public void DoSomething() { Printer p1 = new Printer(); p1.Print("aaa"); Printer p2 = new RedPrinter(); p2.Print("bbb"); GreenPrinter p3 = new GreenPrinter(); p3.Print("ccc"); BluePrinter p4 = new BluePrinter(); (p4 as Printer).Print("ddd"); Console.ReadKey(); }