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CHAPTER 7 CLASSES AND INHERITANCE
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Using References to a Base Class
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An instance of a derived class consists of an instance of the base class, plus the additional members of the derived class. A reference to the derived class points to the whole class object, including the base class part. You can get a reference to the base class part of an object by casting an object reference, to the type of the base class. In the next few sections you will look at accessing an object by using a reference to the base class part of the object. For example, in the following line of code, variable derived contains a reference to an object of type MyDerivedClass. Figure 7-6 illustrates the parts of the object seen by the different variables. Casting the reference in derived to the type of the base class gives a reference to the base class part of the object. The reference to the base class part is stored in variable mybc, on the left side of the assignment operator. The reference to the base class part cannot see the rest of the derived class object. MyBaseClass mybc = (MyBaseClass) derived;
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Figure 7-6. Different views of the same object through a reference to the derived class and a reference to the base class
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CHAPTER 7 CLASSES AND INHERITANCE
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For example, the following code shows the declaration and use of these two classes. Figure 7-7 illustrates the object and references in memory. Main creates an object of type MyDerivedClass and stores its reference in variable derived. Main also creates a variable of type MyBaseClass and uses it to store a reference to the base class portion of the object. When the Print method is called on each reference, the call invokes the implementation of the method that that reference can see, producing different output strings. class MyBaseClass { public void Print() { Console.WriteLine("This is the base class."); } } class MyDerivedClass : MyBaseClass { new public void Print() { Console.WriteLine("This is the derived class."); } } class Program { static void Main() { MyDerivedClass derived = new MyDerivedClass(); MyBaseClass mybc = (MyBaseClass)derived; Cast to base class derived.Print(); // Call Print from derived portion. mybc.Print(); // Call Print from base portion. } } This code produces the following output: This is the derived class. This is the base class.
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Figure 7-7. A reference to the derived class and the base class
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CHAPTER 7 CLASSES AND INHERITANCE
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Virtual and Override Methods
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In the previous section, you saw that when you access an object of a derived class by using a reference to the base class, you get the members from the base class. Virtual methods allow a reference to the base class, to access up into the derived class. You can use a reference to a base class to call a method in the derived class, if the following are true: The method in the derived class and the method in the base class each have the same signature and return type. The method in the base class is labeled virtual. The method in the derived class is labeled override. For example, the following code shows the virtual and override modifiers on the methods in the base class and derived class. class MyBaseClass { virtual public void Print() ... class MyDerivedClass : MyBaseClass { override public void Print() // Base class
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// Derived class
Figure 7-8 illustrates this set of virtual and override methods. Notice how the behavior differs from the previous case, where I used new to hide the base class members. When the Print method is called by using the reference to the base class (mybc), the method call is passed up to the derived class and executed, because of the following: The method in the base class is marked as virtual. There is a matching override method in the derived class. Figure 7-8 illustrates this by showing the arrow coming out the back of the virtual Print method and pointing at the override Print method.
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