bond. if ( SetName ( inName ) == false ) { errorMessage = errorMessage + "Bad name " + inName. string inAddress) { if ( SetName ( inName ) == false ) { throw new Exception ( "Bad name " + inName) . and it complains about my address. which is not a bad thing. when the object is first created. the user of the method will not know this until they have fixed the name and then called the constructor again. it will complete and a new instance will be created. Then I put my name right. It is normally when I'm filling in a form on the web. I hate it when I'm using a program and this happens. It is a fact of programming life that you will (or at least should) spend more time worrying about how things fail than you ever do about how they work correctly. Constructors and Exceptions The only way round this at the moment is to have the constructor throw an exception if it is unhappy. Programmer’s Point: Managing Failure is Hard Work This brings us on to a kind of recurring theme in our quest to become great programmers. which is what we want.
  the graphics adapter is usually a separate device which is plugged into the main board. For this to work properly the people who make main boards and the people who make graphics adapters have had to agree on an interface between two devices. During the specification process you need to establish if the code is ever going to be created in multiple language versions. some parts are not "hard wired" to the system. C# Programming © Rob Miles 2010 104 . Software components are exactly the same. Bank Notes: Constructing an Account The issues revolving around the constructor of a class are not directly relevant to the bank account specification as such. You should be familiar with the way that. The next thing to do is consider how we take a further step back and consider expressing a solution using components and interfaces. Posh people call this "abstraction". since they really relate to how the specification is implemented. and not what the system itself actually does. if you write the program as above this might cause a problem when you install the code in a French branch of the bank. If it is you will need to manage the storage and selection of appropriate messages. Fortunately there are some C# libraries which are designed to make this easier. So. For example. Any main board which contains a socket built to the standard can accept a graphics card. we just say "We need an account here" and then move on to other things. This is the progress that we have made so far: representing values by named locations (variables) creating actions which work on the variables (statements and blocks) putting behaviours into lumps of code which we can give names to. That said.8. if the manager says something like "The customer fills in a form. because it means that I can buy a new graphics adapter at any time and fit it into the machine to improve the performance. and how to design systems using them.1 Components and Hardware Before we start on things from a software point of view it is probably worth considering things from a hardware point of view. and from the point of view of what the Account class needs to do. which signals are outputs and so on. This takes the form of a large document which describes exactly how the two components interact. 4. Later we will come back and revisit the problem in a greater level of detail. for example which signals are inputs.. In this section you will find out the difference between an object and a component.Creating Solutions From Object to Component Programmer’s Point: Consider the International Issues The code above assembles a text message and sends it to the user when something bad happens. This is good.8 From Object to Component I take the view that as you develop as a software writer you go through a process of "stepping back" from problems and thinking at higher and higher levels. from the point of view of hardware. enters their name and address and this is used to create the new account" this gives you a good idea of what parameters should be supplied to the constructor. 4. This is a good thing. components are possible because we have created standard interfaces which describe exactly how they fit together. However. in a typical home computer.
  a system designed without components is exactly like a computer with a graphics adapter which part of the main board. When we are creating a system we work out what each of the parts of it need to do. Our first pass at a bank account interface could be as follows: public interface IAccount { void PayInFunds ( decimal amount ). and compiled in the same way. From the balance management point of view this is all we need. This might happen even after we have installed the system and it is being used. } This says that the IAccount interface is comprised of three methods. This will earn you zero marks. An interface is simply a set of method definitions which are lumped together. I am instead just saying what should be done. In C# we express this information in a thing called an interface. For example. These are usually either text based (the user types in commands and gets responses) or graphical (the user clicks on ―buttons‖ on a screen using the mouse). An interface on the other hand just specifies how a software component could be used by another software component. And quite right too. If you don’t do this your programs will compile fine. Another convention is that the name of an interface should start with the letter I. By describing objects in terms of their interfaces however. since the compiler has no opinions on variable names. in this case what a class must do to be considered a bank account. one to pay money in. An interface is placed in a source file just like a class. another to withdraw it and a third which returns the balance on the account. what it is they have to do. we can use anything which behaves like an Account in this position. It is not possible for me to improve the graphics adapter because it is "hard wired" into the system. instead of starting off by designing classes we should instead be thinking about describing their interfaces. Interfaces and Design So. If everything has been hard wired into place this will be impossible. It sets out a number of methods which relate to a particular task or role. The user interface is the way a person using our program would make it work for them. Please don’t be tempted to answer an exam question about the C# interface mechanism with a long description of how windows and buttons work. 4.Creating Solutions From Object to Component Why we Need Software Components? At the moment you might not see a need for software components. Programmer’s Point: Interface names start with I We have seen that there are things called coding conventions. decimal GetBalance ().e. However. it is unfortunately the case that with our bank system we may have a need to create different forms of bank account class. i.8. and then we create those parts.2 Components and Interfaces One point I should make here is that we are not talking about the user interface to our program. C# Programming © Rob Miles 2010 105 . Well. but you may have trouble sleeping at night. It is not obvious at this stage why components are required. we might be asked to create a "BabyAccount" class which only lets the account holder draw out up to ten pounds each time. bool WithdrawFunds ( decimal amount ). Note that at the interface level I am not saying how it should be done. which set down standards for naming variables.
  here are two: C# Programming © Rob Miles 2010 106 .Creating Solutions From Object to Component 4. irrespective of what it really is: public class CustomerAccount : IAccount { private decimal balance = 0. You can think of me in a whole variety of ways. The only difference is the top line: public class CustomerAccount : IAccount { . If the class does not contain a method that the interface needs you will get a compilation error: error CS0535: 'AccountManagement. public bool WithdrawFunds ( decimal amount ) { if ( balance < amount ) { return false . Implementing an interface is a bit like a setting up a contract between the supplier of resources and the consumer. } public decimal GetBalance () { return balance.IAccount.8. it has a corresponding implementation. 4. This means that the class contains concrete versions of all the methods described in the interface.CustomerAccount' does not implement interface member 'AccountManagement. we have now got something which can be regarded in two ways: as a CustomerAccount (because that is what it is) as an IAccount (because that is what it can do) People do this all the time.amount . so that it can be thought of as an account component. } } The code above does not look that different from the previous account class.PayInFunds(decimal)' In this case I missed out the PayInFunds method and the compiler complained accordingly. If a class implements an interface it is saying that for every method described in the interface. The highlighted part of the line above is where the programmer tells the compiler that this class implements the IAccount interface..3 Implementing an Interface in C# Interfaces become interesting when we make a class implement them. } public void PayInFunds ( decimal amount ) { balance = balance + amount .4 References to Interfaces Once we have made the CustomerAccount class compile. } balance = balance . I am going to create a class which implements the interface. In the case of the bank account.. return true.8.
  And you can use the same methods with any other lecturer (i.(the set of account abilities) rather than CustomerAccount (a particular account class). } public decimal GetBalance () { return balance. From the point of view of the university. rather than Rob Miles the individual. return true. It turns out that this is quite easy: IAccount account = new CustomerAccount(). this means that we want to deal with objects in terms of IAccount. and if it does. rather than the particular type that they are. person who implements that interface).amount .e.Creating Solutions From Object to Component Rob Miles the individual (because that is who I am) A university lecturer (because that is what I can do) If you think of me as a lecturer you would be using the interface that contains methods like GiveLecture. So. but they behave slightly differently because we want all withdrawals of over ten pounds to fail: public class BabyAccount : IAccount { private decimal balance = 0. account. contains the required methods). merely a large number of people who can be referred to as having that particular ability or role. The compiler will check to make sure that CustomerAccount does this. In C# terms this means that we need to be able to create reference variables which refer to objects in terms of interfaces they implement. the compilation is successful. I can create a BabyAccount class which implements the IAccount interface. } if (balance < amount) { return false . The account variable is allowed to refer to objects which implement the IAccount interface. and starting to think about them in terms of what they can do. } } C# Programming © Rob Miles 2010 107 .5 Using interfaces Now that we have our system designed with interfaces it is much easier to extend it. It is simply a way that we can refer to something which has that ability (i. In the case of our bank.PayInFunds(50). Note that there will never be an instance of IAccount interface. which has to manage a large number of interchangeable lecturers. it is much more useful for it to think of me as a lecturer. public bool WithdrawFunds ( decimal amount ) { if (amount > 10) { return false . } public void PayInFunds ( decimal amount ) { balance = balance + amount . There is no such physical thing as a "lecturer". 4.e. This is the same in real life. } balance = balance . This implements all the required methods.8. with interfaces we are moving away from considering classes in terms of what they are.
  When we create the account objects we just have ask if a standard account or a baby account is required. what I really want is a way that I can regard an object in terms of its ability to print.. However. Each of these items will be implemented in terms of a component which provides a particular interface (IWarning.7 Designing with Interfaces If you apply the "abstraction" technique properly you should end up with a system creation process which goes along the lines of: gather as much metadata as you can about the problem. public class BabyAccount : IAccount. You should also have spotted that interfaces are good things to hang tests on. Each interface is a new way in which it can be referred to and accessed. for example warning letters.. IPrintToPaper { . The field of Software Engineering is entirely based on this process. There are also graphical tools that you can use to draw formal diagrams to represent this information. The IAccount interface lets me regard a component purely in terms of its ability to behave as a bank account. If you have a set of fundamental behaviours that all bank accounts must have (for example C# Programming © Rob Miles 2010 108 . For example.Creating Solutions From Object to Component The nice thing about this is that as it is a component we don’t have to change all the classes which use it. before you write any code at all.8.8. I don't want to have to provide a print method in each of these. but using the new kind of account in our existing system is very easy. This is actually very easy. This means that a BabyAccount instance behaves like an account and it also contains a DoPrint method which can be used to make it print out. I create the interface: public interface IPrintToPaper { void DoPrint (). 4. I may want to regard a component in a variety of ways. 4. The rest of the system can then pick up this object and use it without caring exactly what it is. ISpecialOffer for example). This would be reasonable if all I ever wanted to print was bank accounts. special offers and the like.. the bank will want the account to be able to print itself out on paper. what is important to the customer. so that we can make sure that withdrawals of more than ten pounds do fail. However. You might think that all I have to do is add a print method to the IAccount interface. } Now anything which implements the IPrintToPaper interface will contain the DoPrint method and can be thought of in terms of its ability to print. there will be lots of things which need to be printed. We will of course have to create some tests especially for it. A class can implement as many interfaces as it needs.6 Implementing Multiple Interfaces A component can implement as many interfaces as are required.
  The design of the interfaces in a system is just this. Each GUID is unique in the world. interface IAccount { int GetAccountNumber (). This is a very important value. in that it will be fixed for the life of the account and can never be changed. No two accounts should ever have the same number. These are data items which are created based on the date. So this requirement ends up being expressed in the interface that is implemented by the account class. We don't care what the method GetAccountNumber actually does. Nothing in C# allows you to enforce a particular behaviour on a method. we sometimes use this to good effect when building a program. The interface mechanism gives us a great deal of flexibility when making our components and fitting them together. For example. the manager has told us that each bank account must have an account number. Inheritance lets a class pick up behaviours from the class which is its parent. In fact. } This method returns the integer which is the account number for this instance. has resulted in the creation of a set of methods in the C# libraries to create things called Globally Unique Identifiers or GUIDs. which really need to be unique in the world. in that we can create "dummy" components which implement the interface but don't have the behaviour as such. It is a way that we can pick up behaviours from classes and just modify the bits we need to make new ones. it just means that a method with that name exists within the class. If a class is descended from a particular parent class this C# Programming © Rob Miles 2010 109 .9 Inheritance Inheritance is another way we can implement creative laziness.Creating Solutions Inheritance paying in money must always make the account balance go up) then you can write tests that bind to the IAccount interface and can be used to test any of the bank components that are created. that is down to how much you trust the programmer that made the class that you are using. Once we have set out an interface for a component we can then just think in terms of what the component must do. and how good your tests are. From the point of view of interface design this means that the account number will be set when the account is created and the account class will provide a method to let us get the value (but there will not be a method to set the account number). It can also be used at the design stage of a program if you have a set of related objects that you wish to create. This is the real detail in the specification. Just because a class has a method called PayInFunds does not mean that it will pay money into the account. but we have not actually described how this should be done. They state that we have a need for behaviours. By placing it in the interface we can say that the account must deliver this value. The need for things like account numbers. It means that once we have found out what our bank account class needs to hold for us we can then go on to consider what we are going to ask the accounts to do. as long as it always returns the value for a particular account. In this respect you can regard it as a mechanism for what is called code reuse. We could use these in our Account constructor to create a GUID which allows each account to have a unique number. and more a promise. Programmer’s Point: Interfaces are just promises An interface is less of a binding contract. I have to add comments to give more detail about what the method does. but they do not necessarily state how they are made to work. time and certain information about the host computer. not precisely how it does it. 4. You can regard an interface as a statement by a class that it has a set of behaviours because it implements a given interface.
  Baby accounts are only allowed to draw up to 10 pounds out at a time.e. It turns out that we can do this in C# using inheritance.1 Extending a parent class We can see an example of a use for inheritance in our bank account project. but not all. and only once. This means that the PayInFunds method from the CustomerAccount class is used at this point. Try not to do this. We have solved this problem from a design point of view by using interfaces. I can now write code like: BabyAccount b = new BabyAccount(). they implement the interface). We can even create brand new accounts at any time after the system has been deployed.IAccount { } The key thing here is the highlighted part after the class name. but not exactly the same.PayInFunds(50). it gets everything from its parent. make it a method. But this does make things a bit tiresome when we write the program. You might think that after such a huge number of years in the job I get everything right every time. instances of the BabyAccount class have abilities which they pick up from their parent class. b. BabyAccount can do. the parent class does. although BabyAccount does not have a PayInFunds method. So. Then I change most. What we really want to do is pick up all the behaviours in the CustomerAccount and then just change the one method that needs to behave differently. In fact. A great programmer writes every piece of code once. We need to create a BabyAccount class which contains a lot of code which is duplicated in the CustomerAccount class.Creating Solutions Inheritance means that it has a set of behaviours because it has inherited them from its parent. Wrong. In short: Interface: "I can do these things because I have told you I can" Inheritance: "I can do these things because my parent can" 4. Customer accounts can draw out as much as they want. I write some code and find that I need something similar. in another part of the program. So I use block copy. And a lot of the mistakes that I make are caused by improper use of block copy. "This is not a problem" you probably think "I can use the editor block copy to move the program text across". the BabyAccount class has no behaviours of its own. of the new code and find that my program doesn't work properly.9. This means that everything that CustomerAccount can do. When I create the BabyAccount class I can tell the compiler that it is based on the CustomerAccount one: public class BabyAccount : CustomerAccount. These can be introduced and work alongside the others because they behave correctly (i. C# Programming © Rob Miles 2010 110 . But: Programmer’s Point: Block Copy is Evil I still make mistakes when I write programs. I have put the name of the class that BabyAccount is extending. This works because. We have already noted that a BabyAccount must behave just like a CustomerAccount except in respect of the cash withdrawal method. By separating the thing that does the job from the description of the job (which is what an interface lets you do) we can get the whole banking system thinking in terms of IAccount and then plug in accounts with different behaviours as required. at the moment. If you need to use it in more than one place.
  This means that code like: BabyAccount b = new BabyAccount(). public class CustomerAccount : IAccount { private decimal balance = 0. b.9. return true. The call of PayInFunds will use the method in the parent (since that has not been overridden) but the call of WithdrawFunds will use the method in BabyAccount. This is called overriding a method. } } The keyword virtual means ―I might want to make another version of this method in a child class‖. The next thing we need to be able to do is change the behaviour of the one method that we are interested in. C# Programming © Rob Miles 2010 111 . you definitely can’t. The C# compiler needs to know if a method is going to be overridden.WithdrawFunds(5). To make the overriding work correctly I have to change my declaration of the method in the CustomerAccount class. You don’t have to override the method. } } The keyword override means "use this version of the method in preference to the one in the parent".PayInFunds(50). This is because it must call an overridden method in a slightly different way from a "normal" one. public virtual bool WithdrawFunds ( decimal amount ) { if ( balance < amount ) { return false . Virtual Methods Actually.Creating Solutions Inheritance 4. but if you don’t have the word present. } if (balance < amount) { return false .amount . In other words. } balance = balance .amount . In the BabyAccount class I can do it like this: public class BabyAccount : CustomerAccount. return true.2 Overriding methods We now know that we can make a new class based on an existing one.IAccount { public override bool WithdrawFunds (decimal amount) { if (amount > 10) { return false . We want to replace the WithdrawFunds method with a new one. the above code won't compile properly because the compiler has not been told that WithDrawFunds might be overridden in classes which are children of the parent class. b. there is one other thing that we need to do in order for the overriding to work. } balance = balance .
  4. Fortunately the designers of C# have thought of this and have provided a way that you can call the base method from one which overrides it..3 Using the base method Remember that programmers are essentially lazy people who try to write code only once for a given problem. We carefully made it private so that methods in other classes can’t get hold of the value and change it directly. You use virtual to mark a method as able to be overridden and override to actually provide a replacement for the method. it looks as if we are breaking our own rules here. in that it stops the BabyAccount class from being able to change the value. in that it means that the balance value has to be made more exposed that we might like. In other words. Well. this protection is too strict. } I’m not terribly happy about doing this. It also has access to all the protected members of the classes above it.9. public class CustomerAccount : IAccount { protected decimal balance = 0. A class hierarchy is a bit like a family tree. This makes the member visible to classes which extend the parent. This calls for more metadata to be gathered from the customer and used to decide which parts of the behaviour need to be changed during the lift of the project.. We have already noted that we don’t like this much. for now making this change will make the program work. And we would have written this down in the specification. Of course this should be planned and managed at the design stage. This is because the balance value in the CustomerAccount class is private. I can use this to make the WithDrawFunds method in my BabyAccount much simpler: C# Programming © Rob Miles 2010 112 . Bank Notes: Overriding for Fun and Profit The ability to override a method is very powerful. However. methods in the BabyAccount class can see and use a protected member because they are in the same class hierarchy as the class containing the member. in that the WithDrawFunds method in the BabyAccount class contains all the code of the method in the parent class. Protection of data in class hierarchies It turns out that the code above still won’t work. Every class has a parent and can do all the things that the parent can do. the balance is very important to me and I’d rather that nobody outside the CustomerAccount class could see it. Later we will see better ways to manage this situation. It means that we can make more general classes (for example the CustomerAccount) and customise it to make them more specific (for example the BabyAccount)... We would have made the WithDrawFunds method virtual because the manager would have said ―We like to be able to customise the way that some accounts withdraw funds‖. To get around this problem C# provides a slightly less restrictive access level called protected. .Creating Solutions Inheritance This makes override and virtual a kind of matched pair. The word base in this context means ―a reference to the thing which has been overridden‖. However.
  This is because in this situation there is no overriding. and then it is fixed for all the classes which call back to it. If I leave it out (and leave out the override too) the program seems to work fine. This makes it more difficult to pick up behaviours from parent classes. It is important that you understand what I’m doing here. } } The problem with this way of working is that you are unable to use base. C# Programming © Rob Miles 2010 113 . } return base. the one that the method overrides. in the top level class. By making this change I can put the balance back to private in the CustomerAccount because it is not changed outside it. If you play around with C# you will find out that you don’t actually seem to need the virtual keyword to override a method.e.WithdrawFunds(amount). IAccount { public override bool WithdrawFunds (decimal amount) { if (amount > 10) { return false . The use of the word base to call the overridden method solves both of these problems rather beautifully.4 Making a Replacement Method This bit is rather painful.amount . but don’t worry too much since it actually does make sense when you think about it. and why I’m doing it: I don’t want to have to write the same code twice I don’t want to make the balance value visible outside the CustomerAccount class. return true. i.Creating Solutions Inheritance public class BabyAccount : CustomerAccount. If I need to fix a bug in the behaviour of the WithDrawFunds method I just fix it once.IAccount { public new bool WithdrawFunds (decimal amount) { if (amount > 10) { return false . } } The very last line of the WithDrawFunds method makes a call to the original WithDrawFunds method in the parent class. Note that there are other useful spin-offs here. } balance = balance . 4. you have just supplied a new version of the method (in fact the C# compiler will give you a warning which indicates that you should provide the keyword new to indicate this): public class BabyAccount : CustomerAccount. } if (balance < amount) { return false .9. Because the method call returns a bool result I can just send whatever it delivers.
