11.3 Polymorphism

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.

11.3.1 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.

11.3.1.1 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 Example 11-1 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.

In the following excerpt from Example 11-2 (which appears later in this section), ListBox derives from Window and implements its own version of DrawWindow():

public override void DrawWindow()
{
   base.DrawWindow();  // invoke the base method
   Console.WriteLine ("Writing string to the listbox: {0}", 
      listBoxContents);
}

The keyword override tells the compiler that this class has intentionally overridden how DrawWindow() works. Similarly, you'll override this method in another class, Button, also derived from Window.

In the body of Example 11-2, you'll create three objects: a Window, a ListBox, and a Button. You'll then call DrawWindow() on each:

Window win = new Window(1,2);
ListBox lb = new ListBox(3,4,"Stand alone list box");
Button b = new Button(5,6);
win.DrawWindow();
lb.DrawWindow();
b.DrawWindow();

This works much as you might expect. The correct DrawWindow() method is called for each. So far, nothing polymorphic has been done. The real magic starts when you create an array of Window objects.

Example 11-2 uses an array, which is a collection of objects that are all the same type. You create an array by indicating the type of objects to hold and then allocating space for a given number of those objects. For example, the following code declares winArray to be an array of three Window objects: Window[] winArray = new Window[3]; You access the members of the array with square brackets. The first element is accessed with winArray[0], the second with winArray[1], and so forth. Arrays are explained in detail in Chapter 15.

Because a ListBox is a Window, you are free to place a ListBox into an array of Windows. You can also place a Button into an array of Window objects because a Button is also a Window:

Window[] winArray = new Window[3];  
winArray[0] = new Window(1,2);  
winArray[1] = new ListBox(3,4,"List box in array");
winArray[2] = new Button(5,6);

The first line of code declares an array named winArray that will hold three Window objects. The next three lines add new Window objects to the array. The first adds a Window. The second adds a ListBox (which is a Window because ListBox derives from Window), and the third adds a Button (Button also derives from Window).

What happens when you call DrawWindow() on each of these objects?

for (int i = 0;i < 3; i++)
{
   winArray[i].DrawWindow();
}

This code uses i as a counter variable. It calls DrawWindow() on each element in the array in turn. The value i is evaluated each time through the loop, and that value is used as an index into the array.

All the compiler knows is that it has three Window objects and that you've called DrawWindow() on each. If you had not marked DrawWindow() as virtual, Window's original DrawWindow() method would be called three times.

However, because you did mark DrawWindow() as virtual, and because the derived classes override that method, when you call DrawWindow() on the array, the right thing happens for each object in the array. Specifically, the compiler determines the runtime type of the actual objects (a Window, a ListBox, and a Button) and calls the right method on each. This is the essence of polymorphism.

The runtime type of an object is the actual (derived) type. At compile time you do not have to decide what kind of objects will be added to your collection, so long as they all derive from the declared type (in this case Window). At runtime the actual type is discovered and the right method is called. This allows you to pick the actual type of objects to add to the collection while the program is running.

The complete code for this example is shown in Example 11-2.

Example 11-2. Virtual methods

using System;

public class Window
{
    // constructor takes two integers to
    // fix location on the console
    public Window(int top, int left)
    {
        this.top = top;
        this.left = left;
    }

    // simulates drawing the window
    public virtual void DrawWindow()
    {
        Console.WriteLine("Window: drawing Window at {0}, {1}",
            top, left);
    }

    // these members are protected and thus visible
    // to derived class methods. We'll examine this 
    // later in the chapter
    protected int top;
    protected int left;
    
}

// ListBox derives from Window
public class ListBox : Window
{
    // constructor adds a parameter
    public ListBox(
        int top, 
        int left, 
        string contents):
        base(top, left)  // call base constructor
    {
 
        listBoxContents = contents;
    }
    
    // an overridden version (note keyword) because in the
    // derived method we change the behavior
    public override void DrawWindow()
    {
        base.DrawWindow();  // invoke the base method
        Console.WriteLine ("Writing string to the listbox: {0}", 
            listBoxContents);
    }

    private string listBoxContents;  // new member variable
}

public class Button : Window
{
    public Button(
        int top,
        int left):
        base(top, left)
    {
    }

    // an overridden version (note keyword) because in the
    // derived method we change the behavior
    public override void DrawWindow()
    {
        Console.WriteLine("Drawing a button at {0}, {1}\n",
            top, left);
    }
}

public class Tester
{
    static void Main()
    {
        Window win = new Window(1,2);
        ListBox lb = new ListBox(3,4,"Stand alone list box");
        Button b = new Button(5,6);
        win.DrawWindow();
        lb.DrawWindow();
        b.DrawWindow();

        Window[] winArray = new Window[3];
        winArray[0] = new Window(1,2);
        winArray[1] = new ListBox(3,4,"List box in array");
        winArray[2] = new Button(5,6);

        for (int i = 0;i < 3; i++)
        {
            winArray[i].DrawWindow();
        }
    }
}
Output:
Window: drawing Window at 1, 2
Window: drawing Window at 3, 4
Writing string to the listbox: Stand alone list box
Drawing a button at 5, 6

Window: drawing Window at 1, 2
Window: drawing Window at 3, 4
Writing string to the listbox: List box in array
Drawing a button at 5, 6

Note that throughout this example, the overridden methods are marked with the keyword override:

public override void DrawWindow()

The compiler now knows to use the overridden method when treating these objects polymorphically. The compiler is responsible for tracking the real type of the object and for handling the late binding so that ListBox.DrawWindow() is called when the Window reference really points to a ListBox object.

11.3.2 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.

Now suppose that one of the programmers for Company B decides to add a Sort() method to ListBox:

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

This presents no problems until Company A, the author of Window, releases Version 2 of its Window class, and the programmers in Company A also add a Sort() method to their public class Window:

public class Window
{
   // ...
   public virtual void Sort() {...}
}

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:

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

To avoid this warning, it might be tempting to add the new keyword to all your virtual methods. This is a bad idea. When new appears in the code, it ought to document the versioning of code. It points a potential client to the base class to see what it is that you are not overriding. Using new scattershot undermines this documentation. The warning exists to help identify a real issue.