Programming C#, Third Edition

Abstract Classes

Every subclass of Window should implement its own DrawWindow( ) method—but nothing requires that it do so. To require subclasses to implement a method of their base, you need to designate that method as abstract.

An abstract method has no implementation. It creates a method name and signature that must be implemented in all derived classes. Furthermore, making one or more methods of any class abstract has the side effect of making the class abstract.

Abstract classes establish a base for derived classes, but it is not legal to instantiate an object of an abstract class. Once you declare a method to be abstract, you prohibit the creation of any instances of that class.

Thus, if you were to designate DrawWindow( ) as abstract in the Window class, you could derive from Window, but you could not create any Window objects. Each derived class would have to implement DrawWindow( ). If the derived class failed to implement the abstract method, that class would also be abstract, and again no instances would be possible.

Designating a method as abstract is accomplished by placing the keyword abstract at the beginning of the method definition, as follows:

abstract public void DrawWindow( );

(Because the method can have no implementation, there are no braces; only a semicolon.)

If one or more methods are abstract, the class definition must also be marked abstract, as in the following:

abstract public class Window

Example 5-3 illustrates the creation of an abstract Window class ...