  This goes well with a design process which means that as you move down the ―family tree‖ of classes you get more and more specific. the customer will not have particularly strong opinions on how you use things like sealed in your programs.. In fact. It means that a programmer can just change one tiny part of a class and make a new one with all the behaviours of the parent. 4. If you want to have a policy of allowing programmers to make custom versions of classes in this way it is much more sensible to make use of overriding since this allows a well-managed way of using the method that you over-rid.9.5 Stopping Overriding Overriding is very powerful. For a programming course at this level it is probably a bit heavy handed of me to labour this point just right now. No matter how much cash is drawn out.Creating Solutions Inheritance Programmer’s Point: Don’t Replace Methods I am very against replacing methods rather than overriding them.e.. Bank Notes: Protect Your Code As far as the bank application is concerned. it always returns a balance value of a million pounds! A naughty programmer could insert this into a class and give himself a nice spending spree. One of the unfortunate things about this business is that you will have to allow for the fact that people who use your components might not all be nice and trustworthy.IAccount { . What this means is that we need a way to mark some methods as not being able to be overridden. This means that the class cannot be extended.. } This is the banking equivalent of the bottle of beer that is never empty. C# does this by giving us a sealed keyword which means ―You can’t override this method any more‖. I’m wondering why I mentioned this at all. public sealed class BabyAccount : CustomerAccount. However. it cannot be used as the basis for another class. This is never going to need a replacement. The rules are that you can only seal an overriding method (which means that we can’t seal the GetBalance virtual method in the CustomerAccount class) and you can still replace a sealed method.. is that you can mark a class as sealed. Consider the GetBalance method. This means that you should take steps when you design the program to decide whether or not methods should be flagged as virtual and also make sure that you seal things when you can do so. i. Unfortunately this is rather hard to use. C# Programming © Rob Miles 2010 114 . which has a bit more potential. overriding/replacing is not always desirable. And yet a naughty programmer could write their own and override or replace the one in the parent: public new decimal GetBalance () { return 1000000. } The compiler will now stop the BabyAccount from being used as the basis of another account. just remember that when you create a program this is another risk that you will have to consider. Another use for sealed. But they will want to have confidence in the code that you make. and if it didn’t all make sense there is no particular need to worry..
  I can use it to force a set of behaviours on items in a class hierarchy. This C# Programming © Rob Miles 2010 115 . balance = inBalance. They must make sure that at each level in the creation process a constructor is called to set up the class at that level. I think of these things as a bit like the girders that you erect to hold the floors and roof of a large building. The result of this is that programmers must take care of the issue of constructor chaining.9. the first a string and the second a decimal value. Constructor Chaining When considering constructors and class hierarchies you must therefore remember that to create an instance of a child class an instance of the parent must first be created. In other words. "Hull"). the proper version of the customer account constructor is as follows: public CustomerAccount (string inName. The keyword base is used to make a call to the parent constructor.6 Constructors and Hierarchies A constructor is a method which gets control during the process of object creation. to create a CustomerAccount you must first create an Account. It is used by a programmer to allow initial values to be set into an object: robsAccount = new CustomerAccount("Rob Miles". In this situation the constructor in the child class will have to call a particular constructor in the parent to set that up before it is created.Creating Solutions Inheritance 4. 4.9. However. inBalance) { } The base keyword is used in the same way as this is used to call another constructor in the same class. decimal inBalance) { name = inName. This means that a constructor in the parent must run before the constructor in the child. For example. It is of course very important that you have these designs written down and readily available to the development team. It is part of the overall architecture of the system that you are building. The code above will only work if the CustomerAccount class has a constructor which accepts two strings. In other words. it is also possible to use overriding in a slightly different context. In other words. They tell programmers who are going to build the components which are going to implement the solution how to create those components. You might think that I could solve this by writing a constructor a bit like this: public CustomerAccount (string inName. the name and the address of the new customer.7 Abstract methods and classes At the moment we are using overriding to modify the behaviour of an existing parent method. in the context of the bank application we might want to provide a method which creates a warning letter to the customer that their account is overdrawn. Programmer’s Point: Design your class construction process The means by which your class instances are created is something you should design into the system that you build. decimal inBalance) : base ( inName. The constructor above assumes that the Account class which CustomerAccount is a child of has a constructor which accepts two parameters. } But this class is an extension of the Account class. And the account is the class which will have a constructor which sets the name and the initial balance. to make a CustomerAccount I have to make an Account. If there are some things that an account must do then we can make these abstract and then get the child classes to actually provide the implementation.
  This can be useful because it means you don’t have to repeatedly implement the same methods in each of the components that implement a particular interface. The methods in the second category must be made abstract. This is true.. so you can only pick up the behaviours of one class. It is not possible to make an instance of an abstract class. This means that at the time we create the bank account system we know that we need this method. We could just provide a ―standard‖ method in the CustomerAccount class and then rely on the programmers overriding this with a more specific message but we then have no way of making sure that they really do provide the method. If you want to make an instance of a class based on an abstract parent you must provide implementations of all the abstract methods given in the parent. This leads us to a class design a bit like this: C# Programming © Rob Miles 2010 116 . you may have to repeat methods as well. The problem is that you can only inherit from one parent. abstract classes are different in that they can contain fully implemented methods alongside the abstract ones. This means that the method body is not provided in this class. An instance of Account would not know what to do it the RudeLetterString method was ever called. but we don’t know precisely what it does in every situation. Abstract classes and interfaces You might decide that an abstract class looks a lot like an interface. An abstract class can be thought of as a kind of template.. in that an interface also provides a ―shopping list‖ of methods which must be provided by a class. } The fact that my new Account class contains an abstract method means that the class itself is abstract (and must be marked as such). C# provides a way of flagging a method as abstract. If you want to implement interfaces as well. If you think about it this is sensible. Perhaps at this point a more fully worked example might help.
  public abstract string RudeLetterString(). } public virtual decimal GetBalance () { return balance. decimal GetBalance (). public virtual bool WithdrawFunds ( decimal amount ) { if ( balance < amount ) { return false . } balance = balance . } } public class BabyAccount : Account { public override bool WithdrawFunds ( decimal amount ) { if (amount > 10) { return false .Creating Solutions Inheritance public interface IAccount { void PayInFunds ( decimal amount ). string RudeLetterString(). } public override string RudeLetterString() { return "Tell daddy you are overdrawn". } public abstract class Account : IAccount { private decimal balance = 0. } public void PayInFunds ( decimal amount ) { balance = balance + amount . } } C# Programming © Rob Miles 2010 117 .WithdrawFunds(amount). bool WithdrawFunds ( decimal amount ). } return base. } } public class CustomerAccount : Account { public override string RudeLetterString() { return "You are overdrawn" .amount . return true.
  for example all the different kinds of bank account you might need to deal with. This might seem useful. 4. current account etc. However.8 Designing with Objects and Components For the purpose of this part of the text you now have broad knowledge of all the tools that can be used to design large software systems. Then I have added customised methods into the child classes where appropriate. methods) which a component can be made to implement.9. Note how I have moved all the things that all accounts must do into the parent Account class. A concrete example of this would be something like IPrintHardCopy. If their accounts are software components too (and they should be) then all we have to do is implement the required interfaces at each end and then our systems understand each other. as we can consider something as an ―account‖ rather than a BabyAccount. then the best way to do this is to set up a parent class which contains abstract and non-abstract methods. Use Interface References One important consideration is that even if you make use of an abstract parent class I reckon that you should still make use of interfaces to reference the data objects themselves. get their account objects (whatever they are called) to implement the interface and. This gives you a degree of flexibility that you can use to good effect. hey presto. Interfaces let me describe a set of behaviours which a component can implement. allowing them to present different faces to the systems that use them. The child classes can make use of the methods from the parent and override the ones that need to be provided differently for that particular class. Abstract: lets you create a parent class which holds template information for all the classes which extend it. Broadly: Interface: lets you identify a set of behaviours (i. deposit account. If you want to create a related set of items. References to abstract classes References to abstract classes work just like references to interfaces. That is because. even though I now have this abstract structure I still want to think of the account objects in terms of their ―accountness‖ rather than any particular specific type. including credit card. Bank Notes: Making good use of interface and abstract If our bank takes over another bank and wants to share account information we might need a way to use their accounts. In other words the other bank must create the methods in the IAccount interface. This would be much more difficult if my entire system thought in terms of a parent Account class – since their classes would not fit into this hierarchy at all. Objects can implement more than one interface. I much prefer it if you manage references to abstract things (like accounts) in terms of their interface instead.Creating Solutions Inheritance This code repays careful study. Then our printer can just regard each of the instances that implement this interface purely in this way. If you have an understanding of what an interface and abstract classes are intended to achieve this will stand you in very good stead for your programming career. A reference to an Account class can refer to any class which extends from that parent.e. I can now use their accounts. Lots of items in my bank system will need to do this and so we could put the behaviour details into the interface for them to implement in their own way. Once a component can implement an interface it can be regarded purely in terms of a component with this ability. C# Programming © Rob Miles 2010 118 . Note also though that I have left the interface in place. Any component which implements the interface can be thought of in terms of a reference of that interface type.
  9. issues that relate to how we use objects in our programs. If I write the code: int i = 99.this is equivalent to writing: public class Account : object { The object class is a part of C#. In other words. It is in fact a child of the object class. This has a couple of important ramifications: Every object can do what an object can do. (in fact we have seen that the whole basis of building programs is to decide what the objects should do and then make them do these things). in that we know how they can be used to design and implement large software systems.9 Don’t Panic This is all deep stuff. Now it is time to find out how this is achieved.Creating Solutions Object Etiquette 4. We have also seen that you can extend a parent object to create a child which has all the abilities of the parent. Interfaces let me describe what each component can do.10 Object Etiquette We have considered objects "in the large".1 Objects and ToString We have taken it as read that objects have a magical ability to print themselves out. The important point to bear in mind is that the features are all provided so that you can solve one problem: Create software which is packaged in secure. 4.10. These features of C# are tied up with the process of software design which is a very complex business. What we need to do now is take a look at some smaller. Now we are going to see how these abilities of objects are used in making parts of the C# implementation itself work. interchangeable components. and also how we can give our own objects the same magical ability. don’t worry. C# Programming © Rob Miles 2010 119 . It turns out that this is all provided by the "objectness" of things in C#. We know that an object can contain information and do things for us. The Object class When you create a new class this is not actually created from nowhere. This will print out: 99 The integer somehow seems to know how to print itself out. if I write: public class Account { . A reference to an object type can refer to any class. and everything is a child of the object class. plus the new ones that we add. Console. but very important. 4. If you don’t get it now. And that is it. Class hierarchies let me re-use code inside those components.WriteLine(i).
  the object.WriteLine(o. } C# Programming © Rob Miles 2010 120 . . 25). whenever you design a class you should provide a ToString method which provides a text version of the content of that class. public override string ToString() { return "Name: " + name + " balance: " + balance.WriteLine(a).would print out: Name: Rob balance: 25 So. } } In the tiny Account class above I've overridden the ToString method so that it prints out the name and balance value. the first point is the one which has the most importance here. Console.WriteLine(o).would print out: System. . This means that we can override it to make it behave how we would like: class Account { private string name. The ToString method The system knows that ToString exists for every object. Console. In other words: object o = new object(). and so if it ever needs the string version of an object it will call this method on the object to get the text.Creating Solutions Object Etiquette For the purpose of this part of the notes. you can use the base mechanism to do this. decimal inBalance) { name = inName. This means that the code: Account a = new Account("Rob". Getting the string description of a parent object If you want to get hold of a string description of the parent object. If you look inside the actual code that makes an object work you will find a method called ToString. The nice thing about ToString is that it has been declared virtual. from the point of view of good etiquette.and the results would be exactly the same. } public Account (string inName. private decimal balance. The object implementation of ToString returns a string description of the type of that object.ToString() + " Parent : " + parentName. .ToString()). This is sometimes useful if you add some data to a child class and want to print out the content of the parent first: public override string ToString() { return base.Object You can call the method explicitly if you like: Console. It means that all classes that are made have a number of behaviours that they inherit from their ultimate parent. balance = inBalance.
  y = 2. We know that objects are managed by named tags which refer to unnamed items held in memory somewhere.Creating Solutions Object Etiquette The method above is from a ChildAccount class. which means that the equals test will fail. 4. This is because I'm not that concerned about protecting them. The nice thing about this is that if the behaviour of the parent class changes. We can express this position as a coordinate or point. } This is my point class. Even though I have put the spaceship and the missile at the same place on the screen the word Bang is not printed.y = 2. with an x value and a y value. We then might want to test to see if two items have collided. I can now create instances of Point and use them to manage my game objects: Point spaceshipPosition = new Point(). spaceshipPosition.e. The code above uses the ToString method in the parent object and then tacks the name of the parent on the end before returning it. To do this we must override the standard Equals behaviour and add one of our own: C# Programming © Rob Miles 2010 121 . missilePosition.x = 1. take a look at the test that is being performed. the ChildAccount class does not have to change. This class extends the CustomerAccont and holds the name of the child's parent. i. class Point { public int x. When we perform an equals test the system simply checks to see if both references refer to the same location.2 Objects and testing for equals We have seen that when object references are compared a test for equals does not mean the same as it does for values. Programmer’s Point: Make sure you use the right equals As a programmer I must remember that when I'm comparing objects to see if they contain the same data I need to use the Equals method rather than the == operator. spaceshipPosition. they are not located at the same address in memory. This is because although the two Point objects hold the same data. since it will just make use of the upgraded method. In the game the various objects in it will be located at a particular place on the screen. If you find that things that contain the same data are not being compared correctly.WriteLine("Bang"). if ( spaceshipPosition == missilePosition ) { Console.x = 1. Point missilePosition = new Point().10. new members are added or the format of the string changes. To see how this might cause us problems we need to consider how we would implement a graphical computer game (makes a change from the bank for a while). public int y. Some of the nastiest bugs that I've had to fix have revolved around programmers who have forgotten which test to use. but I do want my program to run quickly. missilePosition. The problem is that the above program code does not work. Adding your own Equals method The way that you solve this problem is to provide a method which can be used to compare the two points and see if they refer to the same place. } Note that I've made the x and y members of the Point class public.
  there is a very good reason why you might find an equals behaviour very useful. If the bank ever contains two identical accounts this would cause serious problems. If they are both the same the method returns true to the caller. However. why should I waste time writing code to compare them in this way?” However. if ( ( p. Bank Notes: Good Manners are a Good Idea It is considered good manners to provide the Equals and ToString methods in the classes that you create. This means that I can write code like: if ( missilePosition. It is also very useful to make appropriate use of this when writing methods in classes. We need to do this because we want to get hold of the x and y values from a Point (we can't get them from an object). Note that this reference is supplied as a reference to an object. You will expect all programmers to write to these standards if they take part in the project.WriteLine("Bang").. The object is cast into a Point and then the x and y values are compared. In fact.x == x ) && ( p. When you start to create your bank account management system you should arrange a meeting of all the programmers involved and make them aware that you will be insisting on good mannered development like this. C# Programming © Rob Miles 2010 122 . for a professional approach you should set out standards which establish which of these methods you are going to provide. Programmer’s Point: Equals behaviours are important for testing The customer for our bank account management program is very keen that every single bank account that is created is unique. These will allow your classes to fit in with others in the C# system. } } The Equals method is given a reference to the thing to be compared. } else { return false. the Equals method is sometimes used by C# library methods to see if two objects contain the same data and so overriding Equals makes my class behave correctly as far as they are concerned.Creating Solutions Object Etiquette public override bool Equals(object obj) { Point p = (Point) obj. The first thing we need to do is create a reference to a Point. The fact that everything in the system is required to be unique might lead you to think that there is no need to provide a way to compare two Account instances for equality. In this situation an equals behaviour is important. Note that I didn't actually need to override the Equals method. } This test will work. and so I reckon you should always provide one.y == y ) ) { return true. because the Equals method actually compares the content of the two points rather than just their references.Equals(spaceshipPosition) ) { Console. I could have written one called TheSame which did the job.
  Each time the Count method is called the counter is made one larger. then don't worry about it for now.Creating Solutions Object Etiquette 4. In other words. } } We can add a this. Consider: public class Counter { public int Data=0. I can use the class as follows: Counter c = new Counter(). (dot) means "follow the reference to the object and then use this member of the class". If I have a member of my class which contains methods. Perhaps the best way to get around the problem is to remember that when I use the word this I mean "a reference to the currently executing instance of a class". the "proper" version of the Counter class is as follows: public class Counter { public int Data=0. Console. this as a reference to the current instance I hate explaining this. Confusion with this I reckon that using this is a good idea. in front of each use of a member of the class. I end up writing code like this: C# Programming © Rob Miles 2010 123 . public void Count () { this.Count(). in this situation we are not passing an account into the bank.Data + 1. A new bank account could add store itself in a bank by passing a reference to itself to a method that will store it: bank.WriteLine("Count : " + c. If this hurts your head. Passing a reference to yourself to other classes Another use for this is when an instance class needs to provide a reference to itself to another class that wants to use it.Data = this.Store(this). Of course. instead we are passing a reference to the account. The data is a counter. When a method in a class accesses a member variable the compiler automatically puts a this. When the Store method is called it is given a reference to the currently executing account instance. and I want to use one of those methods.Data). This calls the method and then prints out the data. if we like to make it explicit that we are using a member of a class rather than a local variable. public void Count () { Data = Data + 1. The Store method accepts this and then stores this somehow.3 Objects and this By now you should be used to the idea that we can use a reference to an object to get hold of the members of that object. } } The class above has a single data member and a single method member. However it might take a bit of getting used to. We know that in this context the . I want to use the word this to mean this. but this also has a special meaning within your C# programs.10. It means that people reading my code can tell instantly whether I am using a local variable or a member of the class. c.
  4. You can sort out the case of the text. never allowing a thing to be changed by making a new one each time it is required. You might ask the question: "Why go to all the trouble?" It might seem all this hassle could be saved by just making strings into value types. It is very important that you understand what happens when you transform a string. Consider the situation where you are storing a large document in the memory of the computer. trim spaces off the start and end and extract sub-strings using these methods. So. This saves memory and it also makes searching for words much faster.Creating Solutions The power of strings and chars this. take a look at section 4. A string can be regarded as either an object (referred to by means of a reference) or a value (referred to as a value). The thing that s1 is referring to is unchanged. C# Programming © Rob Miles 2010 124 . The methods will return a new string. because C# regards an instance of a string type in a special way. 4. 4. This hybrid behaviour is provided because it makes things easier for us programmers. In other words. However it does mean that we have to regard strings as a bit special when you are learning to program. It calls it immutable. If you think you know about objects and references.1. after the assignment s1 and s2 no longer refer to the same object. This is because. To find out more about this.WriteLine(s1 + " " + s2). This is because it gives you a lot of nice extra features which will save you a lot of work. This is because programmers want strings to behave a bit like values in this respect. when the system sees the line: s2 = "different". although strings are objects. . is how the system implements immutable. These are exposed as methods which you can call on a string reference.2 Immutable strings The idea is that if you try to change a string.11. no. s2 = "different". string s2=s1. A bat can be regarded as either an animal or a bird in many respects.SetName("Rob"). This behaviour. string s1="Rob".4. This means "I have a member variable in this class called account. so changing s2 should change the object that s1 refers to as well.it makes a new string which contains the text "different" and makes s2 refer to that.account. you should be expecting s1 to change when s2 is changed.11 The power of strings and chars It is probably worth spending a few moments considering the string type in a bit more detail. Well. transformed in the way that you ask. By using references we only actually need one string instance with the word ―the‖ in it. they don't actually behave like objects all the time. This does not happen though.11. I regard them as a bit like bats. the C# system instead creates a new string and makes the reference you are "changing" refer to the changed one. Call the SetName method on that member to set the name to 'Rob'". The second statement made the reference s2 refer to the same object as s1. Console. All the occurrences in the text can just refer to that one instance.1 String Manipulation Strings are rather special. because they seem to break some of the rules that we have learnt.
