csharp

public class A { protected int num; public A(int n) { num = n; }

public int GetNum()
{
    return num;
}

public virtual bool F(A a)
{
    Console.WriteLine("F in A");
    return num == a.num * 2;
}

}// end of class A

public class B : A { public B(int n): base(n) { }

public bool F(B b)
{
  Console.WriteLine("F in B");
    return num == b.num;
}

}// end of class B

public class C : A { public C (int n): base(n) { }

public override bool F(A a)
{
  Console.WriteLine("F in C");
    return a is C && num == ((C)a).num;
}

} //end of class C

public class Driver { public static void Main(string[] args) { A y1 = new B(10); B y2 = new B(10); A z1 = new C(10); C z2 = new C(10); (****) } }