using System;

​

public class Program

{

    public static void Main()

    {

        var a = new A();

        var b = new B();

        var c = new C();

        b.WriteFrom(c);

    }

​

    class A

    {

        protected internal virtual void Write()

        {

            Console.Write("A");

        }

    }

​

    class B : A

    {

        public void WriteFrom(A other)

        {

            other.Write();

        }

    }

​

    class C : A

    {

        protected internal override void Write()

        {

            Console.Write("C");

        }

    }

}