Tip static methods are also called class methods. However, static elds aren t usually called class in VS .NET

Generating PDF-417 2d barcode in VS .NET Tip static methods are also called class methods. However, static elds aren t usually called class

Tip static methods are also called class methods. However, static elds aren t usually called class
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elds; they re just called static elds (or sometimes static variables).
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Creating a static Field by Using the const Keyword
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By pre xing the eld with the const keyword, you can declare that a eld is static but that its value can never change. const is short for constant. A const eld does not use the static keyword in its declaration but is nevertheless static. However, for reasons that are beyond the scope of this book, you can declare a eld as const only when the eld is an enumeration, a numeric type such as int or double, or a string. (You learn about enumerations in
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Part II
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Understanding the C# Language
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9, Creating Value Types with Enumerations and Structs. ) For example, here s how the Math class declares PI as a const eld:
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class Math { ... public const double PI = 3.14159265358979323846; }
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static Classes
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Another feature of the C# language is the ability to declare a class as static. A static class can contain only static members. (All objects that you create using the class share a single copy of these members.) The purpose of a static class is purely to act as a holder of utility methods and elds. A static class cannot contain any instance data or methods, and it does not make sense to try to create an object from a static class by using the new operator. In fact, you can t actually create an instance of an object using a static class by using new even if you want to. (The compiler will report an error if you try.) If you need to perform any initialization, a static class can have a default constructor as long as it is also declared as static. Any other types of constructor are illegal and will be reported as such by the compiler. If you were de ning your own version of the Math class, one containing only static members, it could look like this:
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public static class Math { public static double Sin(double x) {...} public static double Cos(double x) {...} public static double Sqrt(double x) {...} ... }
Note, however, that the real Math class is not de ned this way because it actually does have some instance methods. In the nal exercise in this chapter, you will add a private static eld to the Point class and initialize the eld to 0. You will increment this count in both constructors. Finally, you will write a public static method to return the value of this private static eld. With this eld, you can nd out how many Point objects have been created.
Write static members, and call static methods c c
1. Using Visual Studio 2008, display the Point class in the Code and Text Editor window.
7
Creating and Managing Classes and Objects
2. Add a private static eld called objectCount of type int to the end of the Point class. Initialize it to 0 as you declare it, like this:
class Point { ...; private static int objectCount = 0; }
Note You can write the keywords private and static in any order. The preferred order is
private rst, static second.
3. Add a statement to both Point constructors to increment the objectCount eld, as shown in bold type in the following code example. Each time an object is created, its constructor is called. As long as you increment the objectCount in each constructor (including the default constructor), objectCount will hold the number of objects created so far. This strategy works only because objectCount is a shared static eld. If objectCount were an instance eld, each object would have its own personal objectCount eld that would be set to 1. The Point class should now look like this:
class Point { public Point() { this.x = -1; this.y = -1; objectCount++; } public Point(int x, int y) { this.x = x; this.y = y; objectCount++; } private int x, y; private static int objectCount = 0; }
Notice that you cannot pre x static elds and methods with the this keyword because they do not belong to the current instance of the class. (They do not actually belong to any instance.) The question now is this: How can users of the Point class nd out how many Point objects have been created At the moment, the objectCount eld is private and not available outside the class. A poor solution would be to make the objectCount eld
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