  Substring(2).Length). } If s1 and s2 were proper reference types this comparison would only work if they referred to the same object.WriteLine ( "Length: " + s1. .WriteLine("Still the same"). s1=s1. In this respect strings are just like arrays. The Length property gives the number of characters in the string.Creating Solutions The power of strings and chars 4.5 String Length All of the above operations will fail if you try to do something which takes you beyond the length of the string. . 4. You can leave out the second parameter if you like. You do this by calling methods on the reference to get the result that you want: s1=s1.6 Character case Objects of type string can be asked to create new.11.11. you can't change the characters: name[0] = 'R'. in which case all the characters up to the end of the string are copied: string s1="Miles".3 String Comparison The special nature of strings also means that you can compare strings using equals and get the behaviour you want: if ( s1 == s2 ) { Console.Equals(s2) ) { Console.11. modified. versions of themselves in slightly different forms. But in C# the comparison works if they contain the same text.would cause a compilation error because strings are immutable. You can pull a sequence of characters out of a string using the SubString method: string s1="Rob".would leave "les" in s1. C# Programming © Rob Miles 2010 125 .11.WriteLine("The same").(remember that strings are indexed starting at location 0).4 String Editing You can read individual characters from a string by indexing them as you would an array: char firstCh = name[0]. You can use an equals method if you prefer: if ( s1.Substring(1. However. } 4. 4. s1=s1. It is also possible to use the same methods on them to find out how long they are: Console.2).ToUpper(). This would set the character variable firstCh to the first character in the string. This would leave the string "ob" in s1. The first parameter is the starting position and the second is the number of characters to be copied.
  The reason you are suffering is that strings are really design to hold strings. They are a bit like the switch keyword.IsDigit(ch) char. tab or newline You can use these when you are looking through a string for a particular character.IsLower(ch) char.Trim(). You should look this up if you have a need to do some proper editing. It works a lot like a string.11. which lets us select things easily. but C# provides them because they make programs slightly easier to write and simpler to read. These are static methods which are called on the character class and can be used to test characters in a variety of ways: char.IsUpper(ch) char. s1=s1. We don't need properties.7 Trimming and empty strings Another useful method is Trim. If you don't trim the text you will find equals tests for the name will fail. 4.11.9 String Twiddling with StringBuilder If you really want to twiddle with the contents of your strings you will find the fact that you can't assign to individual characters in the string a real pain. No other characters in the string are changed.11.Creating Solutions Properties The ToUpper method returns a version of the string with all the letters converted to UPPER CASE. 4. This is useful if your users might have typed " Rob " rather than "Rob". If you trim a string which contains only spaces you will end up with a string which contains no characters (i. but you can assign to characters and even replace one string with another.Text namespace.12 Properties Properties are useful.8 Character Commands The char class also exposes some very useful methods which can be used to check the values of individual characters. its length is zero). This removes any leading or trailing spaces from the string.IsLetter(ch) char. For proper string editing C# provides a class called StringBuilder. There are TrimStart and TrimEnd methods to just take off leading or trailing spaces if that is what you want.IsPunctuation(ch) char. C# Programming © Rob Miles 2010 126 . 4. It is also very easy to convert between StringBuilder instances and strings. There is a corresponding ToLower method as well. 4.IsLetterOrDigit(ch) char.e. not provide a way that they can be edited. It is found in the System.
  4.2 Creating Get and Set methods To get control and do useful things we can create get and set methods which are public.GetAge() ). but we need to make the member value public. public int GetAge() { return this.WriteLine ( "Age is : " + s. The problem is that we have already decided that this is a bad way to manage our objects. the bank may ask us to keep track of staff members. Console. An age property for the StaffMember class would be created as follows: C# Programming © Rob Miles 2010 127 . } } } We now have complete control over our property. } public void SetAge( int inAge ) { if ( (inAge > 0) && (inAge < 120) ) { this. but we have had to write lots of extra code. This is very naughty.age. s.12.12.3 Using Properties Properties are a way of making the management of data like this slightly easier.Age = -100210232. I can do it like this: public class StaffMember { public int Age. For example. s.age = inAge. We have seen that we can use a member variable to do this kind of thing. These provide access to the member in a managed way. just by giving the name of the member. There is nothing to stop things like: s. One of the items that they may want to hold is the age of a member of staff.12. } The class contains a public member.Creating Solutions Properties 4.1 Properties as class members A property is a member of a class that holds a value. We then make the Age member private and nobody can tamper with it: public class StaffMember { private int age.SetAge(21). I can access a public member of a class directly. 4. I can get hold of this member in the usual way: StaffMember s = new StaffMember().Age = 21. Programmer’s who want to work with the age value now have to call methods: StaffMember s = new StaffMember(). but because the Age member is public we cannot stop it.
  } } } The age value has now been created as a property. Console. They are packaged as a list of methods which a class must contain if it implements the interfaces. it returns the age in months. I can do other clever things too: public int AgeInMonths { get { return this.Age ). s. The really nice thing about properties is that they are used just as the class member was: StaffMember s = new StaffMember().ageValue*12. called AgeInMonths. This gives us all the advantages of the methods.Creating Solutions Properties public class StaffMember { private int ageValue. } } This is a new property. 4. since it does not provide a set behaviour. You do this by leaving out the statements which are the body of the get and set behaviours: interface IStaff { int Age { get.Age = 21. These equate directly to the bodies of the get and set methods that I wrote earlier.4 Properties and interfaces Interfaces are a way that you can bring together a set of behaviours. Write only properties are also possible if you leave out the get.12. When the Age property is given a value the set code is run. However.ageValue = value. public int Age { set { if ( (value > 0) && (value < 120) ) { this.ageValue. The keyword value means ―the thing that is being assigned‖. You can also provide read-only properties by leaving out the set behaviour. This means that you can provide several different ways of recovering the same value. It is also possible to add properties to interfaces. It can only be read. When the Age property is being read the get code is run. but they are much easier to use and create. } } C# Programming © Rob Miles 2010 128 . } } get { return this.WriteLine ( "Age is : " + s. Note how there are get and set parts to the property. set. based on the same source value as was used by the other property.
  but it has no way of telling the user of the property that this has happened. A SetAge method on the other hand could return a value which indicates whether or not it worked: public bool SetAge( int inAge ) { if ( (inAge > 0) && (inAge < 120) ) { this. } C# Programming © Rob Miles 2010 129 . which is not a good way to go on. I suppose that it would be possible to combine a set method and a get property. This is OK. If there is a situation where a property assignment can fail I never expose that as a property.Creating Solutions Building a Bank This is an interface that specifies that classes which implement it must contain an Age property with both get and set behaviours. Properties Run Code When you assign a value to a property you are actually calling a method. But there are a few things that you need to be aware of when deciding whether or not to use them: Property Assignment Failure We have seen that the set behaviour can reject values if it thinks they are out of range. But it can also be made very confusing. What we now need is a way of storing a large number of accounts. This looks very innocent.13 Building a Bank We are now in a situation where we can create working bank accounts. but could result in a thousand lines of code running inside the set property. and they are used throughout the classes in the . This container class will provide methods which allow us to find a particular account based on the name of the holder. 4. With our Age property above the code: s.age = inAge. the person performing the assignment would not be aware of this.NET libraries. since there is no way that the property can return a value which indicates success or failure. However. We can express the behaviour that we need from our bank in terms of an interface: interface IBank { IAccount FindAccount (string name). Unfortunately there is no way you can tell that from the code which does the assignment: s. This gives us a way of creating new types of account as the bank business develops.Age = 121. . 4. It is possible to use this to advantage. We can use interfaces to define the behaviour of a bank account component. } A programmer setting the age value can now find out if the set worked or failed.would fail. but I reckon that would be madness. in that an object can react to and track property changes. bool StoreAccount (IAccount account). return true.Age = 99.12.5 Property problems Properties are really neat. The only way that a property set method could do this would be to throw and exception. } return false.
  they are all set to null. We never place an account "in" the array. The method to add an account to our bank has to find the first empty location in the array and set this to refer to the account that has been added: C# Programming © Rob Miles 2010 130 .Creating Solutions Building a Bank A class which implements these methods can be used for the storage of accounts. What we have created is an array of references. The light coloured elements are set to null. The element at the start of the array contains a reference to the account instance.1 Storing Accounts in an array The class ArrayBank is a bank account storage system which works using arrays. public ArrayBank( int bankSize ) { accounts = new IAccount[bankSize]. We have not created any accounts at all. When we add an account to the bank we simply make one of the references refer to that account instance. } Note that it is very important that you understand what has happened here. accounts Name : "Rob" Balance : 0 The diagram shows the state of the accounts array after we have stored our first account. instead we put a reference to that account in the array. printing out a message if the storage worked. IAccount account = new CustomerAccount("Rob". Each reference in the array can refer to an object which implements the IAccount interface. } The code above creates a bank and then puts an account into it.WriteLine ( "Account stored OK" ). if (friendlyBank. I can put an account into it and then get it back again: IBank friendlyBank = new ArrayBank (50). In the code above we are creating a bank with enough room for references to 50 accounts. But at the moment none of the references refer anywhere. 0). When you create an instance of an ArrayBank you tell it how many accounts you want to store by means of the constructor.13. The constructor creates an array of the appropriate size when it is called: private IAccount [] accounts . 4.StoreAccount(account)) { Console.
  } } return false.2 Searching and Performance The solution above will work fine. Each time we add a new account the searching gets slower as the FindAccount method must look through more and more items to find that one. If it finds one it sets this to refer to the account it has been asked to store and then returns true.13. On a bank with only fifty accounts this is not a problem. for (position=0 . C# Programming © Rob Miles 2010 131 . In the array based implementation of the bank this is achieved by means of a simple search: public IAccount FindAccount ( string name ) { int position=0 . If it finds an element which contains a null reference it skips past that onto the next one.GetName() == name ) { return accounts[position]. position++) { if (accounts[position] == null) { accounts[position] = account. otherwise it returns a reference to the account that it found. 4. } This code works its way through the accounts array looking for an entry with a name which matches the one being looked for. If it does not find a null element before it reaches the end of the array it returns false to indicate that the store has failed. position<accounts. it becomes very slow as the size of the bank increases.Length . 4. but if there are thousands of accounts this simple search will much to slow.13. } if ( accounts[position]. When we want to work on an account we must first find it.FindAccount("Rob"). for (position = 0. If it reaches the end of the array without finding a match it returns null. This will either return the account with the required name. } } return null. position++) { if ( accounts[position] == null ) { continue.3 Storing Accounts using a Hash Table Fortunately for us we can use a device called a "hash table" which allows us to easily find items based on a key This is much faster than a sequential search.Length. position<accounts. since it uses a technique which will take us straight to the required item. The bank interface provides a method called FindAccount which will find the account which matches a particular customer name: IAccount fetchedAccount = arrayBank. However.Creating Solutions Building a Bank public bool StoreAccount (IAccount account) { int position = 0. and could be used as the basis of a bank. } This method works through the array looking for an element containing null. return true. or null if the account cannot be found.
  C# Programming © Rob Miles 2010 132 .Add(account. look up the ASCII code for each letter and then add these values up. When we want to find a given item we use the hash to take us to the starting position for the search and then look for the one with the matching name. We want to return a reference to an instance which implements the IAccount interface. rather than always looking from the beginning of the array in our simple array based code above. we could take all the letters in the account name string. } } The Hashtable class can be found in the System. In short. It also allows you to use a reference to the key (in this case the name) to locate an item: return bankHashtable[name] as IAccount. The as operator has the advantage that if bankHashtable[name] does not return an account.GetName(). The as operator is a form of casting (where we force the compiler to regard an item as being of a particular type). return true. 4. We can do clever things with the way we combine the values to reduce the chances of this happening. the as operator will generate a null reference.4 Using the C# Hashtable collection Fortunately for us the designers of the C# have created a hash table class for us to use. at run time the bank hash table returns the wrong thing) our program will fail with an exception. Clashes can be resolved by adding a test when we are storing an account. If a cast fails (i. the hash function gives us a starting point for our search. or returns something of the wrong type. the name "Rpa" would give the same total (82+112+97) and refer to the same location.e. but we can't completely avoid clashes.Creating Solutions Building a Bank The idea is that we do something mathematical (called "hashing) to the information in the search property to generate a number which can then be used to identify the location where the data is stored. This is poetically referred to as a "hash clash". The name "Rob" could be converted to 82+111+98 = 291. If we find the location we would like to use is not null we simply work our way down looking for the first free location on from that point. Since this is just what the caller of our FindAccount method is expecting this can be returned back directly. } public bool StoreAccount(IAccount account) { bankHashtable. This will store items for us based on a particular object which is called the key. account). Of course this hash code is not foolproof. It provides a method called Add which is given the value of the key and a reference to the item to be stored on that hash code.Collections namespace. public IAccount FindAccount(string name) { return bankHashtable[name] as IAccount. For example. It turns out to be very easy to create a bank storage mechanism based on this: class HashBank : IBank { Hashtable bankHashtable = new Hashtable(). It is used in preference to the code: return (IAccount) bankHashtable[name].13. We could look in location 291 for our account. We have to use the "as" part of this coded as the collection will return a reference to an object. This greatly speeds up access to the data.
  which is the name of the account holder. and there may well be more than one. When gathering metadata about a system you will often need to consider which of the properties of an item will be key fields. In a real bank the key will be more complex. C# Programming © Rob Miles 2010 133 . Bank Notes: Key properties are important The action that is being performed here is exactly the same as what happens when you use your cash card to draw some money from the bank. In our simple bank we only have a single key. so that they can search for an account holder by name. address or account number depending on the information they have available. we can very easily change the way that the account storage part of the program works without changing anything else. The cash machine uses information on the card as a key to find your account record so that it can check your balance value and then update it when the money has been withdrawn.Creating Solutions Building a Bank It is interesting to note that because we have implemented our bank behaviour using an interface.
  because the array size was set at 10. C# Programming © Rob Miles 2010 134 . I think the root of Generics is probably "general". It lets us create something very useful. in that the idea of them is that you specify a general purpose operation and then apply it in different contexts in a way appropriate to each of them. at least amongst those who can't spell very well. although there are overloaded constructors that let you give this information and help the library code along a bit: ArrayList storeFifty = new ArrayList(50). Fortunately the C# libraries provide a number of solutions. They sound a bit scary. Creating an ArrayList It is very easy to create an ArrayList: ArrayList store = new ArrayList().001st customer to the bank is not actually a cause for celebration. but we aren't that keen on that because it means that for smaller banks the program might be wasting a lot of memory. but it can hold more or less as required.Advanced Programming Generics and Collections 5 Advanced Programming 5.1. in that it lives in the same Systems.Add(robsAccount). telling people you learned about "generics" probably conjures up images of people with white coats and test tubes. Perhaps the best way to talk about generics is to see how they can help us solve a problem for the bank. If it doesn't. not the thing itself. then it might be worth re-reading those bits of the book until they make sense. The class provides an Add method: Account robsAccount = new Account ().1 The ArrayList class The ArrayList is a cousin of the HashTable we have just seen. in that what it actually does is add a reference. where adding the 10. This statement probably doesn't tell you much about them or what they do. We are not putting an Account into the arraylist. starting with the simple ArrayList. This leads to a "straw that breaks the camel's back" problem. It is important to remember what is going on here. If this sounds a bit like abstraction and inheritance you are sort of on the right track. We have just seen that we can store Account references in an Array. But I digress. and some very clever code in the library makes sure that this works.000 and our program crashes. One way to solve this problem is to use a really big array. But we know that when an array is created the programmer must specify exactly how many elements it contains. Adding Items to an ArrayList Adding items to an ArrayList is also very easy. Note that you don't have to set the size of the ArrayList. In this respect the word Add can be a bit misleading. store. but it does indicate that they are very useful. we are instead making one element of the arraylist refer to that account. We can always add new elements to the ArrayList. The ArrayList called storeFifty is initially able to store 50 references.Collections namespace. 5.1 Generics and Collections Generics are very useful. an array that can grow in size.
  } C# Programming © Rob Miles 2010 135 . cast it into an Account class and then pay fifty pounds into it. This puts a reference to a KitchenSink instance into our bank storage. it is just no longer on that list. To get a properly typesafe Account storage we have to take a look at generics in detail a bit later.PayInFunds(50). a. it is not necessarily destroyed. It would be very nice if we could just get hold of the account from the arraylist and use it. if I ran the code above when store did not contain robsAccount) this does not cause an error. Unfortunately this won't work.WriteLine("The bank is empty"). If you think about it. You will no doubt remember from the discussion of object hierarchies that an object reference can refer to an instance of any class (since they are all derived from object) and so that is the only kind of reference that the arraylist works with.Remove(robsAccount). an item "in" an arraylist is never actually in it. It will have a list of all the customers. Account a = store[0]. If you remove an item the size of the arraylist is decreased. The designers of the ArrayList class had no idea precisely what type of object a programmer will want to use it with and so they had to use the object reference. This will cause problems (and an exception) if we ever try to use it as an Account. Note that if the reference given is not actually in the arraylist (i. So. This would get the element at the start of the arraylist.Add(k). If the store contained more than one reference to robsAccount then each of them could be removed individually. just like your name can appear on multiple lists in the real world. but with a tricky twist. This means that I can't be sure that an arraylist you give me has nothing other than accounts in it: KitchenSink k = new KitchenSink(). Accessing Items in an ArrayList Items in arraylists can be accessed in just the same way as array elements. Finding the size of an ArrayList You can use the property Count to find out how many items there are in the list: if (store. because a program can use a cast to change the type of the item the arraylist returns: Account a = (Account) store[0]. a.Count == 0) { Console. the list actually contains a reference to that item. If you write this code you will get a compilation error. store. Removing Items from an ArrayList It is not really possible to remove things from an array. When an item is removed from an arraylist. this is the only thing that it could hold.e. This is given a reference to the thing to be removed: store. This removes the first occurrence of the reference robsAccount from the store arraylist. along with a list of "special" customers and perhaps another list of those who owe it the most money. This is not actually a huge problem. The reason for this is that an arraylist holds a list of object references.Advanced Programming Generics and Collections Remember that it would be perfectly possible to have robsAccount in multiple arraylists. but the arraylist class provides a really useful Remove behaviour.PayInFunds(50). A slightly larger problem is that an arraylist is not typesafe. It might be that the bank has many lists of customers.
  It is newer than the ArrayList class. They also throw exceptions if you try to access elements that are not in the array. not any particular class.1. with the advantage that it is also typesafe. if (a. float or Account array the job that it does is exactly the same. that is not what the designers of C# did. the fundamental behaviours of arrays are always the same. Whether you have an int. the only problem that you have is that an arraylist will always hold references to objects. However. It does this by using references to objects. If you wish you can use arraylists in place of arrays and have the benefits of storage that will grow and shrink as you need it. To understand how it works you have to understand a bit of generics. ArrayLists and Arrays Arraylists and arrays look a lot the same. and because it is not quite as much a part of the language as an array is. You could of course write this behaviour yourself. but this can lead to programs which are dangerous. However. C# can make sure that an array always holds the appropriate type of values. They both let you store a large number of items and you can use subscripts (the values in square brackets) to get hold of elements from either. string. if you give me an ArrayList there is no way I can be sure that accounts are all it contains. but having it built into the class makes it much easier. there is just no way that you can take an Account reference and place it in element in an array of integers.WriteLine("Rob is in the bank").Advanced Programming Generics and Collections Checking to see if an ArrayList contains an item The final trick I'm going to mention (but there are lots more things an arraylist can do for you) is the Contains method. And because each array is declared as holding values of a particular type. Generics and Behaviours If you think about it. so that it because as much a part of C# as the array is. Instead they introduced a new language feature. If you give me an array of Accounts I can be absolutely sure that everything in the array is an account. generics. This is all to the good. it has to use a compromise to allow it to hold references to any kind of item in a program.2 The List class The List class gives you everything that an arraylist gives you. and provides you with some means to work with them. 5. And I only really find out when I start trying to process elements as Accounts and my program starts to go wrong. being based on the generic features provided by a more recent version of the C# language. The way that the system works. The abilities an array gives you with an array of integers are exactly the same as those you have with an array of Accounts. It holds a bunch of things in one place. It doesn't matter what the thing I'm dealing with is. this can be fixed when you move over to the List class. The ArrayList was added afterwards. } The Contains method is given a reference to an object and returns true if the arraylist contains that reference. This is simply a quick way of finding out whether or not an arraylist contains a particular reference. but the ArrayList breaks things a bit. One way to solve this problem would have been to find a way of making the ArrayList strongly typed. It could contain a whole bunch of KitchenSink references for all I know. we can sort that out when we actually want to work with something. C# Programming © Rob Miles 2010 136 . in that there is nothing to stop references of any type being added to an arraylist.Contains(robsAccount)) { Console. Generics let me write code that deal with objects as "things of a particular type". However.
