public class test4 : test2
{
  public test4 ()
  {
    this.name = "test4";
  }

  public override void DoIt()
  {
    // call the base methods DoIt first
    base.DoIt();
    // then do this
    System.Console.WriteLine("test 4 derived" );
  }
}

public class test3 : test2
{
  public test3 ()
  {
    this.name = "test3";
  }

  // override means it has overriden a base class virtual
  // method
  public override void DoIt()
  {
    System.Console.WriteLine("test3 derived" );
  }
}

public class test2
{
  public string name { get; protected set; }
  public test2 ()
  {
    this.name = "test2";
  }

  // virtual means that this method can be overriden
  public virtual void DoIt()
  {
    System.Console.WriteLine("test2 base" );
  }
}

public class test
{
  static void Main ()
  {
    test3 t3 = new test3();
    test4 t4 = new test4();

    System.Console.WriteLine(t3.name);
    t3.DoIt();

    System.Console.WriteLine(t4.name);
    t4.DoIt();
  }
}