Inheritance in C#
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Inheritance and polymorphism are important concepts in object-oriented programming. In this first of two parts, you will learn about inheritance. This article is excerpted from chapter 11 of
Learning C# 2005, Second Edition, written by Jesse Liberty and Brian MacDonald (O'Reilly, 2006; ISBN: 0596102097). Copyright © 2006 O'Reilly Media, Inc. All rights reserved. Used with permission from the publisher. Available from booksellers or direct from O'Reilly Media.
In Chapter 7, you learned how to create new types by declaring classes, and in Chapter 6, you saw a discussion of the principle object relationships of association, aggregation, and specialization. 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, and a discussion of the root of all classes, the Object class.
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.
One of the most important relationships among objects in the real world is specialization, which can be described as the is-a relationship. 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 to the familiar characteristics of canis domesticus. A cat is also a mammal. As such, we expect it to share certain characteristics with the dog that are generalized in Mammal, but to differ in those characteristics that are specialized in cats.
The specialization and generalization relationships are both reciprocal and hierarchical. Specialization is just the other side of the generalization coin: Mammal generalizes what is common between dogs and cats, and dogs and cats specialize mammals to their own specific subtypes.
These relationships are hierarchical because they create a relationship tree, with specialized types branching off from more generalized types. As you move “up” the hierarchy, you achieve greater generalization. You move up toward Mammal to generalize that dogs, cats, and horses all bear live young. As you move “down” the hierarchy you specialize. Thus, the cat specializes Mammal in having claws (a characteristic) and purring (a behavior).
Similarly, when you say thatListBoxandButtonare Windows, you indicate that there are characteristics and behaviors of Windows that you expect to find in both of these types. In other words, Window generalizes the shared characteristics of bothListBoxandButton, while each specializes its own particular characteristics and behaviors.
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 This article is excerpted from chapter 11 of Learning C# 2005, Second Edition, written by Jesse Liberty and Brian MacDonald (O'Reilly, 2006; ISBN: 0596102097). Check it out today at your favorite bookstore. Buy this book now.
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