De ning Classes in Visual C#.NET

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De ning Classes
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When you define a new class, you define the characteristics of all objects of that class, as well as their behaviors. For example, if you create your own windowing operating system, you might want to create screen widgets (known as controls in Windows). One control of interest might be a listbox, a control that is very useful for presenting a list of choices to the user and enabling the user to select from the list. Listboxes have a variety of characteristics: height, width, location, and text color, for example. Programmers have also come to expect certain behaviors of listboxes they can be opened, closed, sorted, and so on. Object-oriented programming allows you to create a new type, ListBox, which encapsulates these characteristics and capabilities. To define a new type or class, you first declare it and then define its methods and fields. You declare a class using the class keyword. The complete syntax is:
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[attributes] [access-modifiers] class identifier [:base-class] {class-body}
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Attributes are used to provide special metadata about a class (that is, information about the structure or use of the class). You won t need to use attributes in this book, but you may run into them if you venture into more advanced topics.
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7: Classes and Objects
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Access modifiers are discussed later in this chapter. (Typically, your classes will use the keyword public as an access modifier.) The identifier is the name of the class that you provide. Typically, C# classes are named with nouns (Dog, Employee, ListBox). The naming convention (not required, but strongly encouraged) is to use Pascal notation. In Pascal notation, you don t use underbars or hyphens, but if the name has two words (Golden Retriever), you push the two words together, each word beginning with an uppercase letter (GoldenRetriever). As we mentioned earlier, inheritance is one of the pillars of object-oriented programming. The optional base class is key to inheritance, as we ll explain in 11. The member definitions that make up the class body are enclosed by open and closed curly braces ({}):
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class Dog { int age; // the dog's age int weight; // the dog's weight Bark( ) { //... } Eat( ) { // ... } }
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Methods within the class definition of Dog define all the things a dog can do. The fields (member variables) such as age and weight describe all of the dog s attributes or state.
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Instantiating Objects
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To make an actual instance, or object, of the Dog class, you must declare the object and allocate memory for the object. These two steps combined are necessary to create, or instantiate, the object. Here s how you do it. First, you declare the object by writing the name of the class (Dog) followed by an identifier (name) for the object or instance of that class:
Dog milo; // declare milo to be an instance of Dog
This is not unlike the way you create a local variable; you declare the type (in this case, Dog), followed by the identifier (milo). Notice also that by convention, the identifier for the object is written in Camel notation. Camel notation is just like Pascal notation except that the very first letter is lowercase. Thus, a variable or object name might be myDog, designatedDriver, or plantManager. The declaration alone doesn t actually create an instance, however. To create an instance of a class, you must also allocate memory for the object using the keyword new:
milo = new Dog( ); // allocate memory for milo
Defining Classes |
You can combine the declaration of the Dog type with the memory allocation into a single line:
Dog milo = new Dog( );
This code declares milo to be an object of type Dog and also creates a new instance of Dog. You ll see what the parentheses are for in Constructors later in this chapter. In C#, everything happens within a class. No methods can run outside a class, not even Main( ). The Main( ) method is the entry point for your program; it is called by the operating system, and it is where execution of your program begins. Typically, you ll create a small class to house Main( ), because like every method, Main( ) must live within a class. Some of the examples in this book use a class named Tester to house Main( ):
public class Tester { public static void Main( ) { //... } }
Even though Tester was created to house the Main( ) method, you ve not yet instantiated any objects of type Tester. To do so, you would write:
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