using System;

​

    public class A

    {

        public virtual void fun(C c) 

        {

            c.fun(this);

        }

    }

​

    public class B:A

    {

        public override void fun(C c) 

        {

            c.fun(this);

        }

    }

​

    public class C

    {

        public void fun(A a)

        {

            Console.WriteLine("that was A");

        }

​

        public void fun(B b)

        {

            Console.WriteLine("that was B");

        }

    }

​

    public class Program

    {

        public static void Main()

        {

            A a = new A(), b = new B();

            C c = new C();

            a.fun(c);

            b.fun(c);

        }

    }