  What the list stores is not important when making the list. cakes or even cars. Something like: "Divide the number of marbles by the number of friends and then distribute the remainder randomly". since the acountList variable is declared as holding a list of Account references. we might want to use the name of an account holder as a way of locating a particular account. robsAccount). The sharing algorithm could have been invented without worrying about the type of the thing being shared. This allows the key to your hashtable. We can now add items into our dictionary: accountDictionary.1. However.Generic namespace and works like this: List<Account> accountList = new List<Account>(). Because we have told the compiler the type of things the list can hold it can perform type validation and make sure that nothing bad happens when the list is used: KitchenSink k = new KitchenSink(). In this respect. The List class lives in the System. we have a string as the key and an Account reference as the value. You now have a system that lets you share marbles. For example. using a Dictionary has the advantage that we can only add Account values which are located by means of a string. Since the compiler knows that AccountList holds Account references. In other words. The type between the < and > characters is how we tell the list the kind of things it can store. In other words statements like this would be rejected. 5.Add(k).Account> accountDictionary = new Dictionary<string. and the items it stores.PayInFunds(50). generics can be regarded as another form of abstraction. accountList.Account>(). There is no need to cast the item in the list as the type has already been established. cousin called List. Generics and the List In the case of generics. If you wanted to share out marbles amongst you and your friends you could come up with way of doing that. it can be a general sharing behaviour that is then applied to the things we want to share.3 The Dictionary class Just as ArrayList has a more powerful. We can create a dictionary to hold these key/value pairs as follows: Dictionary<string. how about an example.Add("Rob". Count. this means that you can write code like this: accountList[0]. just as long as you have a way of telling it what to store. You could take your universal sharing algorithm and use it to share most anything. However. the behaviour you want is that of a list.Advanced Programming Generics and Collections This sounds a bit confusing. you could also use it to share out sweets. This looks just like the way the HashTable was used (and it is). We could create a List that can hold integers in a similar way: List<int> scores = new List<int>(). to be made typesafe. The above statement creates a list called acountList which can hold references to Accounts. The above statements would cause a compilation error. generically enhanced. so the HashTable has a more powerful cousin called Dictionary. C# Programming © Rob Miles 2010 137 . You can do everything with a List (Add. The C# features that provide generics add a few new notations to allow you to express this. Remove) that you can with an ArrayList.Collections. What the system works on is not particularly important. and will not accept the kitchen sink. which makes them the perfect way to store a large number of items of a particular type.
  How these features are used to create generic classes is a bit beyond the scope of this text. 5. k). } The Dictionary class is simply wonderful for storing keys and values in a typesafe manner.1. a lot easier. We can express the required account behaviour in terms of the following interface: public interface IAccount { void PayInFunds ( decimal amount ). Programmer’s Point: Use Dictionary and List You don't really have to know a lot about generics to be able to spot just how useful these two things are.Add("Glug". This means that our data storage requirements are that we should load all the accounts into memory when the program starts.PayInFunds(50). The account that we are going to work with only has two members but the ideas we are exploring can be extended to handle classes containing much larger amounts of data. make a List. If you want to be able to find things on the basis of a key. accountDictionary. Every time you need to hold a bunch of things. particularly library routines. but you are strongly advised to find out more about generics as it can make writing code. and I recommend it strongly to you. bool WithdrawFunds ( decimal amount ). } All the accounts in the bank are going to be managed in terms of objects which implement this interface to manage the balance and read the name of the account owner. decimal GetBalance (). We can create a class called CustomerAccount which implements the interface and contains the required methods.2 Storing Business Objects For our bank to really work we have to have a way of storing the bank accounts and bringing them back again.ContainsKey("Rob")) { Console. The programmers who wrote these things are very clever folks . You can get around this by asking the dictionary if it contains a particular key: if (d. Then we can move on to consider the code which will save a large number of them.4 Writing Generic Code The List and Dictionary classes were of course written in C# and make use of the generic features of the language. and then save them when the program completes. The only problem with this use of a dictionary is that if there is no element with the key "Rob" the attempt to find one will result in a KeyNotFoundException being thrown. To start with. It also has a constructor which allows the initial name and balance values to be set when the account is created. Don't have any concerns about performance. A further advantage is that we do not need to cast the results: d["Rob"]. string GetName(). If we want to our program to be able to process a large number of accounts we know that we have to create an array to manage this.WriteLine("Rob is in the bank"). This would find the element with the hash key "Rob" and then pay fifty pounds into it. 5. use a Dictionary. we'll consider how we save one account.Advanced Programming Storing Business Objects KitchenSink k = new KitchenSink(). C# Programming © Rob Miles 2010 138 .
  this is the only way that we can save an account. but it is just here to illustrate how the data in the class can be saved and restored. private string name. } public decimal GetBalance () { return balance.Advanced Programming Storing Business Objects public class CustomerAccount : IAccount { public CustomerAccount( string newName. 5.2. public virtual bool WithdrawFunds ( decimal amount ) { if ( balance < amount ) { return false. since any save mechanism would need to save data in the account which is private and therefore only visible to methods inside the class. if you think about it. In fact. } public string GetName() { return name. decimal initialBalance) { name = newName. so it is not what I would call "production" code. balance = initialBalance.amount . We can add a Save method to our CustomerAccount class: C# Programming © Rob Miles 2010 139 . } } Note that this version of the class does not perform any error checking of the input values. } public void PayInFunds ( decimal amount ) { balance = balance + amount . } private decimal balance = 0.1 Saving an Account The best way to make achieve the saving behaviour is to make an account responsible for saving itself. return true. } balance = balance .
  Close(). A way to get around this is to write a static method which will create an account given a filename: public static CustomerAccount Load(string filename) { CustomerAccount result = null. } catch { return false.2.Advanced Programming Storing Business Objects public bool Save ( string filename ) { try { System.Parse(balanceText).IO. try { textIn = new System. textOut. } This method is given the name of the file that the account is to be stored int. When we save an account we have an account which we want to save.WriteLine(name).ReadLine(). result = new CustomerAccount(nameText.Save ("outputFile.TextWriter textOut = new System.balance). string nameText = textIn. } This would ask the account referred to by Rob to save itself in a file called "outputFile. The Load method above takes great pains to ensure that if anything bad happens it does a number of things: It does not throw any exceptions which the caller must catch. } finally { if (textIn != null) textIn. textOut. } This method opens a file. } return true. decimal balance = decimal.ReadLine(). System. C# Programming © Rob Miles 2010 140 . Note that I have written the code so that if the file output fails the method will return false. fetches the balance value and then creates a new CustomerAccount with the balance value and name.IO.2 Loading an Account Loading is slightly trickier than saving. So I could do things like: if (Rob.TextReader textIn = null. 5. string balanceText = textIn.StreamWriter(filename). to indicate that the save was not successful.IO.txt".WriteLine(balance).WriteLine ("Saved OK").txt")) { Console.Close() . } return result. When we load there will not be an account instance to load from.StreamReader(filename). textOut. } catch { return null.IO. It writes out the name of the customer and the balance of the account.
  and is something you should aim for when writing code you are going to sell. since that makes sure that a failure is brought to the attention of the system much earlier. Using streams A better solution is to give the file save method a stream to save itself.txt" ). you can argue this either way. but this would be confusing and inefficient. Actually. However. It makes sure that the file it opens is always closed.IO. rather than a filename. with large numbers of file open and close actions being required (bear in mind that our bank may contain thousands of accounts). A stream is the thing that the C# library creates when we open a connection to a file: System. If the factory fails (because the file cannot be found or does not contain valid content) it will return a null result. And they will probably try and blame me for the problem (which would be very unfair).2. We could create a new file for each account.TextWriter textOut = new System. in that it creates an instance of a class for us. } Programmer’s Point: There is only so much you can do Note that if an incompetent programmer used my Load method above they could forget to test the result that it returns and then their program would follow a null reference if the customer is not found. textOut. which we can test for: if (test == null) { Console.WriteLine(balance).WriteLine(name). The way around this is to make sure that you document the null return behaviour in big letters so that users are very aware of how the method behaves. your part doesn’t break.IO. We can create a save method which accepts the stream reference as a parameter instead of a filename: public void Save(System.WriteLine( "Load failed" ). at the moment we can only store one account in a file.TextWriter textOut) { textOut.3 Multiple Accounts The above code lets us store and load single accounts.Advanced Programming Storing Business Objects It returns null to indicate that the load failed if anything bad happens. This kind of method is sometimes called a ―factory‖ method.txt. 5. You can also get code analysis tools which are bit like "compilers with attitude" (a good one is called FxCop). you just have to make sure that. whatever happens. This is what I would call a "professional" level of quality. The reference textOut refers to a stream which is connected to the file Test. This means that their program will fail in this situation. We can use this as follows: test = CustomerAccount. as I grow older I become more inclined to make my programs throw exceptions in situations like this.StreamWriter( "Test.Load( "test.txt" ). } This save method can be called from our original file save method: C# Programming © Rob Miles 2010 141 . These scan programs for situations where the results of methods are ignored and flag these up as potential errors. the best place to do it is at the pub.IO. At the end of the day though there is not a great deal you can do if idiots use your software. However.
  try { textOut = new System.TextWriter textOut = null. } catch { return false.IO. decimal balance = decimal. Save(textOut). Saving and loading bank accounts Now that we have a way of saving multiple accounts to a single stream we can write a save method for the bank. not just files on a disk.ReadLine(). Note that this is an example of overloading in that we have two methods which share the same name. public static CustomerAccount Load( System.ReadLine(). } finally { if (textOut != null) { textOut. balance).IO.Parse(balanceText). This will open a stream and save all the accounts to it: C# Programming © Rob Miles 2010 142 . The load method for our bank account can be replaced by one which works in a similar way. } return result. try { string name = textIn. For example. It reads the name and the balance from this stream and creates a new CustomerAccount based on this data. } } return true.StreamWriter(filename). } catch { return null. you can create a stream which is connected to a network port. string balanceText = textIn. The C# input/output system lets us connect streams to all manner of things.Close(). Programmer’s Point: Streams are wonderful Using streams is a very good idea. result = new CustomerAccount(name. This means that if you make your business objects save and load themselves using streams they can then be sent over network connections with no extra work from you.Advanced Programming Storing Business Objects public bool Save ( string filename ) { System. } This method creates a stream and then passes it to the save method to save the item.IO.TextReader textIn) { CustomerAccount result = null. } This method is supplied with a text stream.
  TextReader textIn) { HashBank result = new HashBank().ReadLine().4 Handling different kinds of accounts The code above will work correctly if all we want to save and load are accounts of a particular type.IO. in that only the CurrentAccount class is responsible for the content. and supplies each item in turn.Count).Add(account.WriteLine(bankHashtable. Note that we are using a new C# loop construction here. Bank Notes: Large Scale Data Storage What we have done is created a way that we can store a large number of bank account values in a single file.Values) { account. This is very useful when dealing with collections of data. i++) { CustomerAccount account = CustomerAccount. The Load method for the entire bank is as follows: public static HashBank Load(System. A production version of the program would check that each account was loaded correctly before adding it to the hash table. } return result. However. I don't expect you to understand this at first reading. Note that before it writes anything it writes out the number of customers in the bank..GetName(). } This reads the size of the bank and then reads each of the accounts in turn and adds it to the hash table. in that everything is written in terms of the CurrentAccount class. The reason that it is here is for completeness. foreach (CustomerAccount account in bankHashtable.Load(textIn). account).bankHashtable. in this case the Values property of the bankHashtable.IO.2. Health Warning This is complicated stuff. This can be obtained via the Count property of the Hashtable. We do this is so that when the bank is read back in the load method knows how many accounts are required.TextWriter textOut) { textOut. 5. It works its way through a collection. for (int i = 0. foreach. we have seen that our customer needs the program to be able to handle many different kinds of account. i < count.Parse(countString). We have done this without sacrificing any cohesion. We could just write out the data and then let the load method stop when it reaches the end of the file.Advanced Programming Storing Business Objects public void Save(System. We have also been very careful to make sure that whenever we save and load data we manage the way that this process can fail. Note that this load method does not handle errors. string countString = textIn. } } This is the Save method which would be added to our Hashtable based bank. result. int count = int.Save(textOut). This material is provided to give you an idea of C# Programming © Rob Miles 2010 143 . but this would not allow us to detect if the file had been shortened. It gets each account out of the hash table and saves it in the given stream.
  and overrides the WithdrawFunds method to provide the new behaviour. Note that I have created a constructor which is supplied with the name of the account holder. decimal initialBalance. the starting balance and the name of the parent. As an example. This makes use of the constructor in the parent to set the balance and name. a special account for young people which limits the amount that can be withdrawn to no more than 10 pounds. but it assumes a very good understanding of class hierarchies. } public override bool WithdrawFunds(decimal amount) { if (amount > 10) { return false. } return base. The customer has also asked that the BabyAccount class contains the name of the "parent" account holder. initialBalance) { parentName = inParentName.Advanced Programming Storing Business Objects how you really could make a working bank. I can create a BabyAccount as follows: BabyAccount babyJane = new BabyAccount ("Jane". } public BabyAccount( string newName.WithdrawFunds(amount). This turns out to be very easy. The good news is that when you do understand this stuff you really can call yourself a fully-fledged C# programmer. string inParentName) : base(newName. This would create a new BabyAccount instance and set the reference babyJane to refer to it. Saving a child class When we want to save a BabyAccount we also need to save the information from the parent class. and then sets the parent name. parentName. 20. method overriding/overloading and constructor chaining. "John"). It contains an additional property. some of which will be based on others. } } This is a complete BabyAccount implementation. Banks and Flexibility We know that when our system is actually used in a bank there will be a range of different kinds of account class. as long as we use the save method which sends the information to a stream: C# Programming © Rob Miles 2010 144 . we have previously discussed the BabyAccount. We could implement such behaviour by creating an account which extends the CustomerAccount class and adds the required behaviours and properties: public class BabyAccount : CustomerAccount { private string parentName. public string GetParentName() { return parentName.
  In the code above there is a constructor for the BabyAccount class which accepts the three data items that the BabyAccount holds and then sets the instance up with these values.ReadLine(). but when it runs it will not work correctly. Loading a child class We could create a static load method for the BabyAccount which reads in the information and constructs a new BabyAccount: public static BabyAccount Load( System. it first calls the overridden method in the parent to save the CustomerAccount data. This is very good design. I'm not particularly keen on this approach. string parent = textIn.TextReader textIn) { BabyAccount result = null. textOut. I can therefore write code like this: C# Programming © Rob Miles 2010 145 . } This constructor sets up the new CustomerAccount instance by reading the values from the stream that is supplied with. string balanceText = textIn. What we really want to do is make the CustomerAccount responsible for loading its data and the BabyAccount just look after its content. balance. We know that a constructor is a method which gets control when an instance of a class is being created. result = new BabyAccount (name.. } However. balance = decimal. } return result. } This method overrides the Save method in the parent CustomerAccount. Then it performs the save behaviour required by the BabyAccount class.ReadLine(). as it means that if the data content and save behaviour of the parent class changes we don't need to change the behaviour of the child. If I forget to do this the program will compile.ReadLine().Advanced Programming Storing Business Objects public override void Save(System.IO. parent). What we do is create constructors for the CustomerAccount and BabyAccount classes which read their information from a stream that is supplied to them: public CustomerAccount(System.Save(textOut).Parse(balanceText). try { string name = textIn. and it can be passed information to allow it to do this. The way to do this is to go back to the construction process. A constructor can be used to set up the data in the instance.Parse(balanceText).IO. However.WriteLine(parentName). } catch { return null. in a similar way to the way the save method uses the base keyword to save the parent object.ReadLine().ReadLine().IO. decimal balance = decimal.TextWriter textOut) { base. string balanceText = textIn.TextReader textIn) { name = textIn. This method breaks one of the rules of good design.
  result = new CustomerAccount(textIn). decimal GetBalance ().Values) { textOut.TextReader textIn) : base (textIn) { parentName = textIn.TextWriter textOut) { textOut. If the behaviour of the CustomerAccount constructor changes we do not have to change the BabyAccount at all.IO. account. this is reasonable behaviour. since the account container will not have to behave differently depending on what kind of account it is saving.IO.WriteLine(account. textIn.Count). it just has to call the save method for the particular instance. However. This creates a new CustomerAccount from the stream. bool Save(string filename).GetType(). void Save(System. in that when we are loading the accounts we don't actually have an instance to call any methods on. Note that these constructors do not do any error checking. This is very useful when we store our collection of accounts. they just throw exceptions if something goes wrong. The save method for our bank could look like this: public void Save(System. bool WithdrawFunds ( decimal amount ). This is a good idea.IO.Save(textOut).IO. The solution to this problem is to identify the type of each instance in the steam when we save the classes. bearing mind we are reading from a stream of data. but it could be updated to include them: public interface IAccount { void PayInFunds ( decimal amount ).Close(). You might think it would be sensible to add load methods to the IAccount interface. there is a problem here. Bearing in mind that the only way that a constructor can fail is to throw an exception anyway.Advanced Programming Storing Business Objects System. } } C# Programming © Rob Miles 2010 146 . things get a little trickier. Also.StreamReader(filename). Interfaces and the save operation When we started this account development we set out an interface which describes all the things which an instance of an account should be able to do.IO.Name).TextReader textIn = new System. } We can now ask any item which implements the IAccount interface to save itself to either a file or a stream.TextWriter textOut). Now I can create a constructor for the BabyAccount which uses the constructor of the parent class: public BabyAccount(System. so that we can ask instances of a particular kind of account to load themselves. At the start this did not include the save behaviours. Loading and factories When it comes to loading our classes back.ReadLine(). string GetName(). } This removes the dependency relationship completely.WriteLine(bankHashtable. we don't actually know what kind of item we are loading. foreach (CustomerAccount account in bankHashtable.
  for (int i = 0. This writes out the name of the class. The neatest way to do this is to create a factory which will produce instances of the required class: class AccountFactory { public static IAccount MakeAccount( string name. It makes use of the method GetType().Add(account.Advanced Programming Storing Business Objects This looks very like our original method. It involves writing code which will search for C# classes which implement particular interfaces and creating them automatically. result.bankHashtable. In other words. IAccount account = AccountFactory. string countString = textIn. the name of the class to be created and a stream to read from. However this kind of stuff is beyond the scope of this text. } } } This class only contains a single method. except that it uses the factory to make account instances once it has read the name of the class from the stream. which is static. } return result. If we ever add a new type of account we need to update the behaviour of the factory so that it contains a case to deal with the new account type.IO.IO. creates one. } Again.TextReader textIn) { switch (name) { case "CustomerAccount": return new CustomerAccount(textIn).ReadLine(). Factory Dependencies Note that we now have a genuine dependency between our system and the factory class. It uses the name to decide which item to make. i++) { string className = textIn.TextReader textIn) { HashBank result = new HashBank().Parse(countString). this looks very like our original load method.GetName(). if the account is of type CustomerAccount the program will output: CustomerAccount Rob 100 The output now contains the name of the type of each class that has been written.MakeAccount(className. This means that when the stream is read back in this information can be used to cause the correct type of class to be created. C# Programming © Rob Miles 2010 147 .ReadLine(). which has been highlighted. int count = int. account). The bank load method can use this factory to create instances of accounts as they are loaded: public static HashBank Load(System. except that it has an additional line. and then returns that to the caller. textIn). System. If the name is not recognized it returns null. which can be called on an instance of a class to get the type of that class. There is in fact a way of removing the need to do this. i < count. default: return null. case "BabyAccount": return new BabyAccount(textIn). Having got the type we can then get the Name property of this type and print that. The method is given two parameters.
  Programmer’s Point: Production Code From now on all the code examples are going to be given in the context of production code. The job of the business object is to make sure that the data that it holds is always correct. However. The new name must be a valid one. The way that we manage the saving of items (by using a method in the class) loading (by using a constructor) is not symmetrical. We now also know how to save the information in a class instance. 5. This might make the examples a bit more complex that you might like..3. If it is going to reject names it would be very useful to the user if they could be told why a given name was not valid.Advanced Programming Business Objects and Editing Bank Notes: Messy Code You might think that the solutions above are rather messy. When you want to move from objects to storage and back you will find that you hit these issues and this solution is as good as any I've found. there is more to it than just changing one string for another. are just those that "real" programmers are up against. A good software engineer would provide something like this: C# Programming © Rob Miles 2010 148 . and also perform this for a large number of items of a range of different types. This means that the account must be able to reject names it doesn't like. Now we need to consider how we can make our account management system genuinely useful. there is no dishonour in this way of working. and the problems you are grappling with. 5. It is important that this is always stored safely but people may want to change their name from time to time. This means that the business object must provide a way that the name can be changed. This is code which I would be happy to have supplied in a product. Managing a bank account name As an example.1 The role of the Business Object We have taken a very strict line in our banking system to stop any of the bank account components from actually talking to the user. so the validation process should provide feedback as to why it did not work. It is not their job to talk to users. they are strictly concerned with keeping track of the information in the bank account of the customer.3 Business Objects and Editing We have seen how to design a class which can be used to hold information about the customers in our bank. but I reckon this is worth doing since it is important you start to understand that what you are writing now. However. This is because classes like CustomerAccount are what is called business objects. consider the name of the bank account holder.
