The Stochastic Game
Ramblings of General Geekery

The extension method trick

If you’ve been writing C# code lately you should be pretty familiar with extension methods: it’s a nice compiler feature added to C# 3.0 which lets you “attach” methods to an existing type.

So for instance, if you define the following extension method:

    static class ConsoleExtensions
    {
        public static void WriteReversed(this TextWriter writer, string message)
        {
            writer.WriteLine(new string(message.Reverse().ToArray()));
        }
    }

You can write:

    Console.Out.WriteReversed("hello world!");

You effectively “extended” the TextWriter type with a new WriteReversed method in which you get a pseudo-this reference. Before extension methods, these types of methods would have been in a ConsoleHelper class of some kind – a lot less discoverable for the clients of your library.

Now, the (not so-)peculiar thing about it is that the pseudo-this variable can be null. Indeed, since this whole feature is only syntactic sugar, as they say, it’s just a more elegant way to write the following, which is a plain old function call:

    ConsoleExtensions.WriteReversed(Console.Out, "hello world!");

But it means it would look like you’re calling a method on a null object without getting slapped in the face with the infamous NullReferenceException.

slap!

You just need to add some simple parameter validation code to your extension method:

    static class ConsoleExtensions
    {
        public static void WriteReversed(this TextWriter writer, string message)
        {
            if (writer != null)
                writer.WriteLine(new string(message.Reverse().ToArray()));
        }
    }
    TextWriter writer = null;             // woah, are you crazy!?
    writer.WriteReversed("hello world!"); // woah, this is crazy!!

This trick can be pretty useful if you have an API that aims to be as minimalist as possible for the user and null is a valid case for one of the objects exposed publicly. You can already handle this with various well known techniques like wrapping or the NullObject pattern, but the extension method is a nice new alternative that’s enough in some cases and requires a minimum of code.

I’ve seen that trick used only once so far, in the excellent MvcMiniProfiler:

    using (MiniProfiler.Current.Step("Getting the answer to the ultimate question"))
    {
        var result = DeepThought.Compute();
        ViewBag.UltimateAnswer = result;
    }

Here, it’s totally OK if you disabled profiling in your web application, which means MiniProfiler.Current would return null. That’s because the Step() method is an extension method that checks for a null value provided as “this”, and doesn’t do much in that case.

Sure, they could have abstracted the profiler behind an IProfiler interface and have the MiniProfiler.Current return a NullProfiler when profiling is disabled, or something like that, but that would have been a lot more code than just using an extension method.