Trying to emulate covariant return types

CovariantReturnTypes.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace CovariantReturnTypes
{
    interface IA<out This> 
        where This : IA<This>
    {
        This m();
    }

    class A<This> : IA<This>
        where This : A<This>
    {
        public This m()
        {
            return this as This; // this cast is unfortunately needed
        }

        public void top()
        {
            Console.WriteLine("top");
        }
    }
    class A : A<A> { } // non-generic class needed
                       // (cannot instantiate A<A<A<A<...>>> otherwise)

    class B<T> : A<T>
        where T : B<T>
    {
        public void hello()
        {
            Console.WriteLine("B.hello()");
        }
    }
    class B : B<B> {}


    class C<T> : B<T>
        where T : C<T>
    {
    }
    class C : C<C> {}

    
    class Program
    {
        static void Main(string[] args)
        {
            //A<A<A<...>>> a = new A<...>();
            A a = new A();
            B b = new B();
            b.m().hello();

            C c = new C();
            Console.WriteLine(c.m() == c);


            //      common subtype?
            //       /     |     \
            //      /      |      \
            //     /       |       \
            //   IA<C>   IA<B>    IA<A>
            //     |       |        |
            //    A<C>    A<B>     A<A>
            //     |       |        |
            //    B<C>    B<B>      A
            //     |       |
            //    C<C>     B
            //     |
            //     C

            Console.ReadKey();
        }
    }
}