using System;

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

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

   public class C {
      public static void Main(string[] args) {
         A a1 = new A(); a1.F();
         //B b1 = new B(); b1.F();
         //A a2 = new B(); a2.F();
      }
   }
}