C# 5.0 Programmer's Reference

248 ❘ CHAPTER 11 OOP Concepts
A second analogy compares a class definition to a cookie cutter. After you create the cookie cutter, you
can use it to make any number of cookies (instances). The cookie cutter (class) defines the cookies’ size
and shape. Specific instances might have different properties such as thickness, dough type (chocolate
chip, sugar, gingerbread, and so on), and frosting type (none, single color, and patterned).
Classes, Classes Everywhere
The .NET Framework is full of classes. Every type of control and component (Form,
TextBox, Label, Timer, Window, and so forth) is a class. Their parent classes Control
and Component are classes. The Framework also includes classes that represent containers,
queues, messages, random number generators, web pages, XML documents,
XPS documents, cryptographic functions, printers, operating system features, and all
sorts of other things.
Even Object, from which all other classes derive, is a class. Whenever you work
with any of these (getting or setting properties, calling methods, and responding to
events), you are working with instances of classes. In total, the .NET Framework
contains somewhere on the order of 10,000 public classes.
Actually, every piece of code you write in C# must be contained in a class. There’s
no escaping classes!
Because all classes ultimately derive from the Object class, every instance of every class is in some
sense an Object, so they are often simply called objects. If you don’t know or don’t care about an
item’s class, you can simply refer to it as an object.
Outstanding Objects
When you read the section “Polymorphism” later in this chapter, you’ll see that this
makes technical, as well as intuitive, sense. Because all classes eventually derive from
the Object class, all instances of all classes are actually Objects.
The following sections provide more details about the more important features provided by OOP
languages in general and C# in particular.
Encapsulation
A class’s public interface is the set of properties, methods, and events that are visible to code outside of
the class. The class may also have private properties, methods, and events that it uses to do its job. For
example, the Job class described in the previous section provides an AssignJob method. That method
might call a private FindQualifiedEmployee method that looks through an employee database to
find someone who has the skills and equipment necessary to do the job. That routine is not used outside
of the class, so it can be declared private.
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