Programming C# 3.0, 5th Edition

Chapter 4. Classes and Objects

In Chapter 3, we discussed the myriad 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.

One very powerful example of the use of "programmer defined types" is the .NET Framework that facilitates common tasks, such as interacting with databases and web sites. The Framework provides entire suites of controls, such as those used to interact with users, and those used to display data, as well as other programmer-defined types that manipulate data but have no visible aspect.

It is this ability to use and create powerful new types that characterizes an object-oriented language. You specify a new type in C# by defining a class. (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, (eat), bark, (eat some more), and sleep. A particular dog ...