barcode vb.net source code 11: Inheritance and Polymorphism in C#.NET

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11: Inheritance and Polymorphism
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There are two powerful aspects to inheritance. One is code reuse. When you create a ListBox class, you re able to reuse some of the logic in the base (Control) class. What is arguably more powerful, however, is the second aspect of inheritance: polymorphism. Poly means many and morph means form. Thus, polymorphism refers to being able to use many forms of a type without regard to the details. When the phone company sends your phone a ring signal, it does not know what type of phone is on the other end of the line. You might have an old-fashioned Western Electric phone that energizes a motor to ring a bell, or you might have an electronic phone that plays digital music. As far as the phone company is concerned, it knows only about the base type phone and expects that any derived instance of this type knows how to ring. When the phone company tells your phone to ring, it, effectively, calls your phone s ring method, and old-fashioned phones ring, digital phones trill, and cutting-edge phones announce your name. The phone company doesn t know or care what your individual phone does; it treats your telephone polymorphically.
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Because a ListBox is a Control and a Button is a Control, you expect to be able to use either of these types in situations that call for a Control. For example, a form might want to keep a collection of all the derived instances of Control it manages (buttons, lists, and so on) so that when the form is opened, it can tell each of its Controls to draw itself. For this operation, the form does not want to know which elements are ListBoxes and which are Buttons; it just wants to tick through its collection and tell each one to draw. In short, the form wants to treat all its Control objects polymorphically. You implement polymorphism in two steps: 1. Create a base class with virtual methods. 2. Create derived classes that override the behavior of the base class s virtual methods. To create a method in a base class that supports polymorphism, you mark the method as virtual. For example, to indicate that the method DrawControl( ) of class Control in Example 11-1 is polymorphic, add the keyword virtual to its declaration, as follows:
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public virtual void DrawControl( )
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Each derived class is free to inherit and use the base class s DrawControl( ) method as is, or to implement its own version of DrawControl( ). If a derived class does override the DrawControl( ) method, that overridden version will be invoked for each instance of the derived class. You override the base class virtual method by using the keyword
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override in the derived class method definition, and then add the modified code for
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that overridden method. Example 11-2 shows how to override virtual methods. The Control and ListBox classes are back, and they ve brought along a Button class, which also derives from Control.
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using using using using System; System.Collections.Generic; System.Linq; System.Text;
namespace Example_11_2_ _ _ _Polymorphism { public class Control { // these members are protected and thus visible // to derived class methods. protected int top; protected int left; // constructor takes two integers to // fix location on the console public Control (int top, int left) { this.top = top; this.left = left; } // simulates drawing the control public virtual void DrawControl( ) { Console.WriteLine("Control: drawing Control at {0}, {1}", top, left); } } // ListBox derives from Control public class ListBox : Control { private string listBoxContents; // new member variable // constructor adds a parameter // and calls the base constructor public ListBox( int top, int left, string contents) : base(top, left) { listBoxContents = contents; }
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