How does C# resolve extension methods call?

Question

The part of my question is answered in this answer.

However, there is another situation:

public class A { }

public static class ExtendedA
{
    public static void Ext(this A a) { }
}

public static class ExtendedB
{
    public static void Ext(this A a) { }
}

public static class App
{
    public static void Main()
    {
        A a = new A();
        a.Ext();
    }
}

How does the C# compiler chooses the method to call?

@Jamiec : Yes you can, the question is about Extension methods and this is exactly how they work. — Ian, Jul 27 '11 at 07:53
I imagine it won't compile because the call will be ambiguous between the two. — Ian, Jul 27 '11 at 07:53
See http://stackoverflow.com/questions/5283583/extension-methods-conflict — linepogl, Jul 27 '11 at 07:53
@V4Vendetta: It seems I have made an error in my code... I somehow excluded the call, so I didn't receive the error from the compiler. — Kornelije Petak, Jul 27 '11 at 08:04
-1: Your question is invalid. The code doesn't even compile. You could have checked that yourself. — Daniel Hilgarth, Jul 27 '11 at 08:04

score 4 · Answer 1 · answered Jul 27 '11 at 07:54

If you try to compile all that code in the same namespace, you will get a compiler error on a.Ext(), saying that Ext is an ambiguous function call that cannot be resolved. In order to fix this error, you'll either have to move the extension classes into different namespaces and import only one of them, or invoke the method as a regular static one.

Worded as a direct answer to your question: The C# compiler doesn't choose. It forces you to.

score 2 · Answer 2 · answered Jul 27 '11 at 07:54

2

It doesn't -- if the call is completely ambiguous, you get compiler error CS0121:

The call is ambiguous between the following methods or properties: 'ExtendedA.Ext(A)' and 'ExtendedB.Ext(A)'

answered Jul 27 '11 at 07:54

Rowland Shaw

37,700
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score 2 · Answer 3 · answered Jul 27 '11 at 07:55

If you try your own code, it gives following error:

Error 1 The call is ambiguous between the following methods or properties: 'ConsoleApplication3.ExtendedA.Ext(ConsoleApplication3.A)' and 'ConsoleApplication3.ExtendedB.Ext(ConsoleApplication3.A)' c:\temp\trash\ConsoleApplication3\ConsoleApplication3\Program.cs 28 4 ConsoleApplication3

So clearly the compiler tried to resolve the right call like it would have done for overloads but at the end gave up because two methods caused ambiguity.

so, it seems it tries to resolve just like it does for overloaded methods.

score 1 · Answer 4 · answered Jul 27 '11 at 08:13

1

You can't add 2 extension functions with the same signature as this will lead to an ambiguity error as the compiler can't distinguish which one to use.

answered Jul 27 '11 at 08:13

Chris Snowden

4,982
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How does C# resolve extension methods call?

4 Answers4