What's the actual use of interface?

1. the virtual keyword is used in order to allow a method (or a property) to be implemented in a derived class otherwise than it was in the base class. Doing so, you can call from a derived class the method either in its base class implementation or in the overriden version. To override a method you must use the "override" keyword when implementing it. 2. you use the abstract keyword when declaring a class intended to be a base class for other derived classes. An abstract class cannot be instantiated otherwise than using a derived class member. The methods of an abstract class can also be abstract (and have no actual body, but just the declaration) which involves that all the derived classes must implement those methods. 3. interfaces is somehow the .Net's way to use multiple inheritance. In an interface you can declare methods, properties, events, etc. Then, a class (or a struct) can inherit from one or multiple interfaces, implementing their methods and/or poperties. You can use interfaces to communicate between classes (actually, that's why it's called interface). For sample code, you'll find plenty in VS.Net Help, MSDN or Google.