Autocompleting console input Yesterday I stumbled upon a Stack Overflow question that asked if it was possible to implement Tab-triggered auto-completion in a console application.

I thought that was an interesting idea, so I went ahead and wrote a small program that does just that, given a hard-coded string[] - I might eventually refactor it into its own class and reuse it in my own projects if I ever actually need auto-completion in a console app, but before I do that I'd like some feedback on the way it's implemented, given static aside, the logic itself is pretty much the way I'll have it regardless of whether it's in a dedicated class as part of something much bigger, or right there in a console application that does nothing but verify that the code works.

Pressing Tab when there's no input will change the current line to say Bar; because there's a word in data that starts with Bar, pressing Tab again will autocomplete to Barbec; another Tab will make it Barbecue, and then any subsequent Tab will have no effect, because nothing in the data starts with Barbecue - but then you could Backspace until the input is Ba, type a t to make it Bat, and when you press Tab then it autocompletes to Batman.

In other words, it all works exactly as it should. But does it look right?

class Program
{
    static void Main(string[] args)
    {
        var data = new[]
        {
            "Bar",
            "Barbec",
            "Barbecue",
            "Batman",
        };

        var builder = new StringBuilder();
        var input = Console.ReadKey(intercept:true);

        while (input.Key != ConsoleKey.Enter)
        {
            if (input.Key == ConsoleKey.Tab)
            {
                HandleTabInput(builder, data);
            }
            else
            {
                HandleKeyInput(builder, data, input);
            }

            input = Console.ReadKey(intercept:true);
        }
        Console.Write(input.KeyChar);
    }

    /// &lt;remarks&gt;
    /// https://stackoverflow.com/a/8946847/1188513
    /// &lt;/remarks&gt;&gt;
    private static void ClearCurrentLine()
    {
        var currentLine = Console.CursorTop;
        Console.SetCursorPosition(0, Console.CursorTop);
        Console.Write(new string(' ', Console.WindowWidth));
        Console.SetCursorPosition(0, currentLine);
    }

    private static void HandleTabInput(StringBuilder builder, IEnumerable&lt;string&gt; data)
    {
        var currentInput = builder.ToString();
        var match = data.FirstOrDefault(item =&gt; item != currentInput &amp;&amp; item.StartsWith(currentInput, true, CultureInfo.InvariantCulture));
        if (string.IsNullOrEmpty(match))
        {
            return;
        }

        ClearCurrentLine();
        builder.Clear();

        Console.Write(match);
        builder.Append(match);
    }

    private static void HandleKeyInput(StringBuilder builder, IEnumerable&lt;string&gt; data, ConsoleKeyInfo input)
    {
        var currentInput = builder.ToString();
        if (input.Key == ConsoleKey.Backspace &amp;&amp; currentInput.Length &gt; 0)
        {
            builder.Remove(builder.Length - 1, 1);
            ClearCurrentLine();

            currentInput = currentInput.Remove(currentInput.Length - 1);
            Console.Write(currentInput);
        }
        else
        {
            var key = input.KeyChar;
            builder.Append(key);
            Console.Write(key);
        }
    }
}
Testing


Personally I think it’s conceptually easier to test this code if you are returning a string – rather than a void: what if you decided to change the out put device from a console window to your printer – it would not be easy to change that. What if sometimes you wanted a console window and other times you wanted to send the same thing via text message? I guess it’s easier to separate what is being done vs how it is being done. E.g. Output key is the abstract concept but this could be done a number of different ways: (i) to the console, or (ii) through a voice synthesiser etc. I wrote some tests but then i threw it once it broke when i changed the API.


Naming


What is data? It’s harder to think of a more abstract name. I’ve changed this to keywords.
Personally I like to see the type along with the variable declaration.


Refactoring the code:


I simplified and extracted some methods.
The The HandleKeyInput and the HandleTabInput both seem to be doing conceptually similar things: a key is inputted and then a result is obtained. If it’s a tab then do X, but if it’s a key then do Y. There’s a common abstraction here that can be extracted.
What I want to do is to make the two methods: HandleTabInput and HandleKeyInput to look and be absolutely identical.
Data (renamed to keywords) can be a field so there’s no need to pass it in as a variable.
I want to make two of your methods to basically be the same. They’re kinda doing this same things. After some refactoring this is what I got:

public void HandleTabInput(StringBuilder builder, ConsoleKeyInfo keyInput)
{
        // Perform calculation
    string match = ExtractMatch(builder);

        // Alter the builder
    builder.Clear();
    builder.Append(match);

        // Print Results
    PrintMatch(match);
}



And then this:

    private void HandleKeyInput(StringBuilder builder, ConsoleKeyInfo keyInput)
    {
        if (keyInput.Key == ConsoleKey.Backspace &amp;&amp; builder.ToString().Length &gt; 0)
        {
                // Perform Calculation (nothing here)

                // Alter the builder
            builder.Remove(builder.Length - 1, 1);

                // Print Results
            ClearCurrentLine();                
            Console.Write(builder.ToString().Remove(builder.ToString().Length - 1));
        }
        else
        {
                // Perform calculation (nothing here)

                // Alter the Builder
            var key = keyInput.KeyChar;
            builder.Append(key);

                // Print Reuslts
            Console.Write(key);
        }
    }


We can abstract out the calculation, and then the printing. After some work this was the result. It’s starting to look pretty similar:

    private void HandleKeyInput(StringBuilder builder, ConsoleKeyInfo keyInput)
    {
        if (keyInput.Key == ConsoleKey.Backspace &amp;&amp; builder.ToString().Length &gt; 0)
        {
            Program.KeyInput backSpaceKey = new Program.KeyInput.BackspaceInput(builder, keywords);
            backSpaceKey.AlterBuilder();
            backSpaceKey.PrintResult();                
        }
        else
        {
            KeyInput input = new KeyInput.StandardKeyInput(builder, keywords, keyInput.KeyChar);
            input.AlterBuilder();
            input.PrintResult();
        }
    }


You could probably tidy things up a little more.

The final Result:

    public void RunProgram()
    {
        StringBuilder builder = new StringBuilder();
        ConsoleKeyInfo capturedCharacter = Console.ReadKey(intercept: true);

        while (EnterIsNotThe(capturedCharacter))
        {
            KeyInput key = KeyInput.GetKey(builder, keywords, capturedCharacter);
            builder = key.UpdateBuilder();
            key.Print();

            capturedCharacter = Console.ReadKey(intercept: true);
        }

        Console.Write(capturedCharacter.KeyChar);
    }


Concluding Thoughts


It reads marginally better. Still If you want to reuse the code you’ll have to make changes, but at least it will be a little easier to do when the time comes.
Encapsulation could be a little better – typically speaking you want the KeyInput class to be instantiated via the factory only.


A link to see the full code.