0.5 Xna0108-inheritance in c#  (Page 3/7)

An orderly hierarchy

The C# inheritance mechanism allows you build an orderly hierarchy of classes to supplement the classes that are already in the class library.

When several of your abstract data types have characteristics in common, you can design their commonalities into a single superclass and separate theirunique characteristics into unique subclasses. This is one of the purposes of inheritance.

The airship hierarchy

For example, suppose you are building a program dealing with airships. All airships have altitude and range properties in common. Therefore, you couldbuild a base Airship class containing data and methods having to do with range and altitude.

From this superclass, you could derive a Balloon class and an Airplane class.

The Balloon class

The Balloon class might add properties and methods dealing with passenger capacity and what makes it go up (helium, hydrogen, or hot air) . Objects of the Balloon class would then be able to deal with altitude, range, passenger capacity, and what makes it go up.

The Airplane class

The Airplane class might add properties and methods dealing with engine type (jet or propeller) and cargo capacity. Objects of the Airplane class could then deal with altitude, range, engine type, and cargo capacity.

Three types of objects

Having created this hierarchy of classes, you could instantiate objects of type Airship , Balloon , and Airplane with the objects of each type having properties and methods to deal with those specialcharacteristics of the flying machine indicated by the name of the class.

From the general to the specialized

You may have noticed that in this hierarchical class structure, inheritance causes the structure to grow in a direction from most general to morespecialized. This is typical.

Single and multiple inheritance

C++ and some other object-oriented programming languages allow for multiple inheritance. This means that a new class can extend more than one superclass.This has advantages in some cases, but can lead to difficulties in other cases.

C# does not support multiple inheritance. Instead it supports a different mechanism called an interface that provides most of the benefits ofmultiple inheritance without most of the problems. I will explain the C# interface in a future module.

The ISA relationship

You will sometimes hear people speak of the ISA relationship when discussing OOP (such as in he is a hero) . The source of this terminology is more fundamental than you may at first suspect.

Object-oriented designers often strive to use inheritance to model relationships where a subclass "is a kind of" the superclass. For example, a car "is a kind of" vehicle. A programmer "is a kind of" employee which in turn "is a kind of" person. An airplane "is a kind of" airship and so is a hot-air balloon.

This relationship is called the ISA relationship. It's that simple.

The HASA relationship

If you were to define a class that more fully represents an airplane, you might choose to break certain parts of the airplane out into separate objectsand to incorporate them by reference into your Airplane class.