using System;

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            D d = new D();
            A a = d;
            B b = d;
            C c = d;
            a.F();
            b.F();
            c.F();
            d.F();

            Console.ReadKey();
        }

    }

    class A
    {
        public virtual void F()
        {
            Console.WriteLine("A.F ");
        }
    }

    class B : A
    {
        public override void F()
        {
            Console.WriteLine("B.F ");
        }
    }

    class C : B
    {
        public virtual void F()
        {
            Console.WriteLine("C.F ");
        }
    }

    class D : C
    {
        public override void F()
        {
            Console.WriteLine("D.F ");
        }
    }
}