  I provide the validate method so that users of my class can check their names to make sure that they are valid. } string trimmedName = name. once we have these methods.Trim(). public string GetName() { return this. } public bool SetName ( string inName ) { string reply . The reasons why a name is invalid are of course part of the metadata for the project. reply = ValidateName(inName). the natural thing to do is create tests for them: C# Programming © Rob Miles 2010 149 . the string is rejected.Advanced Programming Business Objects and Editing private string name. } return "". } this. since I also use the method myself when I validate the name. or just contains spaces. Testing Name Handling Of course. I’ve also made this method static so that names can be validated without needing to have an actual instance of the account. There may be several reasons why a name is not valid. if ( trimmedName.Length > 0 ) { return false.Length == 0 ) { return "No text in the name". This does not involve me in much extra work. } public static string ValidateName ( string name ) { if ( name == null ) { return "Name parameter null".Trim(). if ( reply. return true. The validate method returns a string which gives an error message if the string is rejected.name. The validate method is called by the set method so that my business object makes sure that an account never has an invalid name.name = inName. The validate method will reject a name string which is empty. It trims off all the spaces before and after the name text and then checks to see if the length of the resulting string is empty. } The name is stored as a private member of the account class. another to set it and another to validate it. at the moment I've just given two. There is a method to get the name. If it is. The programmer has provided three methods which let the users of my Account class deal with names.
  errorCount++.Advanced Programming Business Objects and Editing int errorCount=0. if (reply != "Name parameter null") { Console. } if ( a. errorCount++. And that is the matter of error handling. 50). if (reply != "No text in the name") { Console. Then I make sure that I can set the name on an account.WriteLine("Empty name test failed").WriteLine("Pete GetName failed"). You should consider issues like this as part of the metadata in your project. if (!a. Programmer’s Point: Use Numbers Not Messages There is one issue which I have not addressed in my sample programs which stops them from being completely perfect. string reply. errorCount++. Editing the Name We now have a business object that provides methods which let us edit the name value. } reply = CustomerAccount.WriteLine("Null name test failed"). errorCount++. errorCount++. The fact that the system must work in France and Germany is something you need to be aware of right at the start.GetName() != "Jim" ) { Console. All I would need is a lookup table to convert the message number to an appropriate string in the currently active language.GetName() != "Pete" ) { Console. } if (errorCount > 0 ) { SoundSiren().ValidateName(" "). reply = CustomerAccount. } These are all the tests I could think of. At the moment the errors supplied by my validation methods are strings.SetName("Jim")) { Console.WriteLine("Blank string name test failed"). errorCount++. } reply = CustomerAccount.WriteLine("Pete trim SetName failed"). Finally I check that the space trimming for the names works correctly.WriteLine("Jim SetName failed").ValidateName(null). } if ( a.SetName(" Pete ")) { Console.ValidateName(""). if (reply != "No text in the name") { Console. This is because they are much easier to compare in tests and it also means that my program could be made to work in a foreign language very easily. } if (!a. I can now write code to edit this property: C# Programming © Rob Miles 2010 150 . First I test ValidateName to make sure that it rejects both kinds of empty string.WriteLine("Jim GetName failed"). In a genuine production environment the errors would be numeric values. } CustomerAccount a = new CustomerAccount("Rob". errorCount++.
  Advanced Programming Business Objects and Editing while (true) { Console. string reply.account = inAccount. public AccountEditTextUI(Account inAccount) { this. When I want to edit an account I create an editor instance and pass it a reference to the account instance: public class AccountEditTextUI { private IAccount account. reply = account. if ( reply.WriteLine( "Invalid name : " + reply ).ReadLine(). Now that we have our edit code we need to put it somewhere. There will be a dependency between the editor class and the account class. Creating an Editor class The best way to do this is to create a class which has the job of doing the editing. reply = this. Console. but it does keep the user informed of what it is doing and it does work. } Console. This class will work on a particular account that needs to be edited. but this is something we will just have to live with. } this. This code will perform the name editing for an account instance referred to by account.ReadLine().Write ( "Enter new name : " ) . If the name is not valid the message is printed and the loop repeats.Write ( "Enter new name : " ) . newName = Console. } } C# Programming © Rob Miles 2010 151 .ValidateName(newName). It will read a new name in.account. string reply.Length == 0 ) { break. if ( reply.account. If the name is valid the loop is broken and the name is set. } account. } public void EditName () { string newName. } Console. This is not the most elegant solution. while (true) { Console. newName = Console.ValidateName(newName).SetName(newName). in that if the account class changes the editor may need to be updated.WriteLine( "Name Edit" ).SetName(newName).Length == 0 ) { break.WriteLine( "Invalid name : " + reply ).
  Console.WriteLine ( " Enter pay to pay in funds" ).Write ("Enter command : "). AccountEditTextUI edit = new AccountEditTextUI(a). You will use this technique when you create different editing forms 5. not a references to the account class. This code creates an instance of a customer account.ReadLine(). but I will add other edit methods later. Console. I've extended the account class to manage the account balance and added edit methods which pay in funds and withdraw them. account. edit.2 A Text Based Edit System We now have a single method in our editor class which can be used to edit the name of a bank account.. Note that I trim the command string and convert it to lower case before using it to drive the switch construction which selects the command.Advanced Programming Business Objects and Editing This is my account editor class. Programmer’s Point: Get used to passing references around It is important that you get used to the idea of passing references between methods.GetName() ).EditName(). At the moment it can only edit the name. Note that the editor class is passed a reference to the IAccount interface. Console. It then passes that reference to the service methods which will do the actual work.3. I would use the name editor as follows: CustomerAccount a = new CustomerAccount("Rob". Console.Trim(). Console. do { Console. It then creates an editor object and asks it to edit the name of that account.WriteLine ( "Editing account for {0}". command = Console. public void DoEdit (CustomerAccount account) { string command. switch ( command ) { C# Programming © Rob Miles 2010 152 . I pass it a reference to this account when I construct it.WriteLine ( " Enter exit to exit program" ). My edit class contains a method which does this: This is my edit method which repeatedly reads commands and dispatches them to the appropriate method.ToLower().WriteLine ( " Enter draw to draw out funds" ). The class keeps track of the account it is editing. command = command. This means that anything which behaves like an account can be edited using this class.WriteLine ( " Enter name to edit name" ). The edit method is passed a reference to the account which is being edited. Note also that the editor remembers the account class that is being edited so that when I call EditName it can work on that reference. 50). If I want to "give" a class something to work on I will do this by calling a method in that class and passing the reference as a parameter to this method. command = command.
  The C# libraries contain a set of resources which help you manage the internationalization of your programs. } } while ( command != "exit" ). case "draw" : WithDrawFunds(account). The only thing that can decide on the validity of a name is the account object itself. break.1 Creating a Form If I want a form on the screen for the user to interact with. Everything should be managed in terms of message numbers. As a general rule in a production system you should never write out straight text to your user (this includes the names of commands that the user might type in). This question is not the responsibility of the front end.4. These will range from an operator in a call centre.Windows. an operator on a dial up terminal.Advanced Programming A Graphical User Interface case "name" : EditName(account). The menu for my bank account edit method sample code prints out text which is hard wired to English. 5. Bank Notes: More Than One User Interface The bank may have a whole range of requirements for editing account details. a customer on a mobile phone and a customer at a cash machine. Note that we never do things like allow the user interface code to decide what constitutes a valid name. The object that I create is a Form. case "pay" : PayInFunds(account). I hope that you can see that the only way we can manage all these different ways of interacting with the bank account is to separate the business object (the account itself) from the input/output behaviour. This class can be found in the System. I can create an instance of this class and ask it to do things for me: C# Programming © Rob Miles 2010 153 . break. In a properly written program this would be managed in terms of message numbers to make it easier to change the text which is output. I need to create an instance of an object to do this for me. 5.4 A Graphical User Interface It should come as no surprise that a graphical user interface on the screen is represented by objects.Forms namespace. } Programmer’s Point: Every Message Counts You should remember that every time your program sends text to the user you may have a problem with language.. The user interface must always work on the basis that it will use the business object to perform this kind of validation. break.
  Forms. f.ShowDialog(). It can 'contain' a number of graphical components which are used to build the user interface. f. sets the text property of the label to ―Hello‖ and then adds it to the controls which are contained by the frame. } } The Form class provides a method called ShowDialog. Adding Components to a Form A form is a container. title. Label title = new Label (). This asks the form to show itself and pause the program until the form is closed. it disappears and the program ends.ShowDialog(). to add a label to the form we can do the following: using System. f.Add(title). We must create the components and add them to the form to make our user interface.Text="Hello".Forms. } } This creates a label. class FormsDemo { public static void Main () { Form f = new Form().Advanced Programming A Graphical User Interface using System. For example. class FormsDemo { public static void Main () { Form f = new Form(). The result of this code is a form which looks like this: I can fiddle with the properties of a component to move it around on the screen and do lots of interesting things: C# Programming © Rob Miles 2010 154 . If I run the program above the following appears on the screen: When the window is closed with the red button.Windows.Windows.Controls.
  ForeColor = Color. the pixel at 0.ShowDialog(). } } The position on the screen is given in pixels (a pixel being an individual dot on the screen). it behaves like every text field you've ever used on a windows screen. In short. The origin (i.Advanced Programming A Graphical User Interface using System.Windows. class FormsDemo { public static void Main () { Form f = new Form(). It provides a whole set of static colour values and also lets you create your own colours by setting the intensity of the red. f. title.Forms.BackColor = Color. It lets the user cut and paste text in and out of the box using the Windows clipboard. title.e. title. Label title = new Label (). The user can then enter text into the textbox and modify it. This is a component just like a label. title. green and blue components. The Windows Forms library provides a TextBox component which is used for this.Left=50. The program above would display a form as below: By adding a number of labels on the form and positioning them appropriately we can start to build up a user interface. I can set the text property of the TextBox to the text I want to have edited.Drawing namespace. This is because it is actually the same thing as every text field you've ever used. but it provides text edit behaviour. I can then read the text property back when I want the updated value. Editing Text with a TextBox Component Displaying labels is all very well.Yellow.Red.Drawing.Text="Hello". The TextBox gives me a great deal of functionality for very little effort on my part. The Color class is found in the System.Add(title). I can create and use a TextBox in my program as follows: C# Programming © Rob Miles 2010 155 .Top=50.0) is in the top left hand corner of the screen. but what I want to do is provide a way that the name text can be edited.Controls. It lets the user type in huge amounts of text and scrolls the text around to make it fit in the box. title. f. using System.
  It also lets you change the text in the name for whatever you want. Label nameLabel = new Label(). TextBox nameTextBox = new TextBox().Advanced Programming A Graphical User Interface using System. If you run this program you get a form as follows: This is not particularly flashy. f. class FormsDemo { public static void Main() { Form f = new Form(). I can place it on the form and manage its position just like any other: C# Programming © Rob Miles 2010 156 . nameLabel.Controls. f. This will trigger my program to store the updated name and close the form down. The Button Component What we want is a button that the user can press when they have finished editing the name. just to show how the components work.Top=50. I've also set the name text we are editing to be Rob.Add(nameLabel). A Button is just like any other component. But at the moment we have no way of signalling when the edit has been finished. nameLabel.ShowDialog(). Now we are going to find out how to make one work. For that we need a button which we can press to cause an event.Forms.Top=50.Left=0.Add(nameTextBox). f. nameLabel. nameTextBox.Left=100. nameTextBox.Controls. You have pressed buttons like these thousands of times when you have used programs. nameTextBox.Drawing.Text="Name".Text = "Rob".Windows. but it is not bad for 25 or so lines of code. } } I've fiddled slightly with the position of the label and given it a more sensible name. using System.
  Label nameLabel = new Label(). Up until now our programs have run from beginning to end. Users expect to interact with items on a screen by pressing "buttons" and triggering different actions inside the program. This means that the way our programs work has to change. finishButton.Top=50. nameLabel. f. When the program wants something from the user it waits patiently until that information is supplied (usually by means of a call of the ReadLine method). nameTextBox.Add(finishButton). TextBox nameTextBox = new TextBox(). f. f.2 Events and Delegates Events are things that happen.Left=0.Left=100. 5. (well.Advanced Programming A Graphical User Interface using System. nameLabel. f. using System. } } This gives me a button on the form which I can press: However at the moment nothing happens when the button goes down.Forms.Text="Name".Text="Finished".Text = "Rob".Controls. Button finishButton = new Button(). To get this to work we have to bind a method in our program to the event which is generated when the button is pressed. nameLabel.Add(nameTextBox). nameTextBox. finishButton.4.Top=50. C# Programming © Rob Miles 2010 157 . finishButton. Nowadays program use is quite different.Controls. class FormsDemo { public static void Main() { Form f = new Form().Add(nameLabel).Top=80.Left=100. The windows forms system uses delegates to manage this. Rather than waiting for the user to do something our programs must respond when an event is generated. nameTextBox.Drawing.Controls. The upshot of this is that the programming language must provide some means by which events can be managed.ShowDialog(). At any given instant the program was either running through code or waiting for input. duh).Windows. In the early days of computers this is how they were all used.
  Button Events To get back to the problem we are solving. The button then does its animation so that it moves in and out. The button keeps a list of people to tell about events.Text="Finished".Advanced Programming A Graphical User Interface Events and method calls In C# an event is delivered to an object by means of a call to a method in that object.Forms.Left=100. nameTextBox.Add(nameTextBox). from mouse movement. finishButton. so we can use this class every time we need to create a delegate. using System. f. f. Then it looks for people to tell about the event. From the point of view of our name editing form we would like to have a particular method called when the "Finished" button is pressed by the user. to window closing. nameLabel. f. TextBox nameTextBox = new TextBox(). The form decides which of the components on the form should be given the event and calls a method in that component to tell it "you've been pressed".Add(finishButton). In this respect you can regard an event and a message as the same thing.Controls.Top=50.ShowDialog().Top=80.Left=100. store it if it was OK and then close down the edit form. nameLabel. using System. In the case of our button. Button finishButton = new Button(). f. class FormsDemo { public static void Main() { Form f = new Form(). The good news is that creating a delegate instance for a windows component is such a common task that the forms library has provided a delegate class which does this for us.Left=0.Controls. finishButton. nameTextBox. Label nameLabel = new Label(). nameLabel.Add(nameLabel).Text = "Rob". finishButton.Drawing.Text="Name". finishButton. C# Programming © Rob Miles 2010 158 . We just need to create an instance of this class to get a delegate to give to the button. the delegate is going to represent the method that we want to have called when the button is pressed. So. A delegate is an object which can be used to represent a particular method in an instance of a class. We pass the button a reference to this delegate and it can then call the method it represents when the button is pressed. This method would validate the new name. The actual code to create a delegate and assign it to a method is as follows: using System. The list is actually a list of delegates.Windows.Controls.Top=50. nameTextBox. to get a button to run a method of ours we need to create a delegate object and then pass that to the button so that it knows to use it when the event occurs. when the user clicks on the button on our form this is registered by the Windows operating system and then passed into the form as an event. The signature of the method that is called when the event occurs is the same for every kind of event. to button press.Click += new EventHandler(finishButton_Click).
  5. In the code above I have an event handler method called finishButton_Click which accepts these parameters (but doesn't do anything with them). we might want to use them to provide information such as the mouse coordinates. C# Programming © Rob Miles 2010 159 .WriteLine ("Finish Pressed").3 An Account Edit Form My example code above is OK for showing how the forms work and components are added to them. sometimes. This is because I have made everything by hand and there is no edit object as such. If you remember our text editing code you will recall that we had an object which did the editing for us. for example when we are capturing mouse movement events. The EventHandler delegate class is in the System namespace and can refer to methods which are void and accept two parameters. The idea is that when the form is created the constructor in the form makes all the components and adds them to the form in the appropriate places. To make a proper account editor we need to create a class which will do this job for us. a reference to the object which caused the event and a reference to an EventArgs instance which contains details about the event which occurred. We passed it a reference to our account and it managed the edit process. In the completed editor this method will finish the edit off for us. Our form can then be regarded as a customised version of an empty form. EventArgs e) { Console. returning when the edit was complete.Advanced Programming A Graphical User Interface } private static void finishButton_Click( object sender.4. If you compile the above program and run it you will find that each time you press the Finish button the program prints out a message. often our methods will ignore the values supplied. This will be used in almost exactly the same way as the text editor. but it is not production code. } } The highlighted code is the bit that ties a delegate to the click event. Instead it prints out a message each time the button is pressed. Note that we don’t actually have to use the parameters. However. Extending the Windows Form class It turns out that the best way to create a class to do this job is to extend the Form class to make a new form of our own. This is a standard technique for using windows components. And the way that you create customised versions of classes is to extend them.
  if ( reply.Add(this.Forms.finishButton.Add(this. } this.Top=50. this. IAccount account.Text).EventArgs e) { string reply = account.finishButton).Length > 0 ) { System.nameLabel).finishButton.Windows.Top=80.Click += new System.Windows. private TextBox nameTextBox.Text="Finished".nameTextBox = new TextBox().account. The finishButton_Click method gets the name out of the text box and validates it. return.Form { private Label nameLabel. this.Left=100. It then gets the properties out of the form and displays them for editing. this.Controls. private Button finishButton. this.Advanced Programming A Graphical User Interface using System.Windows.nameTextBox.nameLabel = new Label(). If it is the name of the account class should be set and the form must then dispose of itself. This code is the windows equivalent of the AccountEditTextUI method that we created previously.Left=100.finishButton.Text="Name".Top=50.EventHandler(finishButton_Click).Show(reply) .Text) . this.nameTextBox. } } The form will be passed a reference to the account instance which is being edited.nameTextBox). this. this.nameTextBox. public AccountEditForm (IAccount inAccount) { this. this. this. } private void finishButton_Click(object sender. this.SetName( nameTextBox. this.GetName().Dispose().nameLabel.account. public class AccountEditForm : System.Forms.Forms.finishButton = new Button(). this.MessageBox. this.nameLabel. If the name is invalid it uses a static method in the windows forms system to pop up a message box and report an error: C# Programming © Rob Miles 2010 160 .Left=0.Add(this.Controls. this.ValidateName(nameTextBox. When the user presses the finish button the form must make sure that the name is valid.nameLabel.Controls. using System.Text = this. System.finishButton. this. this.account = inAccount. this. All the form components are created in the constructor for the form.
  the fundamental principles that drive the forms it produces are exactly the same. AccountEditForm edit = new AccountEditForm (a). This piece of test code creates an account and sets the name of the account holder to Rob. It will also look after the creation of delegates for button actions. Visual Studio and Form Editing The Visual Studio application can take care of all the hard work of creating forms and adding components to them. There are a number of different forms of this Show method which you can use to get different forms of message box and even add extra buttons to it. Disposing of forms The Dispose method is used when we have finished with this form and we want it to get rid of it. It then creates an edit form and uses that to edit the name value in the account.SetName("Rob").ShowDialog(). Once we have disposed of a form we can never use it again. Using the Edit form To use the edit form we must first construct an account instance. The Dispose method is important because we must explicitly say when we have finished with the instance. Modal Editing The call of ShowDialog in the edit form is modal. However. Account a = new Account(). Programmer’s Point: Customers really care about the user interface If there is one part of the system which the customer is guaranteed to have strong opinions about it is the user interface. This means that while the form is on the screen the rest of the system is paused. edit. ever assume you know that the user interface should work in a particular way.Advanced Programming A Graphical User Interface This is how we tell the user that the name is not correct. We will need to create a new form instead. We then pass a reference to the account into the constructor of an AccountEditForm.WriteLine ( "Name value : " + a. The design of this should start at the very beginning of the product and be refined as it goes. The good news is that with something like Visual Studio it is very easy to produce quite realistic looking prototypes of the front end of a system. I have had to spend more time re-working user interface code than just about any other part of the system. Never. it is important to remember that although it automates a lot of the actions that I have performed by hand above. All the components on the form are destroyed and the form is removed from the screen. These can then be shown to the customer (get them signed off even) to establish that you are doing the right thing.GetName()). In the code above the WriteLine is not performed until after the edit form has been closed. a. If we just removed a reference to the form object it will still remain in memory until the garbage collector gets around to noticing it. Console. C# Programming © Rob Miles 2010 161 .
