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CLASSES AND INHERITANCE
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Constructor Execution
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In the preceding chapter, you saw that a constructor executes code that prepares a class for use. This includes initializing both the static and instance members of the class. In this chapter, you saw that part of a derived class object is an object of the base class. To create the base class part of an object, a constructor for the base class is implicitly called as part of the process of creating the instance. Each class in the inheritance hierarchy chain executes its base class constructor before it executes its own constructor body.
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For example, the following code shows a declaration of class MyDerivedClass and its constructor. When the constructor is called, it calls the parameterless constructor MyBaseClass() before executing its own body. class MyDerivedClass : MyBaseClass { MyDerivedClass() // Constructor uses base constructor MyBaseClass(). { ... } Figure 7-11 shows the order of construction. When an instance is being created, one of the first things that is done is the initialization of all the instance members of the object. After that, the base class constructor is called. Only then is the body of the constructor of the class itself executed.
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Figure 7-11. Order of object construction
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CLASSES AND INHERITANCE
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For example, in the following code, the values of MyField1 and MyField2 would be set to 5 and 0, respectively, before the base class constructor is called. class MyDerivedClass : MyBaseClass { int MyField1 = 5; int MyField2; public MyDerivedClass() { ... } } class MyBaseClass { public MyBaseClass() { ... } }
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// 1. Member initialized // Member initialized // 3. Body of constructor executed
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// 2. Base class constructor called
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Caution Calling a virtual method in a constructor is strongly discouraged. The virtual method in the base class would call the override method in the derived class while the base class constructor is being executed. But that would be before the derived constructor s body is executed. It would, therefore, be calling up into the derived class before the class is completely initialized.
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CLASSES AND INHERITANCE
Constructor Initializers
By default, the parameterless constructor of the base class is called when an object is being constructed. But constructors can be overloaded, so a base class might have more than one. If you want your derived class to use a specific base class constructor other than the parameterless constructor, you must specify it in a constructor initializer. There are two forms of constructor initializer: The first form uses the keyword base and specifies which base class constructor to use. The second form uses the keyword this and specifies which other constructor from this class should be used.
A base class constructor initializer is placed after a colon following the parameter list in a class s constructor declaration. The constructor initializer consists of the keyword base and the parameter list of the base constructor to call. For example, the following code shows a constructor for class MyDerivedClass. The constructor initializer specifies that the construction process should call the base class constructor with two parameters, where the first parameter is a string and the second parameter is an int. The parameters in the base parameter list must match the intended base constructor s parameter list, in type and order.
Constructor initializer public MyDerivedClass( int x, string s ) : base( s, x ) { ... Keyword When you declare a constructor without a constructor initializer, it s a shortcut for the form with a constructor initializer consisting of base(), as illustrated in Figure 7-12. The two forms are semantically equivalent.
Figure 7-12. Equivalent forms of a constructor
CLASSES AND INHERITANCE
The other form of constructor initializer instructs the construction process (actually, the compiler) to use a different constructor from the same class. For example, the following shows a constructor with a single parameter for class MyClass. That single-parameter constructor, however, uses a constructor from the same class, but with two parameters, supplying a default parameter as the second one. Constructor initializer public MyClass(int x): this(x, "Using Default String") { ... Keyword } Another situation where this comes in particularly handy is where you have several constructors for a class, and they have common code that should always be performed at the beginning of the object construction process. In this case, you can factor out that common code and place it in a constructor that is used as a constructor initializer by all the other constructors. As a matter of fact, this is a suggested practice since it reduces code duplication. You might think that you could just declare another method that performs those common initializations and have all the constructors call that method. This isn t as good for several reasons. The first is that the compiler can optimize certain things when it knows a method is a constructor. The second is that there are some things that can be done only in a constructor and not elsewhere. For example, in the previous chapter you learned that readonly fields can be initialized only inside a constructor. You will get a compiler error if you attempt to initialize a readonly field in any other method, even if that method is called by a constructor only.
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