using System;

public class A {
   protected virtual void foo() {
      Console.WriteLine("a.foo() called.");
   }
};

public class B : A {
   protected override void foo() {
      Console.WriteLine("b.foo() called.");
   }
   public void call_foo() {
       foo();
   }
};

public class MainClass  {
   public static void Main(string[] args) {
      A a = new A();
      B b = new B();    
      b.call_foo();
   }
};