Polymorphism microsoft dot net

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 (Window) 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 "instance" of this type knows how to ring. When the phone company tells your phone to ring, it simply expects the phone to "do the right thing." Thus, the phone company treats your phone polymorphically.

Creating Polymorphic Types

Because a ListBox is a Window and a Button is a Window, you expect to be able to use either of these types in situations that call for a Window. For example, a form might want to keep a collection of all the instances of Window it manages so that when the form is opened, it can tell each of its Windows 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 to "draw." In short, the form wants to treat all its Window objects polymorphically. You implement this polymorphism with polymorphic methods.

Creating polymorphic methods

To create a method that supports polymorphism, you need only mark it as virtual in its base class. For example, to indicate that the method DrawWindow() of class Window in is polymorphic, simply add the keyword virtual to its declaration, as follows: 

public virtual void DrawWindow()

Now each derived class is free to implement its own version of DrawWindow(), and the method will be invoked polymorphically. To do so, simply override the base class virtual method by using the keyword override in the derived class method definition, and then add the new code for that overridden method.

Versioning with new and override

In C#, the programmer's decision to override a virtual method is made explicit with the override keyword. This helps you release new versions of your code; changes to the base class will not break existing code in the derived classes. The requirement to use the override keyword helps prevent that problem.

Here's how: assume for a moment that Company A wrote the Window base class of the previous example. Suppose also that the ListBox and RadioButton classes were written by programmers from Company B using a purchased copy of the Company A Window class as a base. The programmers in Company B have little or no control over the design of the Window class, including future changes that Company A might choose to make.

In other object-oriented languages (such as C++), the new virtual Sort() method in Window would now act as a base method for the virtual Sort() method in ListBox. The compiler would call the Sort() method in ListBox when you intend to call the Sort() in Window. In Java, if the Sort() in Window had a different return type, the class loader would consider the Sort() in ListBox to be an invalid override and would fail to load.

C# prevents this confusion. In C#, a virtual function is always considered to be the root of virtual dispatch; that is, once C# finds a virtual method, it looks no further up the inheritance hierarchy. If a new virtual Sort() function is introduced into Window, the runtime behavior of ListBox is unchanged.

When ListBox is compiled again, however, the compiler generates a warning:

...\class1.cs(54,24): warning CS0114: 'ListBox.Sort()' hides inherited member 'Window.Sort()'. To make the current member override that implementation, add the override keyword. Otherwise add the new keyword.

To remove the warning, the programmer must indicate what she intends. She can mark the ListBox Sort() method new to indicate that it is not an override of the virtual method in Window:

public class ListBox : Window { public new virtual void Sort() {...}

This action removes the warning. If, on the other hand, the programmer does want to override the method in Window, she need only use the override keyword to make that intention explicit:

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