The compiler doesn't find the "closest" extension method

[Chuck]

Test case:

# x-extend.cobra
class A

	pass


class B
	inherits A
	
	pass


class C
	inherits B
	
	pass


extend B

	def n as int
		return 2


extend C

	def n as int
		return 3


class P

	def main is shared
		b = B()
		assert b.n == 2
		c = C()
		assert c.n == 3

Cobra takes the first extension method it can find instead of the closest among all available.

Also, this bug is blocking further extensions being added to Cobra.Lang\ExtendIEnumerable.cobra

[Chuck]

If you find one or more good reference URLs for the C# rules on this, please include them on this ticket.

[Chuck]

Any progress on this?

[Chuck]

I have confirmed that C# takes the closest extension method, not just the first compatible one that appears in the code (as even MSDN docs seem to imply).

// C# 3.0 / VS 2008
using System;

class A {
}

class B : A {   
}

class C : B {
}

static class ExtendB {

    public static int N(this B b) {
        return 2;
    }

}

static class ExtendC {
    
    public static int N(this C c) {
        return 3;
    }

}

class P {

    static void Main() {
        var b = new B();
        Console.WriteLine("b.N() == {0}", b.N());
        var c = new C();
        Console.WriteLine("c.N() == {0}", c.N());
    }

}

That prints 2 and then 3. And to prove that ExtendB is compatible with instances of C, you can comment out ExtendC at which point the program will print 2 and then 2.

[Chuck]

Here is article on extension methods that gives a little more detail and a lot more hints about binding. However, it's about VB and it's not clear that the comments about binding apply to the C# compiler.

•  http://msdn.microsoft.com/en-us/magazine/cc163317.aspx

[Chuck]

The C# 3.0 Spec, section 7.5.5.2 "Extension method invocations" describes what's going on. Basically, all the candidates are located, then overload resolution is applied to the candidate set as described in section 7.4.3 with the first argument of any method being the type that is extended.

Cobra's overload resolution is found in Expr.cobra CallExpr?._computeBestOverload.