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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. If you have a reference to a derived class object, you can get a reference to the base class part of the object by casting the reference to the type of the base class by using the cast operator. The cast operator is placed in front of the object reference, and consists of a set of parentheses containing the name of the class being cast to. Casting is covered in detail in 18. The next few sections will cover accessing an object by using a reference to the base class part of the object. We ll start by looking at the two lines of code that follow, which declare references to objects. Figure 7-6 illustrates the code and shows the parts of the object seen by the different variables. The first line declares and initializes variable derived, which then contains a reference to an object of type MyDerivedClass. The second line declares a variable of the base class type, MyBaseClass, and casts the reference in derived to that type, giving 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, because it is looking at it through a reference to the base type. MyDerivedClass derived = new MyDerivedClass(); MyBaseClass mybc = (MyBaseClass) derived; // Create an object. // Cast the reference.
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Figure 7-6. Reference derived can see the entire MyDerivedClass object, while mybc can only see the MyBaseClass part of the object.
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CHAPTER 7 CLASSES AND INHERITANCE
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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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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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