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CHAPTER 6 MORE ABOUT CLASSES
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Instance Constructors
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An instance constructor is a special method that is executed whenever a new instance of a class is created. A constructor is used to initialize the state of the class instance. If you want to be able to create instances of your class from outside the class, you need to declare the constructor public. Figure 6-12 shows the syntax of a constructor. A constructor looks like the other methods in a class declaration, except for the following: The name of the constructor is the same as the name of the class. A constructor cannot return a value.
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Figure 6-12. Constructor declaration For example, the following class uses its constructor to initialize its fields. In this case, it has a field called TimeOfInstantiation that is initialized with the current date and time. class MyClass { DateTime TimeOfInstantiation; ... public MyClass() { TimeOfInstantiation = DateTime.Now; } ... }
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// Field // Constructor // Initialize field
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Note Having finished the section on static properties, take a closer look at the line that initializes
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TimeOfInstantiation. The DateTime class is from the BCL, and Now is a static property of the DateTime class. The Now property creates a new instance of the DateTime class, initializes it with the current date and time from the system clock, and returns a reference to the new DateTime instance.
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CHAPTER 6 MORE ABOUT CLASSES
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Constructors with Parameters
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Like other methods, constructors Can have parameters. The syntax for the parameters is exactly the same as for other methods. Can be overloaded. When creating an instance of a class with the new operator, you are telling it which constructor to use by supplying the class name, along with any actual parameters for the constructor, as the operand to the new operator. For example, in the following code, Class1 has three constructors: one that takes no parameters, one that takes an int, and another that takes a string. Main creates an instance using each one. class Class1 { int MyNumber; string MyName; public Class1() { MyNumber=28; MyName="Nemo"; } // Constructor 0 public Class1(int Value){ MyNumber=Value; MyName="Nemo"; } // Constructor 1 public Class1(String Name) { MyName=Name; } // Constructor 2 public void SoundOff() {Console.WriteLine("MyName {0}, MyNumber {1}", MyName, MyNumber); } } class Program { static void { Class1 a b c
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Main() = new Class1(), = new Class1(7), = new Class1("Bill"); // Call constructor 0. // Call constructor 1. // Call constructor 2.
a.SoundOff(); b.SoundOff(); c.SoundOff(); } } This code produces the following output: MyName Nemo, MyNumber 28 MyName Nemo, MyNumber 7 MyName Bill, MyNumber 0
CHAPTER 6 MORE ABOUT CLASSES
Default Constructors
If no instance constructor is explicitly supplied in the class declaration, then the compiler supplies an implicit, default constructor with No parameters An empty body If the programmer defines any constructor at all, then the compiler does not define any default constructors for the class. For example, Class2 declares two constructors. Because there is at least one explicitly defined constructor, the compiler does not create any additional constructors. In Main, there is an attempt to create a new instance using a constructor with no parameters. Since there is no constructor with zero parameters, the compiler will produce an error message. class Class2 { public Class2(int Value) { ... } public Class2(String Value) { ... } } class Program { static void Main() { Class2 a = new Class2(); ... } }
// Constructor 0 // Constructor 1
// Error! No constructor with 0 parameters
CHAPTER 6 MORE ABOUT CLASSES
Static Constructors
Constructors can also be declared static. While an instance constructor initializes each new instance of a class, a static constructor initializes items at the class level. Generally, static constructors initialize the static fields of the class. Class level items need to be initialized Before any static member is referenced Before any instance of the class is created Like instance constructors The name of the static constructor must be the same as the name of the class. The constructor cannot return a value. Unlike instance constructors Static constructors use the static keyword in the declaration. There can only be a single static constructor for a class, and it cannot have parameters. Static constructors cannot have accessibility modifiers. The following is an example of a static constructor. Notice that its form is the same as that of an instance constructor, but with the addition of the static keyword. class Class1 { static Class1 () { ... } ...
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