Question

I’m trying to call an extension method on my own class, but it fails to compile. Consider the following lines of code:

public interface IHelloWorld
{
}

public static class Extensions
{
    public static string HelloWorld(this IHelloWorld ext)
    {
        return "Hello world!";
    }
}

public class Test : IHelloWorld
{
    public string SaySomething()
    {
        return HelloWorld();
    }
}

Basically I’m extending on the interface. I keep getting this error:

The name 'HelloWorld' does not exist in the current context

Can anybody explains this to me? When I do a cast all seems well:

return ((Test)this).HelloWorld();

Any explanations?

Answer 1

The cast isn’t necessary – the this part is. So this works fine:

return this.HelloWorld();

Section 7.6.5.2 explicitly talks about method invocations of the form

expr.identifier ( )
expr.identifier ( args )
expr.identifier < typeargs > ( )
expr.identifier < typeargs > ( args )

This invocation:

HelloWorld()

isn’t of that form, as there’s no expression involved.

It’s not immediately clear to me why the language was designed that way (i.e. why the “implicit this” was excluded) and maybe Eric Lippert will add an answer to that effect later. (The answer may well be along the lines of “because it would have taken a long time to spec, implement and test, for relatively little benefit.”) However, this answer at least shows that the C# compiler is sticking to the spec…