Duck Typing in CSharp

Currently only GetEnumerator and Deconstruct supports duck typing. Dispose is just not available since you the compiler won't be able to know which one to pick if there's duplication.

using System.Collections;

using var foo = new Foo(); // [!code error] 
foreach (var _ in new Foo()) ;
var (a, b) = new Foo();

class Foo;

static class FooExtension
{
    public static void Dispose(this Foo self) => Console.WriteLine("Disposing Foo");
    public static IEnumerator GetEnumerator(this Foo _)
    {
        for (int i = 0; i < 100; i++)
            yield return i;
    }
    public static void Deconstruct(this Foo self, out object? a, out object? b) => a = b = null;
}

If you do want the type has an overall solution for enumeration, just implement a GetEnumerator method. foreach and GetEnumerator now supports duck typing, meaning that:

• An object implemented GetEnumerator can be enumerated by foreach
• The return type of GetEnumerator can be valid as long as it satisfies the shape of IEnumerator<T>

This feature is typically for ref struct.