  This means I can use it as a kind of method selector. Therefore you should read this text carefully and make sure you understand what is going on. We have seen that things like overriding let us create methods which are specific to a particular type of object.1 Type safe delegates The phrase type safe in this context means that if the method accepts two integer parameters and returns a string. Delegates are useful because they let us manipulate references to methods. When the event occurs (this is sometimes called the event "firing") the method referred to by the event is called to deliver the message.Advanced Programming Using Delegates 5. for example we provided a custom WithdrawFunds for the BabyAccount class which only lets us draw out a limited amount of cash. Note that I've not created any delegates yet. I call a delegate ―A way of telling a piece of program what to do when something happens‖. This word is used to distinguish a delegate from things like pointers which are used in more primitive languages like C. Events are things that happen which our program may need to respond to. Just remember that delegates are safe to use. the delegate for that method will have exactly the same appearance and cannot be used in any other way. Delegates are an important part of how events are managed in a C# program. An example of a method we might want to use with this delegate you could consider this one: C# Programming © Rob Miles 2010 162 . I've just told the compiler what the delegate type CalculateFee looks like. consider the calculation of fees in our bank. If you call the delegate it calls the method it presently refers to. A delegate is a "stand in" for a method.5 Using Delegates Events and delegates are a very important part of C#. It might want to have a way in which a program can choose which fee calculation method to use as it runs.5. In C you can create pointers to methods. This delegate can stand in for a method which accepts a single decimal parameter (the balance on the account) and returns a decimal value (the amount we are going to charge the customer). These techniques are very useful. depending on the type of customer and the status of that customer. The way that C# does this is by allowing us to create instances of delegate classes which we give the event generators. but they are hard wired into the code that I write. In each case we need to tell the system what to do when the event occurs. This means that you can call them in the wrong way and cause your program to explode. A delegate type is created like this: public delegate decimal CalculateFee (decimal balance). We can manage which particular method is going to be called in a given situation in terms of a lump of data which we can move around. Using a Delegate As an example. 5. A posh description would have the form: A delegate is a type safe reference to a method in a class. but the C environment does not know (or even care) what the methods really look like. clocks going tick and messages arriving via the network. Once I've compiled the class the methods in it cannot change. They include stuff like people pressing buttons in our user interface. Don’t worry about this too much. The bank will have a number of different methods which do this.
  In the future. } } This is a rather evil fee calculator. rather than as a good way to write programs. In the same way that you can put more than one train on a C# Programming © Rob Miles 2010 163 . Now when I "call" calc it will run the FriendlyFee method. 5. so I can build structures which contain delegates and I can also pass delegates in and out of methods.6 Threads and Threading If you want to call yourself a proper programmer you need to know something about threads. Delegates are used a lot in event handlers and also to manage threads (as you will see below).1 What is a thread? At the moment our programs are only using a single thread when they run. An instance of a delegate is an object like any other. You can regard a list of delegates as a list of methods that I can call. If I want to use this in my program I can make an instance of CalculateFee which refers to it: CalculateFee calc = new CalculateFee (RipoffFee). The thread usually starts in the Main method and finishes when the end of this method is reached.6. This means that it can be managed in terms of references. where the speed of computer processors is going to be limited by irritating things like the speed of light and the size of atoms. You can visualise a thread as a train running along a track. If you are overdrawn the fee is 100 pounds. they are the way that we will be able to keep on improving the performance of our computer systems. I don’t use them much beyond this in programs that I write. The calc delegate presently refers to a delegate instance which will use the RipoffFee method. but they can also be the source of really nasty bugs. Now I can "call" the delegate and it will actually run the ripoff calculator method: fees = calc(100). Programmer’s Point: Use Delegates Sensibly Allowing programs to change what they do as they run is a rather strange thing to want to do. If you are in credit the fee is 1 pound. The track is the statements that the thread is executing. } else { return 1. They can make programs much easier to create. it is best just to consider them in terms of how we use delegates to manage the response of a program to events. 5. For now however. I can change that by making another delegate: calc = new CalculateFee (FriendlyFee). I’m presenting this as an example of how delegates let a program manipulate method references as objects.Advanced Programming Threads and Threading public decimal RipoffFee (decimal balance) { if ( balance < 0 ) { return 100. This gives us another layer of abstraction and means that we can now design programs which can have behaviours which change as the program runs. This of course only works if FriendlyFee is a method which returns a value of type decimal and accepts a single decimal value as a parameter.
  6. When a thread is not running it is held in a ―frozen‖ state. it makes sense to share this ability amongst several tasks. rather than try interleaving the second task with the first. You have not been aware of this because the operating system (usually Windows) does a very good job of sharing out the processing power. The first computers had only one processor and so were forced to use ―time slicing‖ to support multiple threads.Advanced Programming Threads and Threading single track. Threads are also how Windows forms respond to events. I’m going to give some tips on how to make sure that your programs don’t fall foul of threading related bugs. Bugs caused by threading are amongst the hardest ones to track down and fix because you have to make the problem happen before you can fix it.3 Threads and Processors Consider the following method: static private void busyLoop() { long count. Newer machines now have ―dual core‖ or even ―quad core‖ processors which can actually run multiple threads simultaneously. The first is by rapidly switching between active threads. count < 1000000000000L. for (count = 0. A modern computer can perform many millions of instructions per second. The second way to support multiple threads is to actually have more than one processor.6. The programs that you have been writing and running have all executed as threads in your computer. and the circumstances that cause the fault may only occur every now and then. Programmer’s Point: Threads can be dangerous In the same way that letting two trains share the same railway track can sometimes lead to problems. Your system can play music and download files while your program waits for you to press the next key.2 Why do we have threads? Threads make it possible for your computer to do something useful while one program is held up. 5. As far as the thread is concerned it is running continuously. waiting for its turn to run. in that if we wish to do more than one thing at the same time it is very useful just to send a thread off to perform the task. Threads provide another level of ―abstraction‖. This is how your program can be active at the same time as other programs you may wish to use alongside it. and I suggest that you follow these carefully. The Windows system actually creates a new thread of execution which runs the event handler code. programs that use threads can also fail in spectacular and confusing ways. but in reality a thread may only be active for a small fraction of a second every now and then. 5. A computer can support multiple threads in two ways. These could be performed ―in the background‖ while the user continues to work on the document. count = count+1) { } } C# Programming © Rob Miles 2010 164 . If we wrote a word processor we might find it useful to create threads to perform time consuming tasks like printing or spell checking. a computer can run more than one thread in a single block of program code. You have seen that a ―click‖ event from a Button component can be made to run an event handler method. It is possible for systems to fail only when a certain sequence of events causes two previously well behaved threads to “fight” over a shared item of data. giving each thread the chance to run for a limited time before moving on to run another.
  The easiest way to make sure that we can use all the threading resources is to add a using directive at the start of our program: using System. If I run a program that calls this method the first thing that happens is that the program seems to stop. You can see little graphs under the CPU (Central Processor Unit) Usage History part of the display showing how busy each processor is. If I start up Windows Task Manager I see the picture below: My laptop has four processor cores. We have used namespaces to locate resources before when we used files.Threading. From the look of the above graph. 5. The Thread class provides a link between your program and the operating system.Advanced Programming Threads and Threading This method does nothing.4 Running a Thread An individual thread is managed by your program as an instance of the Thread class. the leftmost processor and the rightmost processor are quite busy but the middle two processors are not doing anything. One job of the operating system is to manage threads on your computer and decide C# Programming © Rob Miles 2010 165 . If I stop my program running I get a display like the one below. as my programs run on multiple processors. The method above can only ever use a maximum of 25% of my system.6. When we start using more than one thread we should see more of the graphs climbing up. which means that it can run four threads at a time. since it only runs on one of the four available processors. Then the fan in my laptop comes on as the processor starts to heat up as it is now working hard. Now all the processors are just ticking over. but it does do it many millions of times. Programmer’s Point: Multiple Threads Can Improve Performance If you can work out how to spread your program over several threads this can make a big difference to the speed it runs at. This class lives in the System namespace.
  This is like telling your younger brother where on the track you’d like him to place your train. Note that at the moment the thread is not running. } } The above code creates a thread called t1 and starts it running the busyLoop method. Creating a Thread Once you have your start point defined you can now create a Thread value: Thread t1 = new Thread(busyLoopMethod). for (count = 0. class ThreadDemo { static private void busyLoop() { long count.Start(). A delegate is a way of referring to a method in a class. t1. This is the point at which the thread begins to run. it is waiting to be started. It also calls busyLoop directly from the Main method.Start(). This delegate type can refer to a method that does not return a value or accept any parameters. This changes the Task Manager display: C# Programming © Rob Miles 2010 166 . count < 1000000000000L. You can create a ThreadStart delegate to refer to this method as follows: ThreadStart busyLoopMethod = new ThreadStart(busyLoop). Selecting where a Thread starts running When you create a thread you need to tell the thread where to start running.Advanced Programming Threads and Threading exactly when each should get to run. The variable t1 now refers to a thread instance. By calling methods provided by the Thread class you can start and stop them. To start the thread running you can use the Start method: t1. This means that there are two processes active. Starting a Thread The Thread class provides a number of methods that your program can use to control what it does. The delegate type used by threads is the ThreadStart type. When we start running t1 it will follow the delegate busyLoopMethod and make a call to the busyloop method. Thread t1 = new Thread(busyLoopMethod). If you take a look at the busyLoop method above you will find that this method fits the bill perfectly. You do this by using delegates. count = count+1) { } } static void Main() { ThreadStart busyLoopMethod = new ThreadStart(busyLoop). busyLoop().
  This is potentially dangerous. Programmer’s Point: Two Many Threads Will Slow Everything Down Note that nothing has stopped your program from starting a large number of threads running. t1. This makes your computer unusable by starting a huge number of threads which tie up the processor. } This loop will create 100 threads. we can create many more threads than this: for (int i = 0. and the CPU usage is up to 100%. Now our computer is really busy: All the processors are now maxed out. all running the busyLoop method. If you run this code you might actually find that the machine becomes slightly less responsive.Advanced Programming Threads and Threading Now more of the usage graphs are showing activity and the CPU usage is up from 25% to 50% as our program is now using more processors. and that all the cooling fans come on full blast. Making lots of Threads From the displays above you can see that if we create an additional thread we can use more of the power the computer gives us. called a Denial of Service attack. This is actually the basis of a type of attack on your computer. C# Programming © Rob Miles 2010 167 . If you run too many threads at once you will slow everything down. Fortunately Windows boosts the priority of threads that deal with user input so you can usually get control and stop such wayward behaviour.Start(). i < 100. i = i + 1) { Thread t1 = new Thread(busyLoopMethod). In fact.
  Before the addition can be performed.Advanced Programming Threads and Threading 5. We could make a tiny change to this code and make the count variable a member of the class: static long count. When they entered the single track they were given the token. As the threads run they are all loading. static object sync = new object(). count < 1000000000000L. What we need is a way of stopping the threads from interrupting each other. Anyone wanting to enter the track had to wait for their turn with the token. This is because the variable is local to the busyLoop method: static private void busyLoop() { long count. Sometime later Thread 1 gets control again. It is now very hard to predict how long this multi-threaded program will take to finish. adds one to it and stores it back in memory. it has overwritten the changes that Thread 20 made. must be allowed to complete without being interrupted. it is as if the data for each thread is held in trucks that are pulled along behind the train. This is potentially disastrous. Using Mutual Exclusion to Manage Data Sharing Allowing two threads to share the same variable is a bit like allowing two trains to share the same piece of track. 4. C# Programming © Rob Miles 2010 168 . 2. Because of the way that Thread 1 was interrupted during its calculation. You can do this by using a feature called mutual exclusion or mutex.5 Threads and Synchronisation You might be wondering why all the threads don’t fight over the value of count that is being used as the loop counter. Both are dangerous. Thread 1 fetches the value of the count variable so that it can add 1 to it. The single track problem was solved by using a single brass token which was held by the engine driver. Consider the following sequence of actions: 1. incrementing and storing the value of count and overwriting changes that they have made. Mutual exclusion works in just the same way by using an instance of an object to play the role of the token. The statement count = count + 1. If you are still thinking about threads as trains. 3. count = count + 1) { } } The method looks very similar. When the train left single track section the track the driver handed the token back. adds 1 to the value it fetched before it was stopped and stores the result back in memory. We can return to our train analogy here. static private void busyLoop() { for (count = 0.6. but now every thread running busyLoop is now sharing the same count variable. Thread 20 fetches the value of count. for (count = 0. count = count+1) { } } The count variable is declared within the body of the method. Thread 1 is stopped and Thread 20 is allowed to run. This means that each method has its own copy of this local variable. count < 1000000000000L.
  Programmer’s Point: Threads can Totally Break your Program If a thread grabs hold of a synchronisation object and doesn’t let go of it. The above call would pause a program for half a second.Abort(). In other words. each thread will end when the call of busyloop finishes.Advanced Programming Threads and Threading This object doesn’t actually hold any data. Another one. and thread b has got object y and is waiting for object x. This asks the operating system to destroy that thread and remove it from memory. count = count + 1. C# Programming © Rob Miles 2010 169 . This method is given the number of milliseconds (thousandth’s of a second) that the execution is to pause. so the first thread to get to the entry point will be the first to get the token. All the increment operations will complete in their entirety. These let you pause threads. This queue is held in order. there is no chance of Windows interrupting the increment statement and switching to another process.Exit(sync). but at the expense of any other threads that want to run. called the “Deadly Embrace” is where thread a has got object x and is waiting for object y. 5. wait for a thread to finish. If you just want your program to pause. I’m going to wait for him to call me”. The code between the Monitor. This is a really good way to make your programs fail.6 Thread Control There are a number of additional features available for thread management. Once execution leaves the statements between the Monitor calls the thread can be suspended as usual. Pausing Threads As you can see above. It is just used as the token which is held by the active process. This will certainly pause your program. perhaps to allow the user to read the output or to wait a little while for something to happen you should use the Sleep method which is provided by the Thread class: Thread. This is the computer version of “I’m not calling him to apologise. You can abort a thread by calling its Abort method: t1.Sleep(500). A thread finishes when it exits the method that was called when it started.Enter(sync). Monitor.Join(). see if a thread is still active and suspend and remove threads. we don’t need to know how it works. Monitor. This would cause the executing thread to wait until the thread t1 finishes.6. In our examples above.Enter and Monitor. Threads that aren’t able to run are ―parked‖ in a queue of waiting threads. A thread instance provides a Join method that can be called to wait for another thread to complete operation. Joining Threads You might want to make one thread wait for another to finish working. t1.Exit calls can only be performed by one thread at a time. Thread Control The Thread class provides a set of methods that you can use to control the execution of a thread. one way to ―pause‖ a thread is to create a loop with an enormous limit value. this will stop other threads from running if they need that object. with the other threads lining up for their turn. The Monitor class looks after all this for us.
  } 5. threads can also be a great source of frustration. The most useful are: ThreadState. Synchronisation problems like the ―Deadly Embrace‖ described above may only appear when two users both request a particular feature at exactly the same time.Suspend ThreadState. However. t1. When you have problems with a multi-threaded system the biggest difficultly is always making the fault occur so you can fix it. but simply make it pause for a while.6.Running ThreadState. waiting to join with another thread or waiting for a Monitor As an example. You should protect any variables shared between threads by using the Monitor class as described above. If variables sometimes get ―out of step‖ then this is a sure sign that you have several threads fighting over data items. is to create a multi-threaded application which starts up a thread to deal with each incoming request. They let your program deal with tasks by firing off threads of execution. This will stop inadvertent data corruption. Of course. This is mostly because we have done something wrong. A program which works fine for 99. or just lock up completely. This makes the code easier to manage and also means that your program can make the best use of the processing power available. Often the only way to investigate the problem is to add lots of write statements so that the program makes a log that you can examine after it has failed. to display a message if thread t1 is running: if ( t1. The best way to create a system that must support many users. You should try to avoid any ―Deadly Embrace‖ situations by making your threads either producers or consumers of data. Unfortunately when you add multi-threading to a solution this is no longer true. We know that our programs sometimes produce errors when they run. vanish completely. Finding the state of a thread You can find out the state of a thread by using its ThreadState property.999% of the time may fail spectacularly. When we get a problem we look at what has happened and then work out what the fault is. if you are very lucky. Up until now our programs have been single threaded and we have been able to put in the data which causes the problem.WaitSleepJoin the thread is running the thread has finished executing the thread method the thread has been suspended the thread is executing a Sleep.Resume(). time spent writing the log output affects the timing of events in the program and can often cause a fault to move or. This will cause the program to fail. If consumers are always waiting for producers C# Programming © Rob Miles 2010 170 .Stopped ThreadState.7 Staying Sane with Threads Threads are very useful.ThreadState == ThreadState.Running ) { Console. you can call the Suspend method on the thread: t1.Advanced Programming Threads and Threading If you don’t want to destroy a thread. at which point we can begin to fix it.Suspend(). for example a web server. The thread is put to sleep until you call the Resume method on that thread. This is an enumerated value which has a number of possible values. when a specific set of timing related events occur.WriteLine("Thread Running").
  The next step up is to have more than one railway. This is actually a very important part of the design process. You can also use the Process class to start programs for you.exe"). 5. This means that potentially badly behaved code like this has to be enclosed in a try – catch construction so that our program can respond sensibly. or a Parse method is given an invalid string. Processes are different from threads. but nothing in the word processor program can ever directly access the variables in the browser. 5. Your word processor may fire off several threads that run within it (perhaps one to perform spell checking). You should also be thinking that when you build a system you should consider how you are going to manage the way that it will fail. The Start method is supplied with the name of the program that you want to start. When something bad happens the program must deal with this in a managed way.Start("Notepad.Diagnostics namespace.Diagnostics. When you are using a word processor and a web browser at the same time on your computer each of them is running on your computer as a different process. Programmer’s Point: Put Threads into your Design I think what I am really saying here is that any thread use should be designed into your program. You can also create Process instances that you can control from within your program in a similar manner to threads.Advanced Programming Structured Error Handling (and producers never wait for consumers) then you can be sure that you never get the situation where one thread waits for a thread that is waiting for it. and not tacked on afterwards. In a C# program you can create a process and start it in a similar way to starting a thread.7 Structured Error Handling By now I hope that you are starting to think very hard about how programs fail. We have been on the receiving end of exceptions already. Now we are going to take a closer look at exceptions and see about creating our own exception types. The key to achieving this is to think about making your own custom exceptions for your system. so you can add a Using statement to make it easier to get hold of the class: using System.6. We have seen that if something bad happens whilst a file is being read.8 Threads and Processes You can regard different threads as all living in the in computer together in the same way that a number of trains can share a particular railway. in that each process has its own memory space which is isolated from other processes. In programming terms this is a move from threads to processes. the system will throw an exception to indicate that it is unhappy.. This class is held in the System. C# Programming © Rob Miles 2010 171 . The above line of C# would start the Notepad program. To start a program on your system you can use the Start method on the Process class: Process. If you want to use threads the way that they are managed and how they communicate should be decided right at the start of your design and your whole system should be constructed with them in mind.
