Building Applications and Components with Visual Basic .NET

Abstract Classes

All classes can be separated into two categories: classes that are creatable and classes that are not creatable. Classes are creatable if they support object instantiation using the New operator. Classes are not creatable if they do not support object instantiation. In object-oriented terminology, creatable classes are referred to as concrete classes, whereas noncreatable classes are referred to as abstract classes.

The term “concrete class” reinforces the notion that you must have a class definition with a concrete implementation to create an object. Conversely, the term “abstract class” implies that the class definition does not have a concrete implementation. Instead, an abstract class defines a programming contract along ...