Why can you not invoke extension methods directly?

Question

Can someone explain to me why in the following the 3rd invocation of DoSomething is invalid? ( Error message is "The name 'DoSomething' does not exist in the current context" )

public class A { }
public class B : A
{
    public void WhyNotDirect()
    {
        var a = new A();
        a.DoSomething();  // OK
        this.DoSomething();  // OK
        DoSomething(); // ?? Why Not
    }
}
public static class A_Ext
{
    public static void DoSomething(this A a)
    {
        Console.WriteLine("OK");
    }
}

Answer 1

Extension methods are still static methods, not true instance calls. In order for this to work you would need specific context using instance method syntax (from Extension Methods (C# Programming Guide))

In your code you invoke the extension method with instance method syntax. However, the intermediate language (IL) generated by the compiler translates your code into a call on the static method. Therefore, the principle of encapsulation is not really being violated. In fact, extension methods cannot access private variables in the type they are extending.

So while normally, both syntaxes would work, the second is without explicit context, and it would seem that the IL generated can't obtain the context implicitly.

Answer 2

Extension methods can be invoked like other static methods.

Change it to A_Ext.DoSomething(this).

If you're asking why it isn't implicitly invoked on this, the answer is that that's the way the spec was written. I would assume that the reason is that calling it without a qualifier would be too misleading.

Answer 3

Because DoSomething takes a parameter.

DoSomething(a) would be legal.

Edit

I read the question a bit wrong here.

Since your calling it a a normal static method, and not a extension method, you need to prefic with the class name.

So A_Ext.DoSomething(a); will work.

If you call it like a normal static method, all the same rules apply.

Your second variant works because B inhetits A, and therefore you still end up calling it as an extension method, but the third does not.

sorry about the first version above that does not work. I'll leave it to keep the comment relevant.

Answer 4

DoSomething requires an instance of A to do anything, and without a qualifier, the compiler can't see which DoSomething you need to invoke. It doesn't know to check in A_Ext for your method unless you qualify it with this.