Chapter 4. Classes and Objects

Chapter 3 discusses the myriad primitive types built into the C# language, such as int, long, and char. The heart and soul of C#, however, is the ability to create new, complex, programmer-defined types that map cleanly to the objects that make up the problem you are trying to solve.

It is this ability to create new types that characterizes an object-oriented language. You specify new types in C# by declaring and defining classes. You can also define types with interfaces, as you will see in Chapter 8. Instances of a class are called objects. Objects are created in memory when your program executes.

The difference between a class and an object is the same as the difference between the concept of a Dog and the particular dog who is sitting at your feet as you read this. You can't play fetch with the definition of a Dog, only with an instance.

A Dog class describes what dogs are like: they have weight, height, eye color, hair color, disposition, and so forth. They also have actions they can take, such as eat, walk, bark, and sleep. A particular dog (such as my dog Milo) has a specific weight (62 pounds), height (22 inches), eye color (black), hair color (yellow), disposition (angelic), and so forth. He is capable of all the actions of any dog (though if you knew him you might imagine that eating is the only method he implements).

The huge advantage of classes in object-oriented programming is that they encapsulate the characteristics and capabilities of an entity in a single, self-contained, and self-sustaining unit of code. When you want to sort the contents of an instance of a Windows list box control, for example, tell the list box to sort itself. How it does so is of no concern; that it does so is all you need to know. Encapsulation, along with polymorphism and inheritance, is one of three cardinal principles of object-oriented programming.

An old programming joke asks, how many object-oriented programmers does it take to change a light bulb? Answer: none, you just tell the light bulb to change itself. (Alternate answer: none, Microsoft has changed the standard to darkness.)

This chapter explains the C# language features that are used to specify new classes. The elements of a class its behaviors and properties are known collectively as its class members. This chapter will show how methods are used to define the behaviors of the class, and how the state of the class is maintained in member variables (often called fields). In addition, this chapter introduces properties, which act like methods to the creator of the class but look like fields to clients of the class.