  7.Exception class: public class BankException : System. I might want to throw an exception if the name of my account holder is an empty string. The default constructor in BankException (which is added automatically by the compiler).Exception class. I can do this with the code: if ( inName. 5.2 Creating your own exception type Creating your exception type is very easy.Exception class has a default constructor which takes no parameters.7. but they are all based on the parent Exception class. However. For example.1 The Exception class The C# System namespace holds a large number of different exceptions. It can be done by simply extending the System. but this means that exceptions produced by your code will get mixed up with those produced by other parts of the system.Length == 0 ) { throw new BankException( "Invalid Name" ). This all (of course) leads back to the metadata which you have gathered. In the code above I make a new instance of the bank exception and then throw that.7.Exception when something goes wrong. there is a version of the Exception constructor which lets us add a message to the exception. 5. can just use this to make an Exception instance. This might be a catch of mine.Advanced Programming Structured Error Handling 5. If you want to throw your own exceptions you are strongly advised to create your own exception type which extends the system one. If the exception is caught by the system it means that my program will be terminated. If I want to use this with my bank exception I have to sort out the constructor chaining for my exception class and write code as follows: public class BankException : System. } The throw keyword is followed by a reference to the exception to be thrown. or it might be one supplied by the system. At this point the execution would transfer to the ―nearest‖ catch construction. The thing that is thrown must be based on the System. along with everything else in your system. You can generate System.Exception { } This works because the System. If you want your code to be able to explicitly handle your errors the best way forward is to create one or more of your own exceptions. This can be picked up and used to discover what has gone wrong. This means that.Exception { public BankException (string message) : base (message) { } } This makes use of the base keyword to call the constructor of the parent class and pass it the string message. you will need to design how your program will handle and generate errors. in that the specification will give you information about how things can fail and the way that the errors are produced. To create a standard exception with a message I pass the constructor a string. However.3 Throwing an Exception The throw keyword throws an exception at that point in the code. I can catch the exception myself as follows: C# Programming © Rob Miles 2010 172 .
  Exception { public BankExceptionBadName (string message) : base (message) { } } public class BankExceptionBadAddress : System. you should seriously consider extending the exception class to make error exceptions of your own which are even more informative. Note that.WriteLine( "Error : " + exception. the catch invoked. This helps a great deal in working with different languages. It can contain a text message which you can display to explain the problem to the user. The Message member of an exception is the text which was given. } The code tries to create a new account. try { a = new Account(newName. If the customer can say “Error number 25” when they are reporting a problem it makes it much easier for the support person to respond sensibly.Message).Advanced Programming Structured Error Handling Account a. depending on how the code fails: C# Programming © Rob Miles 2010 173 . and the message printed. the code in the exception handler is obeyed. i. We can have many catch constructions if we like. However. newAddress). Errors should be numbered. This means that if I try to create an account with an empty name the exception will be thrown. and the catch will be matched up to the type of exception that was thrown: public class BankExceptionBadName : System. If doing this causes an exception to be thrown. 5.e.7.4 Multiple Exception Types It is worth spending time thinking about how the exceptions and errors in your system are to be managed. you need to design in your handling of errors. your exceptions should be tagged with an error number.Exception { public BankExceptionBadAddress (string message) : base (message) { } } Now I can use different catches. Programmer’s Point: Design your error exceptions yourself An exception is an object which describes how things have gone wrong. } catch (BankException exception) { Console. as with just about everything else. The reference exception is set to refer to the exception which has been thrown.
  and most of the time you will be using test data which assumes everything is OK.Advanced Programming Program Organisation Account a. In this section we are going to see how a large program can be broken down into a number of different chunks.Exception exception ) { Console.WriteLine("System exception : " + exception. C# Programming © Rob Miles 2010 174 . This is fine for teeny tiny projects. To do this we have to solve two problems: how we physically spread the code that we write around a number of files how we logically identify the items in our program. This might mean that you have to create special test versions of the system to force errors into the code.WriteLine("Invalid address : " + addrException. As a professional programmer you must make sure that your error handling code is tested at least as hard as the rest of the system. This is especially important when you consider that a given project may have many people working on it. } catch (BankExceptionBadName nameException) { Console. but now we are starting to write much larger programs and we need a better way to organise things.8. They only get run when something bad happens. ""). } catch (BankExceptionBadAddress addrException) { Console.WriteLine("Invalid name : " + nameException.Message).Message). error handlers are rather hard to test. 5. When you create the system you decide how many and what kind of errors you are going to have to manage.1 Using Separate Source Files In a large system a programmer will want to spread the program over several different source files. in that the error handler is supposed to put things right and if it fails it will usually make a bad situation much worse. which we compile and run.Message). try { a = new Account("Rob". Programmer’s Point: Programs often fail in the error handlers If you think about it. Of course this is a recipe for really big disasters. it is worth the effort! Error handling should be something you design in. The two problems are distinct and separate and C# provides mechanisms for both. } Each of the catches matches a different type of exception. At the very end of the list of handlers I have one which catches the system exception. This means that errors are more likely to get left in the error handlers themselves. When you design your solution to a problem you need to decide where all the files live. Believe me. } catch (System.8 Program Organisation At the moment we have put the entire program source that we have created into a single file. 5. This will be called if the exception is not a name or an address one. since the code doesn’t get exercised as much as the rest of the system.
  So. Options are a way of modifying what a command does. However. we might want to create lots of other classes which will deal with bank accounts. So instead I have decided to put the class into a file called "AccountManagement. This is so that classes in other files can make use of it. I use the target option to the compile command. the compiler cannot make an executable file. If I look at what has been created I find that the compiler has not made me an executable. since it does not know where the program should start. You can apply the protection levels to classes in the same way that you can protect class members. } } } This is a fairly well behaved class in that it won't let us withdraw more money than we have in the account. } public bool WithDrawFunds ( decimal amount ) { if ( amount < 0 ) { return false . Instead other programs will run and create accounts when they need them.cs" if we wanted to.cs The compiler will not now look for a Main method. because it has been told to produce a library rather than a program. public void PayInFunds ( decimal amount ) { balance = balance + amount.exe' does not have an entrypoint defined The compiler is expecting to produce an executable program.As I add more account management The problem is that the compiler now gets upset when I try to compile the file: error CS5001: AccountManagement. } if ( balance >= amount ) { balance = balance . return true. } else { return false . Note that I have made the Account class public. Generally speaking your classes will be public if you want them to be used in libraries. We could put it into a file called "Account. Consider a really simple Account class: public class Account { private decimal balance = 0. } public decimal GetBalance () { return balance. Creating a Library I solve this problem by asking the compiler to produce a library instead. The class will be called Account.amount .cs". Our bank account class does not have a main method because the program will never start by actually running an account. instead it has made me a library file: C# Programming © Rob Miles 2010 175 .Advanced Programming Program Organisation In our bank management program we have identified a need for a class to keep track of a particular account. These have an entry point in the form of the Main method. The compiler can tell that it is being given an option because options always start with a slash character: csc /target:library AccountManagement.
  and so it can build the executable program.GetBalance()). Console. The compiler now knows where to find all the parts of the application.dll The language extension dll stands for dynamic link library. Firstly I'm going to create another source file called AccountTest.cs(5. Library References at Runtime We have now made two files which contain program code: AccountManagement.3): error CS0246: The type or namespace name 'test' could not be found (are you missing a using directive or an assembly reference?) AccountTest.exe the library containing the Account class code the executable program that creates an Account instance Both these files need to be present for the program to work correctly. test. } } This makes a new account. Deleting System Components This means that if I do something horrid like delete the Acccountmanagement. puts 50 pounds in it and then prints out the balance.37): error CS0246: The type or namespace name 'test' could not be found (are you missing a using directive or an assembly reference?) The problem is that the compiler does not know to go and look in the file AccountManagement.dll AccountTest. If I try to compile it I get a whole bunch of errors: AccountTest. This means that the content of this file will be loaded dynamically as the program runs.PayInFunds (50). This because of the "dynamic" in dynamic link library.cs to find the Account class.cs(7.dll AccountTest. which causes further errors. not when it is built. Using a Library Now I have a library I next have to work out how to use it.Advanced Programming Program Organisation AccountManagement. This contains a Main method which will use the Account class: using System. To solve the problem I need to tell the compiler to refer to AccountManagement to find the Account class: csc /reference:AccountManagement.cs(6.cs The reference option is followed by a list of library files which are to be used.dll file and then run the program this causes all kinds of nasty things to happen: C# Programming © Rob Miles 2010 176 . It means that library is only loaded when the program runs.cs. class AccountTest { public static void Main () { Account test = new Account(). This means that it fails to create test.WriteLine ("Balance:" + test. which is the library that contains the required class. In this case there is just the one file to look at.3): error CS0246: The type or namespace name 'Account' could not be found (are you missing a using directive or an assembly reference?) AccountTest.
  was not found. Arrgh! We are now heading for real problems.Main() … lots of other stuff This means that we need to be careful when we send out a program and make sure that all the components files are present when the program runs. We don't just want to break things into physical chunks. And here we are again. and mean that at design time we have to make sure that we name all our classes in a way which will always be unique. We have decided that Account is a sensible name for the class which holds all the details of a customer account. Or both. and the number of times that I've installed a new program (or worse yet an upgrade of an existing one) which has broken another program on the computer is too numerous to happily remember. A far better way would be to say that we have a CustomerBanking namespace in which the word Account has a particular meaning. If the two systems ever meet up we can expect a digital fight to the death about what "Account" really means.IO. The bad news is that you have to plan how to use this technology. Unless the fixed code is exactly right there is a good chance that it might break some other part of the program. File name: "AccountManagement" at AccountTest. But this would be messy. Which would be bad. The bad news is that this hardly ever works. The good news is that I can fix the broken parts of the program without sending out an entire new version. But we also have another problem. rubber stamps. Windows itself works in this way. the new version is picked up by AccountTest automatically. and then make sure you use it. Programmer’s Point: Use Version Control and Change Management So many things end up being rooted in a need for good planning and management. consider the situation in our bank. Perhaps the programmers might decide that a sensible name for such a thing would be Account. The good news is that there are ways of making sure that certain versions of your program only work with particular program files. We could solve the problem by renaming our account class CustomerBankAccount. We also want to use logical ones as well. 5. When you think about selling your application for money you must make sure that you have a managed approach to how you are going to send out upgrades and fixes. But if you consider the whole of the bank operations you find that the word "account" crops up all over the place. It may well wish to keep track of these accounts using a computer system. If this sounds boring and pedantic then I'm very sorry. If I modify the AccountManagement class and re-compile it. Of course this only works as long as I don't change the appearance of the classes or methods which AccountTest uses. This makes management of the solution slightly easier. We can also have an C# Programming © Rob Miles 2010 177 . or one of its dependencies. little pens on chains (pre-supplied with no ink in of course) and the like from suppliers. but if you don't do this you will either go mad or bankrupt. "dll hell".Advanced Programming Program Organisation Unhandled Exception: System.FileNotFoundException: File or assembly name AccountManagement. The bank will buy things like paper clips.8. There is a special phrase. If you are not sure what I mean by this. reserved for what happens. Updating System Components Creating a system out of a number of executable components has the advantage that we can update part of it without affecting everything else.2 Namespaces We can use library files to break up our solution into a number of files. You could say that the bank has an account with such a supplier.
  C# Programming © Rob Miles 2010 178 . If I want to use the Account class from the CustomerBanking namespace I have to modify my test class accordingly. A given source file can contain as many namespace definitions and each can contain as many classes as you like.amount . This prevents the two names from clashing. } public decimal GetBalance () { return balance.e.Account(). This is because we have not explicitly set up a namespace in our source files. } } } } I have used the namespace keyword to identify the namespace. A name like this. they are very easy to set up: namespace CustomerBanking { public class Account { private decimal balance = 0.Account test. } else { return false . with the namespace in front. If we want to create an instance of the class we use the fully qualified name again: test = new CustomerBanking. This is followed by a block of classes. is known as a fully qualified name. } public bool WithDrawFunds ( decimal amount ) { if ( amount < 0 ) { return false . one created outside any namespace) can just be referred to by its name. in this case CustomerBanking. If you want to use a class from a namespace you have to give the namespace as well. This creates a variable which can refer to instances of the Account class. Putting a Class in a Namespace Up until now every name we have used has been created in what is called the global namespace.Advanced Programming Program Organisation EquipmentSupplier namespace as well. public void PayInFunds ( decimal amount ) { balance = balance + amount. However. CustomerBanking. in that they are not defined in the same namespace. return true. Every class declared in this block is regarded as part of the given namespace. The Account class we use is the one in the CustomerBanking namespace. Using a Class from a Namespace A Global class (i. } if ( balance >= amount ) { balance = balance .
  The compiler goes "ah.WriteLine ( "Hello World" ).WriteLine ( "Hello World" ). The System namespace is where a lot of the library components are located.3 Namespaces in Separate Files There is no rule that says you have to put all the classes from a particular namespace in a particular file.Console.RudeLetters . and only one. If it finds one.IO . . .Advanced Programming Program Organisation Using a namespace If you are using a lot of things from a particular namespace C# provides a way in which you can tell the compiler to look in that namespace whenever it has to resolve the name of a particular item. When the compiler sees a line like: Account RobsAccount. The System namespace is like this. We can use these by means of fully qualified names: System. If there is it uses that. it is common for most programs to have the line: using System.at the very top. This means that the programmer can just write: Console. However. You have to make sure that the files you want to use contain all the bits are needed. We have already used this technique a lot. Console then all is well and it uses that. you'd probably figured that one out already. We do this with the using keyword: using CustomerBanking .it automatically looks in the CustomerBanking namespace to see if there is a class called Account. You get to use the items in the System. Of course the namespaces that you use should always be carefully designed. But then again..IO namespace with an appropriate include: using System. It is perfectly OK to spread the classes around a number of different source files.8. there is a namespace which is part of the System namespace which is specifically concerned with Input/Output. I'll go and have a look for a thing called Console in all the namespaces that I've been told to use". and this means of course more planning and organising. Nesting Namespaces You can put one namespace inside another. This allows you to break things down into smaller chunks. C# Programming © Rob Miles 2010 179 .
  and the time taken to deal with them. you will not be supplied with a sequence of steps which cause the fault to appear. if a user reports a fault the evidence may well be anecdotal.RudeLetters. Incidentally.Advanced Programming Debugging Programmer’s Point: Fully Qualified Names are Good There are two ways you can get hold of something. those which always happen and those which sometimes happen. It is very rarely that you will see your program fail because the hardware is faulty. The good news is that if you use a test driven approach to your development the number of faults that are found by the users should be as small as possible. some will always be better than others. and we are going to explore some of these here.9. you should set up a process to deal with faults that are reported. this approach will not make you popular with users. i. Not everyone is good at debugging.9 Debugging Some people are born to debug. the steps taken to manifest it will be well recorded. Also. the way in which you manage your fault reports should be considered at the start of the project. Having said that there are techniques which you can use to ease the process. I know that this makes the programs slightly harder to write. but it means that when I'm reading the code I can see exactly where a given resource has come from. I've been programming for many years and only seen this in a handful of occasions. However. In other words. The two types of Fault Faults split into two kinds. the reason why problems with programs are called bugs is that the original one was caused by an actual insect which got stuck in some contacts in the computer. 5.e. If a program fails as part of a test. If I just see the name OverdraftWarning in the code I have no idea where that came from and so I have to search through all the namespaces that I'm using to find it. you will simply be told "There's a bug in the print routine". Of the two I much prefer the fully qualified name. However. debugging is working very hard to find out how stupid you were in the first place. This is not however how must bugs are caused. There is a strong argument for ignoring a fault report if you have not been given a sequence of steps to follow to make it happen. C# Programming © Rob Miles 2010 180 . The sad thing is that most of the bugs in programs have been put there by programmers. The number of faults that are reported. you can perform the sequence which always causes the bug to manifest itself and then use suitable techniques to nail it (see later). In other words. It is important to stress to users that any fault report will only be taken seriously if it is well documented. Faults are uncovered by the testing process or by users.1 Fault Reporting We have already seen that a fault is something which the user sees as a result of an error in your program. assigned to programmers and tracked very carefully. In a large scale development fault reports are managed.OverdraftWarning) or you can get hold a items in a namespace with using. is valuable information about the quality of the thing that you are making. but there will still be some things that need to be fixed. Faults which always happen are easy. if you formalize (or perhaps even automate) the fault reporting process you can ensure that you get the maximum possible information about the problem. causing it to fail. You can spell out the complete location by using a Fully Qualified Name (CustomerBanking. Programmer’s Point: Design Your Fault Reporting Process Like just about everything else I've ever mentioned. 5.
  even if it is just the cat! The act of explaining the problem can often lead you to deduce the answer. add extra code to prove that what you think is happening is really happening. Explain the problem to someone else . You might track this down to the number of pages printed. some folks are just plain good at finding bugs. for loops which contain code which changes the control variable. invalid loop termination's. The manifestation of a fault may be after the error itself has actually occurred. or the amount of text on the page. If you assume that "the only way it could get here is through this sequence" or "there is no way that this piece of code could be obeyed" you will often be wrong. Look for: use of un-initialised members of classes typographical errors (look for incorrectly spelt variable names. or the loading department turn on the big hoist and inject loads of noise into the mains. What this means is that you do not have a definite sequence of events which causes the system to fail. This is particularly important if the bug is intermittent. If the fault changes in nature this indicates problems with program or data being corrupted. It is best if the person you are talking to is highly sceptical of your statements. 5. Surprisingly. for example a program may fail when an item is removed from a database. errors which overwrite memory will corrupt different parts of the program if the layout of the code changes.e. where you put in extra print statements to find out more about the problem. This can lead to the most annoying kind of fault.2 Bugswatting You can split faults into other categories. If the bug appears on Friday afternoons on your UNIX system. find out if another department uses the machine to do a payroll run at that time and fills up all the scratch space. your first test is to change the code and data layout in some way and then re-run the program. incorrect comparison operators. or in code which overwrote the memory where the database lives. but the error may be in the routine which stored the item. Rather than make assumptions. Here are some tips: Don't make any assumptions. and the problem promptly disappears! If you suspect such an error. A fault may change or disappear when the program itself is changed. or the size of the document which is being edited when the print is requested. do – while constructions which never fail. improperly terminated comments) logical errors (look for faults in the sequence of instructions. this can be harder to find. methods that call themselves by mistake and recurse their way into stack overflow an exception that is not caught properly If your program does the wrong thing. wrongly constructed logical conditions) As I said above. This does not mean that there is no sequence (unless you are suffering from a hardware induced transient of some kind – which is rather rare) but that you have not found the sequence yet. An example would be a print function which sometimes crashes and works at other times. Look at all possible (if seemingly unrelated) factors in the manifestation of the bug. C# Programming © Rob Miles 2010 181 . where the program crashes (i. crashes are often easier to fix. They usually point to: a state which is not catered for (make sure that all selection statements have a default handler which does something sensible and that you use defensive programming techniques when values are passed between modules) programs that get stuck into loops.9.Advanced Programming Debugging Faults which sometimes happen are a pain. stops completely) or when it does the wrong thing.
