I am having a design issue regarding .NET properties.interface IX{    Guid Id { get; }    bool IsInvalidated { get; }    void Invalidate();}Problem:This interface has two read-only properties, Id and IsInvalidated. The fact that they are read-only, however, is by itself no guarantee that their values will remain constant.Let's say that it were my intention to make it very clear thatId represents a constant value (which may therefore be safely cached), while IsInvalidated might change its value during the lifetime of an IX object (and so shouldn't be cached).How could I modify interface IX to make that contract sufficiently explicit?My own three attempts at a solution:The interface is already well-designed. The presence of a method called Invalidate() allows a programmer to infer that the value of the similarly-named property IsInvalidated might be affected by it.This argument holds only in cases where the method and property are similarly named.Augment this interface with an event IsInvalidatedChanged:bool IsInvalidated { get; }event EventHandler IsInvalidatedChanged;The presence of a Changed event for IsInvalidated states that this property may change its value, and the absence of a similar event for Id is a promise that that property will not change its value.I like this solution, but it's a lot of additional stuff that may not get used at all.Replace the property IsInvalidated with a method IsInvalidated():bool IsInvalidated();This might be too subtle a change. It's supposed to be a hint that a value is computed freshly each time — which wouldn't be necessary if it was a constant. The MSDN topic Choosing Between Properties and Methods has this to say about it:Do use a method, rather than a property, in the following situations. [] The operation returns a different result each time it is called, even if the parameters do not change.What kind of answers do I expect? I am most interested in entirely different solutions to the problem, along with an explanation how they beat my above attempts.If my attempts are logically flawed or have significant disadvantages not yet mentioned, such that only one solution (or none) remains, I would like to hear about where I went wrong.If the flaws are minor, and more than one solution remains after taking it into consideration, please comment.At the very least, I would like some feedback on which is your preferred solution, and for what reason(s).
How would I design an interface such that it's clear which properties may change their value, and which will remain constant?
c#;design;.net;properties
I would prefer solution 3 over 1 and 2.My problem with solution 1 is: what happens if there is no Invalidate method? Assume an interface with an IsValid property that returns DateTime.Now > MyExpirationDate; you might not need an explicit SetInvalid method here. What if a method affects multiple properties? Assume a connection type with IsOpen and IsConnected properties  both are affected by the Close method.Solution 2: If the only point of the event is to inform developers that a similarly named property may return different values on each call, then I would strongly advice against it. You should keep your interfaces short and clear; moreover, not all implementation might be able to fire that event for you. I'll re-use my IsValid example from above: you would have to implement a Timer and fire an event when you reach MyExpirationDate. After all, if that event is part of your public interface, then users of the interface will expect that event to work.That being said about these solutions: they aren't bad. The presence of either a method or an event WILL indicate that a similarly named property may return different values on each call. All I'm saying is that they alone are not enough to always convey that meaning.Solution 3 is what I'd be going for. As aviv mentioned, this may only work for C# developers though. To me as a C#-dev, the fact that IsInvalidated is not a property immediately conveys the meaning of that's not just a simple accessor, something is going on here. This does not apply to everyone though, and as MainMa pointed out, the .NET framework itself is not consistent here. If you wanted to use solution 3, I would recommend to declare it a convention and have the whole team follow it. Documentation is essential too, I believe. In my opinion it's actually pretty simple to hint at changing values: returns true if the value is still valid, indicates whether the connection has already been opened vs. returns true if this object is valid. It wouldn't hurt at all though to be more explicit in the documentation. So my advice would be:Declare solution 3 a convention and consistenly follow it. Make it clear in the documentation of properties whether or not a property has changing values. Developers working with simple properties will correctly assume them to be non-changing (i.e. only change when object state is modified). Developers encountering a method that sounds like it could be a property (Count(), Length(), IsOpen()) know that someting's going on and (hopefully) read the method docs to understand what exactly the method does and how it behaves.