using System;

public class A {

    public void f1() {

    Console.WriteLine("A—->f1");

    }

}

​

public class B: A{

    public void f1(){

        Console.WriteLine("B—->f1");

    }

}

​

public class Program

{

    public static void Main()

    {       

        A b = new B();

        B a1 =  (B)new A();

        b.f1();

        a1.f1();

    }

}