11.2 Inheritance

In C#, the specialization relationship is implemented using a principle called inheritance. This is not the only way to implement specialization, but it is the most common and most natural way to implement this relationship.

Saying that ListBox inherits from (or derives from) Window indicates that it specializes Window. Window is referred to as the base class, and ListBox is referred to as the derived class. That is, ListBox derives its characteristics and behaviors from Window and then specializes to its own particular needs.

11.2.1 Implementing Inheritance

In C#, you create a derived class by adding a colon after the name of the derived class, followed by the name of the base class:

public class ListBox : Window

This code declares a new class, ListBox, that derives from Window. You can read the colon as "derives from."

The derived class inherits all the members of the base class (both member variables and methods). These members can be treated just as if they were created in the derived class.

The derived class is free to implement its own version of a base class method. This is called hiding the base class method and is accomplished by marking the method with the keyword new.

This is a different use of the keyword new than you've seen earlier in this book. In Chapter 8, new was used to create an object on the heap; here new is used to replace the base class method. Programmers say the keyword new is overloaded, which means that the word has more than one meaning or use.

The new keyword indicates that the derived class has intentionally hidden and replaced the base class method, as shown in the Example 11-1. (The new keyword is also discussed in Section 11.3.2, later in this chapter.)

Example 11-1. Deriving a new class

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 void DrawWindow()
    {
        Console.WriteLine("Drawing Window at {0}, {1}",
            top, left);
    }

    // these members are private and thus invisible
    // to derived class methods; we'll examine this 
    // later in the chapter
    private int top;
    private int left;
}

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

public class Tester
{
    public static void Main()
    {
        // create a base instance
        Window w = new Window(5,10);
        w.DrawWindow();

        // create a derived instance
        ListBox lb = new ListBox(20,30,"Hello world");
        lb.DrawWindow();
    }
}
Output:
Drawing Window at 5, 10
Drawing Window at 20, 30
Writing string to the listbox: Hello world

Example 11-1 starts with the declaration of the base class Window. This class implements a constructor and a simple DrawWindow() method. There are two private member variables, top and left. The program is analyzed in detail in the following sections.

11.2.2 Calling Base Class Constructors

In Example 11-1, the new class ListBox derives from Window and has its own constructor, which takes three parameters. The ListBox constructor invokes the constructor of its parent by placing a colon (:) after the parameter list and then invoking the base class constructor with the keyword base:

public ListBox(
    int theTop, 
    int theLeft, 
    string theContents):
    base(theTop, theLeft)  // call base constructor

Because classes cannot inherit constructors, a derived class must implement its own constructor and can only make use of the constructor of its base class by calling it explicitly.

If the base class has an accessible default constructor, the derived constructor is not required to invoke the base constructor explicitly; instead, the default constructor is called implicitly. However, if the base class does not have a default constructor, every derived constructor must explicitly invoke one of the base class constructors using the base keyword. The keyword base identifies the base class for the current object.

As discussed in Chapter 8, if you do not declare a constructor of any kind, the compiler creates a default constructor for you. Whether you write it yourself or you use the one provided by default by the compiler, a default constructor is one that takes no parameters. Note, however, that once you do create a constructor of any kind (with or without parameters) the compiler does not create a default constructor for you.

11.2.3 Overriding Base Methods

Notice in Example 11-1 that ListBox implements a new version of DrawWindow():

public new void DrawWindow()

The keyword new indicates that the programmer is intentionally creating a new version of this method in the derived class.

In Example 11-1, the DrawWindow() method of ListBox hides and replaces the base class method. When you call DrawWindow() on an object of type ListBox, it is ListBox.DrawWindow() that is invoked, not Window.DrawWindow(). Note, however, that ListBox.DrawWindow() can invoke the DrawWindow() method of its base class with the code:

base.DrawWindow();  // invoke the base method

11.2.4 Controlling Access

You can restrict the visibility of a class and its members through the use of access modifiers, such as public, private, and protected. (See Chapter 8 for a discussion of access modifiers.)

As you've seen, public allows a member to be accessed by the member methods of other classes, while private indicates that the member is visible only to member methods of its own class. The protected keyword extends visibility to methods of derived classes.

Classes as well as their members can be designated with any of these accessibility levels. If a class member has a different access designation than the class, the more restricted access applies. Thus, if you define a class, myClass, as follows:

public class myClass
{
   // ...
   protected int myValue;
}

the accessibility for myValue is protected even though the class itself is public. A public class is one that is visible to any other class that wishes to interact with it.