C# Extension method precedence

Question

I'm a bit confused about how extension methods work.

If I'm reading this correctly http://msdn.microsoft.com/en-us/library/bb383977.aspx and this If an extension method has the same signature as a method in the sealed class, what is the call precedence? .

Then the following should write out "Instance", but instead it writes "Extension method".

interface IFoo
{
}

class Foo : IFoo
{
    public void Say()
    {
        Console.WriteLine("Instance");
    }
}

static class FooExts
{
    public static void Say(this IFoo foo)
    {
        Console.WriteLine("Extension method");
    }
}

class Program
{
    static void Main(string[] args)
    {
        IFoo foo = new Foo();
        foo.Say();
    }
}

Appreciate any help in clarifying the behavior.

Answer 1

The big difference here is that you have defined an extension method for the IFoo interface, and your foo variable is of type IFoo .

If your code was to look like this:

Foo foo = new Foo();
foo.Say()

The Foo.Say() method would be executed, not the extension method.

I wish I could give you a thorough explanation on why this is but I can only cover the basic mechanism. As your variable was of IFoo type and not of Foo , when the compiler tried to determine what methods were available, it looked past any non-interface methods of the Foo class (as it should). However, the extension method Say() was available, so it invoked this.

Answer 2

In your Main , foo is declared as IFoo . When the compiler looks for a method Say , it finds only the extension method. This is because the instance method is declared in Foo , not in IFoo . The compiler doesn't know that the variable foo happens to contain a instance of Foo ; it just looks at the type the variable is declared of.