Programming C#, Third Edition

Chapter 5. Inheritance and Polymorphism

The previous chapter demonstrates how to create new types by declaring classes. The current chapter explores the relationship among objects in the real world and how to model these relationships in your code. This chapter focuses on specialization, which is implemented in C# through inheritance. This chapter also explains how instances of more specialized classes can be treated as if they were instances of more general classes, a process known as polymorphism. This chapter ends with a consideration of sealed classes, which cannot be specialized; abstract classes, which exist only to be specialized; and a discussion of the root of all classes, the class Object.

Tip

VB6 programmers take note: Like VB.NET, C# provides full object-oriented technology, including inheritance, polymorphism, and encapsulation. These are relatively new topics for VB6 programmers. You should study them carefully—they affect your class and application design.

Specialization and Generalization

Classes and their instances (objects) do not exist in a vacuum but rather in a network of interdependencies and relationships, just as we, as social animals, live in a world of relationships and categories.

The is-a relationship is one of specialization. When we say that a Dog is-a mammal, we mean that the Dog is a specialized kind of mammal. It has all the characteristics of any mammal (it bears live young, nurses with milk, has hair), but it specializes these characteristics ...