  5. this is probably the behaviour of a stopper bug. If you have taken some time off from debugging. This is because when people change the program to make one bit of it work the change that they make often breaks other features of the system. or one of your assumptions that it is impossible is wrong! Can you get back to a state where the bug was not present. heaven forbid. Rip it up and start again In some projects it is possible that the effort involved in starting again is less than trying to find out what is wrong with a broken solution that you have created. it is happening. In other words I can write programs that I can guarantee will contain no bugs. just that it will not be perfect. But if it does something like always output the first page twice if you do a print using the Chinese font and a certain kind of laser printer this might be regarded as a problem most users could live with. Alternatively. I have found statistics which indicate that "two for one" is frequently to be expected. I start introducing bugs. with inputs. or sometimes destroys the filestore of the host computer. explained the code to a friend and checked all your assumptions then maybe. Part of the job of a project manager in a development is deciding when a product is good enough to sell. Such efforts are always doomed. I have been nearly moved to tears by the sight of people putting in another loop or changing the way their conditions to "see if this will make the program work". This means that you need to evaluate the impact of the faults that get reported to you. Go off and do something different and you may find that the answer will just appear. This does not mean that every program that I write is useless. in that every bug fix will introduce two brand new bugs.9. This means that either the impossible is happening. When considering faults you must also consider their impact. A stopper is a fault which makes the program un-saleable. Programmer’s Point: Bug Fixes Cause Bugs The primary cause of bugs is probably the bug fixing process. just maybe this might be the best way forward. and whether or not a fault in the code is a "stopper" or not. One of the rules by which I work is that "any useful program will have bugs in it". and then introduce the changes until the bug appears. just like throwing a bunch of electrical components at the wall and expecting a DVD player to land on the floor is also not going to work. Alternatively you may find the answer as soon as you come back to the problem. If the program crashes every third time you run it. outputs and some behaviours. in that it can tell you exactly what changes have been made from one version to the next.Advanced Programming Debugging Leave the problem alone for a while. and then look at the changes made since? If the system is failing as a result of a change or. A good Source Code Control System is very valuable here.3 Making Perfect Software There is no such thing as perfect software. This at least makes sure that the fix has not broken anything important. However. try to move back to a point where the bug is not there. such programs will be very small and therefore not be good for much. The only way round this is to make sure that your test process (which you created as a series of lots of unit tests) can be run automatically after you've applied the fix. You also need to be aware of the C# Programming © Rob Miles 2010 182 . a bug fix. One thing that I should make clear at this point is that the process of debugging is that of fixing faults in a solution which should work. In other words you must know how the program is supposed to work before you try and fix problems with what it actually does. Remember that although the bug is of course impossible. As soon as I create a useful program. look carefully at how the introduction of the feature affects other modules in the system. prioritise them and manage how they are dealt with.
  as it covers the behaviours of a programmer very well indeed: Further Reading Code Complete Second Edition: Steve McConnell Published by Microsoft: ISBN 0-7356-1967-0 Not actually a book about C#. And I've never stopped programming. However.10 The End? This is not all you need to know to be a programmer. but there are quite a few things missing from this text.10. More a book about everything else.edu/howto/HowToBeAProgrammer.mines. 5. 5.10. If you are serious about this business you should be reading at least one book about the subject at any one time. I have been programming for as long as I can remember but I have never stopped learning about the subject. The key to making software that is as perfect as possible is to make sure that you have a good understanding of the problem that you are solving. For example. You should take a look at the following things if you want to become a great C# programmer: serialisation attributes reflection networking 5. How to be a programmer This web site is also worth a read.1 Continuous Development A good programmer has a deliberate policy of constantly reviewing their expertise and looking at new things. If you have any serious intention to be a proper programmer you should/must read/own this book. because we don't have time to teach them all. it is quite a good start. a fault in a video game is much less of a problem than one in an air traffic control system. Read some of the recommended texts at the end of this document for more on this aspect of programming. that you know how to solve it before you start writing code and that you manage your code production process carefully.Advanced Programming The End? context of the development.html C# Programming © Rob Miles 2010 183 . reading books about programming and looking at other people's code. It covers a range of software engineering and programming techniques from the perspective of "software construction". It is not even all you need to know to be a C# programmer.
  but it does not say how they are to be realised. Whenever you need to collect a number of things into a single unit you should think in terms of creating a class. When the end of the method. It is used in a constructor of a child class to call the constructor in the parent. Note that if the thing being given access to is managed by reference the programmer must make sure that it is OK for a reference to the object is passed out. This means that it is a member of the receipt family (i. Base base is a C# keyword which has different meanings depending on the context in which it is given. In the case of component design an abstract class contains descriptions of things which need to be present. cheque receipt. but you can use it as the basis of. We can therefore create an abstract Receipt class which serves as the basis of all the concrete ones. it can be treated as a Receipt) but it works in its own way. C# Programming © Rob Miles 2010 184 . An accessor is implemented as a public method which will return a value to the caller. Call When you want to use a method. starting at the first statement in its body. abstract one.Glossary of Terms The End? 6 Glossary of Terms Abstract Something which is abstract does not have a "proper" existence as such. It can be used to represent a real world item in your program (for example bank account). or template for. we may decide that we need many different kinds of receipt in our transaction processing system: cash receipt. is reached the sequence of execution returns to the statement immediately following the method call. you call it. Each "real" receipt class is created by extending the parent.e. You can't make an instance of an abstract class. or if it contains one or more method which is marked as abstract. Class A class is a collection of behaviours (methods) and data (properties). It is also used in overriding methods to call the method which they have overridden. For example. When a method is called the sequence of execution switches to that method. When writing programs we use the word to mean "an idealised description of something". If the object is not to be changed it may be necessary to make a copy of the object to return to the caller. wholesaler receipt etc. We don't know how each particular receipt will work inside. Accessor An accessor is a method which provides access to the value managed within a class. in that the data is held securely in the class but code in other classes may need to have access to the value itself. or the return statement. but we do know those behaviours which it must have to make it into a receipt. In C# terms a class is abstract if it is marked as such. Effectively the access is read only. a concrete one.
  C# Programming © Rob Miles 2010 185 . This means that rather than being thought of in terms of what it is (for example a BabyCustomerAccount) it is thought of in terms of what it can do (implement the IAccount interface to pay in and withdraw money). which produces the executable file which is later run by the host. A collection class will support enumeration which means that it can be asked to provide successive values to the C# foreach construction. Collection The C# library has the idea of a collection as being a bunch of things that you want to store together. The compiler will produce an executable file which is run. Their interactions are expressed in the interfaces between them. Otherwise the compiler will refuse to compile your program.Glossary of Terms The End? Code Reuse A developer should take steps to make sure that a given piece of program is only written once. When creating a system you should focus on the components and how they interact. takes the source which the user has written and then finds all the individual keywords. This is usually achieved by putting code into methods and then calling them. The final phase is the code generator. Compiler A compiler takes a source file and makes sense of it. the pre-processor. Component A component is a class which exposes its behaviour in the form of an interface. Most compilers work in several phases. Cohesion A class has high cohesion if it is not dependent on/coupled to other classes. rather than repeating the same statements at different parts of a program. Constructor A constructor is a method in a class which is called as a new instance of that class is created. One form of a collection is an array.Collections namespace. You only need to override the methods that you want to update. The collection classes can be found in the System. they used to be written in assembly language but are now constructed in high level languages (like C#!). If a class is a member of a hierarchy. The first phase. identifiers and symbols producing a stream of program source which is fed to the "parser" which ensures that the source adheres to the grammar of the programming language in use. Programmers use constructors to get control when an instance is created and set up the values inside the class. The use of class hierarchies is also a way of reusing code. which allows you to easily find a particular item based on a key value in that item. Whenever you want to store a number of things together you should consider using a collection class to do this for you. it is important when making the child that you ensure the parent constructor is called correctly. Another is the hashtable. A compiler is a large program which is specially written for a particular computer and programming language. and the parent class has a constructor. for example all the players in a football team or all the customers in a bank. Writing compilers is a specialised business.
  is a good example of this. a reference to the instance/class which contains the method and a reference to the method itself. A dependency relationship exists between two classes when a change in code in one class means that you might have to change the other as well. When the event occurs the method is called to deliver notification. Delegates are used to inform event generators (things like buttons. For example a user interface class may be dependent on a business object class (if you add new properties to the business object you will need to update the user interface). Coupling is often discussed alongside cohesion. Delegate A delegate is a type safe reference to a method. timers going tick etc. buttons being pressed. As an example see the discussion of the CustomerAccount and ChildAccount Load method on page 145. Making sure the spec. When a running C# Programming © Rob Miles 2010 186 . Events include things like mouse movement. However. Windows components make use of delegates (a delegate is a type safe reference to a method) to allow event generators to be informed of the method to be called when the event takes place. since it makes it harder to update the system. timers and the like) of the method which is to be called when the event they generate takes place. Exceptions are part of the way that a C# program can deal with errors. However. it is unlikely that changes to the way that the user interface works will mean that the business object needs to be altered. in that you should aim for high cohesion and low coupling. Dependency In general. Dependency is often directional. The fact that a delegate is an object means that it can be passed around like any other. where you try and pick up existing code. It can then be directed at a method in a class which matches that signature. Event An event is some external occurrence which your program may need to respond to. windows being resized. Generally speaking a programmer should strive to have as little coupling in their designs as possible. Many modern programs work on the basis of events which are connected to methods. It usually means that you have not properly allocated responsibility between the objects in your system and that two objects are looking after the same data. Exception An exception is an object that describes something bad that has just happened. too much dependency in your designs is a bad thing. keys being hit. A delegate is created for a particular method signature (for example this method accepts two integers and returns a float). structuring the design so that you can get someone else to do a lot of the work is probably the best example of creative laziness in action. Code reuse. Creative Laziness It seems to me that some aspects of laziness work well when applied to programming. Note that the delegate instance holds two items. is right before you do anything is another way of saving on work.Glossary of Terms The End? Coupling If a class is dependent on another the two classes are said to be coupled.
  whether the exception // was thrown or not } A try – catch construction can also contain a finally clause. The precise path followed depends on the nature of the job and the techniques in use at the developer. Extending the child produces a further level of hierarchy. Immutable An immutable object cannot be changed. amongst other things. all developments must start with a description of what the system is to do.Glossary of Terms The End? program gets to a position where it just can’t continue (perhaps a file cannot be opened or an input value makes no sense) it can give up and ―throw‖ an exception: throw new Exception("Oh Dear").. The Exception object contains a Message property that is a string which can be used to describe what has gone wrong. The classes at the top of the hierarchy should be more general and possibly abstract (for example BankAccount) and the classes at the lower levels will be more specific (for example ChildBankAccount).. GUID creation involves the use of random values and the date and time. which contains code that is executed whether or not the exception is thrown. however. or FDS. Most operating systems and programmer libraries provide methods which will create GUIDs. If the exception is not ―caught‖ the program will end at that point. and is often called the Functional Design Specification. Functional Design Specification Large software developments follow a particular path. It gives an identifier by which something can be referred to. This is the most crucial item in the whole project. In the above example the message would be set to ―Oh Dear‖.. You can make a program respond to exceptions by enclosing code that might throw an exception in a try – catch construction. from the initial meeting right up to when the product is handed over.
  A class which implements an interface must contain code for each of the methods. If a method is public it can be called by code other classes. Methods are used to break a large program up into a number of smaller units. Metadata must be gathered by the programmer in consultation with the customer when creating a system. Interface An interface defines a set of actions. as long as it implements the interface it can be thought of purely in terms of that ability. Each particular range of computer processors has its own specific machine code. The difference between a library and a program is that the library file will have the extension . Interfaces make it possible to create components. Machine code Machine Code is the language which the processor of the computer actually understands. The fact that the age value is held as an integer is metadata. A message is delivered to an object by means of a call of a method inside that object. This gives strings a behaviour similar to value types. Inheritance Inheritance is the way in which a class extends a parent to allow it to make use of all the behaviours and properties the parent but add/customise these for a slightly different requirement. We don't care precisely what the component is. Member A member of a class is declared within that class. The method has a particular name and may return a value. It operates at all kinds of levels. which makes them easier to use in programs.dll (dynamic link library) and will not contain a main method. The fact that it cannot be negative is more metadata. A public method is how an object exposes its behaviours. C# Programming © Rob Miles 2010 188 . It contains a number of very simple operations. For more detail see the description of hierarchy. They are also used to allow the same piece of program to be used in lots of places in a large development. It can either do something (a method) or hold some data (variable).Glossary of Terms The End? remains in memory. Methods are sometimes called behaviours. The actions are defined in terms of a number of method definitions. It may also accept a parameter to work on. A class which implements an interface can be referenced purely in terms of that interface. Library A library is a set of classes which are used by other programs. each of which performs one part of the task. or add one to an item in the processor. The string class is immutable. Method A method is a block of code preceded by a method signature. for example move an item from the processor into memory. Data members are sometimes called properties. which means that machine code written for one kind of machine cannot be easily used on another. Metadata Metadata is data about data.
  Private A private member of a class is only visible to code in methods inside that class. The programmer can then provide methods or C# properties to manage the values which may be assigned to the private members. This may entail providing updated versions of methods in the class. The only reason for not making a data member private is to remove the performance hit of using a method to access the data. Portable When applied to computer software. This is implemented in the form of a public method which is supplied with a new value and may return an error code. year information or by a text string or by a single integer which is the number of days since 1st Jan. Namespace A namespace is an area within which a particular name has a particular meaning. month. for example a date can be set by providing day. A portable application is one which can be transferred to a new processor or operating system with relative ease. not the overridden one in the parent. A fully qualified name of a resource is prefixed by the namespace in which the name exists. High Level languages tend to be portable. C# provides the using keyword to allow namespaces to be "imported" into a program. Namespaces let you reuse names. overloaded. Three different. In that case the SetDate method could be said to have been overloaded. each with the same name. the new method is called. The change will hopefully be managed. in that invalid values will be rejected in some way. A programmer creating a namespace can use any name in that namespace. C# Programming © Rob Miles 2010 189 . Methods are overloaded when there is more than one way of providing information for a particular action. A namespace can contain another namespace. When the method is called on instances of the child class. the more portable something is the easier it is to move it onto a different type of computer. You do this by creating a child class which extends the parent and then overriding the methods which need to be changed. Note that a namespace is purely logical in that it does not reflect where in the system the items are physically located. Computers contain different kinds of processors and operating systems which can only run programs specifically written for them. allowing hierarchies to be set up. Override Sometimes you may want to make a more specialized version of an existing class.Glossary of Terms The End? Mutator A mutator is a method which is called to change the value of a member inside an object. it just gives the names by which they are known. methods could be provided to set the date. You can use the base keyword to get access to the overridden method if you need to. It is conventional to make data members of a class private so that they cannot be changed by code outside the class. machine code is much harder to transfer. Overload A method is overloaded when one with the same name but a different set of parameters is declared in the same class.
  The signature is the name of the method and the type and order of the parameters to that method: void Silly(int a. An example of a property of a BankAccount class would be the balance of the account. 2) . int b) – has the signature of the name Silly and an float parameter followed by an integer parameter. The reference has a particular name. It is conventional to make the method members of a class public so that they can be used by code in other class. . Reference A reference is a bit like a tag which can be attached to an instance of a class. C# uses a reference to find its way to the instance of the class and use its methods and data.0f.would call the first method. Note that the type of the method has no effect on the signature. Public A public member of a class is visible to methods outside the class. One reference can be assigned to another. . This means that the code: Silly(1.Glossary of Terms The End? Property A property is an item of data which is held in an object. Source file You prepare a source file with a text editor of some kind. It lets you designate members in parent classes as being visible in the child classes.would call the second. whereas: Silly(1. int b) – has the signature of the name Silly and two int parameters. void Silly(float a. It is kind of a half way house between private (no access to methods outside this class) and public (everyone has access). The C# language has a special construction to make the management of properties easy for programmers. Static In the context of C# the keyword static makes a member of a class part of a class rather than part of an instance of the class. Another would be the name of the account holder. Signature A given C# method has a particular signature which allows it to be uniquely identified in a program. Protected A protected member of a class is visible to methods in the class and to methods in classes which extend this class. This means that you don’t need to create an C# Programming © Rob Miles 2010 190 . If you do this the result is that there are now two tags which refer to a single object in memory. It is text which you want to pass through a compiler to produce a program file for execution. 2) . A public method is how a class provides services to other classes.
  confusingly. The movement might be to a disk file. This means that the first element in the array must have a subscript value of 0. It is not managed by reference. Syntax Highlighting Some program editors (for example Visual Studio) display different program elements in different colours. Subscripts in C# always start at 0 (this locates. for use in non-static methods running inside that instance. This means that if you create a four element array you get hold of elements in the array by subscript values of 0. Subscript This is a value which is used to identify the element in an array. It is used in a constructor of a class to call another constructor. You put your program into a test harness and then the program thinks it is in the completed system.2 or 3. C# Programming © Rob Miles 2010 191 . for example interest rates for all the accounts in your bank. Static members are useful for creating class members which are to be shared with all the instances. Structures are useful for holding chunks of related data in single units. It must be an integer value. the first element of the array) and extend up to the size of the array minus 1. They are not as flexible as objects managed by reference. Structures are also passed by value into methods. Note that the colours are added by the editor. to make it easier for the programmer to understand the code. A test harness is very useful when debugging as it removes the need for the complete system (for example a trawler!) when testing. Streams remove the need to modify a program depending on where the output is to be sent or input received from. but they are more efficient to use in that accessing structure items does not require a reference to be followed in the same way as for an object.1. The best way to regard a subscript is the distance down the array you are going to move to get the element that you want. Stream A stream is an object which represents a connection to something which is going to move data for us.Glossary of Terms The End? instance of a class to make use of a static member. and there is nothing in the actual C# source file that determines the colour of the text. to a network port or even to the system console. Structure A structure is a collection of data items. strings in red and comments in green. Test harness The test harness will contain simulations of those portions of the input and output which the system being tested will use. It is also used as a reference to the current instance. Keywords are displayed in blue. and structures are copied on assignment. It also means that static members are accessed by means of the name of their class rather than a reference to an instance. This this is a C# keyword which has different meanings depending on the context in which it is given.
  They assume that just because code has been written to do something. In that case I may want to replace (override) the method in the parent with a new one in the child class. For this to take place the method in the parent class must have been marked as virtual. not afterwards when it has crashed.e. Of course. in that the program must look for any overrides of the method before calling it. that thing must be the right thing. Unit tests should be written alongside the development process so that they can be applied to code just after (or in test drive development just before) the code is written. This is why not all methods are made virtual initially. Some other languages are much more relaxed when it comes to combining things. if you really want to impose your will on the compiler and force it to compile your code in spite of any type safety issues you can do this by using casting. Try to put a float value into an int variable and the compiler will get cross at this point. C# Programming © Rob Miles 2010 192 . I think it is important that developers get all the help they can to stop them doing stupid things. i. C# is very keen on this (as am I). x = y causes the value in y to be copied into x. Unit test A unit test is a small test which exercises a component and ensures that it performs a particular function correctly. Value types are passed as values into method calls and their values are copied upon assignment. Making a method virtual slightly slows down access to it. and a language that stops you from combining things in a way that might not be sensible is a good thing in my book.Glossary of Terms The End? Typesafe We have seen that C# is quite fussy about combining things that should not be combined. Sometimes I may want to extend a class to produce a child class which is a more specialized version of that class. Value type A value type holds a simple value. Note that this is in contrast to reference types where the result of the assignment would make x and y refer to the same instance. Only virtual methods can be overridden. Changes to the value in x will not affect the value of y. This kind of fussiness is called type safety and C# is very big on it. One of these mistakes is the use of values or items in contexts where it is either not meaningful to do this (put a string into a bool) or could result in loss of data or accuracy (put a double into a byte).. and work on the basis that the programmer knows best.
  26 data protection 112 default 70 default constructor 98 delegates 162 pointers 162 Dictionary 137 double 20 B base method 112.. 22 / /* 38 . 124 case 125 Glossary of Terms 193 . 21 { { 20 + + 23 A abstract classes and interfaces 116 methods 115 references to abstract classes 118 accessor 93.Glossary of Terms Index ( () 20. ..
  159 Dispose 161 modal 161 fridge 6 fully qualified name 73 G Generics 134. 98 H hash table 131 Hashtable 132 I identifier 17. 136 global namespace 178 gozzinta 21 graphical user interface 153 GUID 109 N namespace 19.. 52 base method 112 Main 17 overriding 111 replace 113 sealed 114 stopping overriding 114 virtual 111 mutator 91. 53 parenthesis 23 Parse 22 pause 169 plumber 9 pointers 162 print formatting 50 Glossary of Terms 194 .while 43 for 44 while 44 P parameters 22. 177 global 178 nesting 179 separate files 179 using 179 namespaces 73 narrowing 34 nested blocks 58 nesting namespaces 179 new 85. 35 loops 43 break 46 continue 46 do . 31 if 39 immutable 124 information 7 inheritance 109 integers 27 interface abstraction 104 design 108 implementing 106 implementing multiple 108 reference 106 O object class 119 object oriented 15 objects 83. 89.Glossary of Terms operands 33 operators 33 M member protection 91 MessageBox 161 metadata 11 methods 17.
  182 programming languages 13 properties 90. 103 Data Structures are Important 89 Delegates are strong magic 164. 67. 93. 85.. 124 comparison 125 editing 125 immutable 124 Length 125 literal 23 StringBuilder 126 structures 79 accessing 80 defining 79 subscripts 62 switch 68. 94 data 95 methods 96 story telling 37 stream 71 streams 141 StreamWriter 72 string 29. 78 Every Message Counts 153 Flipping Conditions 47 Fully Qualified Names are Good 180 Give Your Variables Sensible Names 32 Good Communicators 13 Great Programmers 16 Importance of Hardware 8 Importance of Specification 10 Interfaces are just promises 109 Internationalise your code 104. 87 parameters 56 to abstract class 118 replacing methods 113 return 53 S scope 76. 87 sealed 114 searching 131 semicolon 21 source files 174 Star Trek 7 statement 17 returning values 49 static 19... 69 case 70 System namespace 19 Glossary of Terms 195 .. 126 in interfaces 128 public 92 punctuation 25 R ReadLine 21 recipie 16 reference 84.Glossary of Terms print placeholders 50 priority 33 private 91.. 26 arrays 61 assignment 32 bool 31 char 29 declaring 26 double 20 float 28 list 20 string 30 structures 79 text 29 types 26 verbatim 30 virtual methods 111 void 19 W widening 34 WriteLine 23 Glossary of Terms 196 .