Section 11.2. Inheritance

11.2 Inheritance

In VB.NET, 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 VB.NET, you create a derived class by adding the Inherits keyword after the name of the derived class, followed by the name of the base class:

 Public Class ListBox    Inherits Window 

Or you might combine these two lines onto one as follows :

 Public Class ListBox : Inherits Window 

This code declares a new class, ListBox, that derives from Window. You can read the Inherits keyword 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, as shown in Example 11-1.

Example 11-1. Deriving a new class

 Option Strict On Imports System Public Class Window     ' constructor takes two integers to     ' fix location on the console     Public Sub New(ByVal top As Integer, ByVal left As Integer)         Me.top = top         Me.left = left     End Sub 'New     ' simulates drawing the window     Public Sub DrawWindow( )         Console.WriteLine("Drawing Window at {0}, {1}", top, left)     End Sub 'DrawWindow     ' these members are private and thus invisible     ' to derived class methods; we'll examine this      ' later in the chapter     Private top As Integer     Private left As Integer End Class 'Window ' ListBox derives from Window Public Class ListBox     Inherits Window     ' constructor adds a parameter     Public Sub New(ByVal top As Integer, ByVal left As Integer, ByVal theContents As         MyBase.New(top, left) ' call base constructor         mListBoxContents = theContents     End Sub 'New     ' a shadow version (note keyword) because in the     ' derived method we change the behavior     Public Shadows Sub DrawWindow( )         MyBase.DrawWindow( ) ' invoke the base method         Console.WriteLine("Writing string to the listbox: {0}", mListBoxContents)     End Sub 'DrawWindow     Private mListBoxContents As String ' new member variable End Class 'ListBox Module Module1     Sub Main( )         ' create a base instance         Dim w As New Window(5, 10)         w.DrawWindow( )         ' create a derived instance         Dim lb As New ListBox(20, 30, "Hello world")         lb.DrawWindow( )     End Sub End Module 

  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 ( top , left , and theContents ). The ListBox constructor invokes the constructor of its parent by calling MyBase.New and passing in the parameters (using the ByVal keyword, as described in Chapter 9):

 Public Sub New( _    ByVal top As Integer, _    ByVal left As Integer, _    ByVal theContents As String)  MyBase.New(top, left) ' call base constructor  mListBoxContents = theContents End Sub 'New 

Because classes cannot inherit constructors, a derived class must implement its own constructor and can make use of the constructor of its base class only 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 MyBase keyword. The keyword MyBase 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 will create 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 Shadowing Base Methods

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

 Public Shadows Sub DrawWindow( ) 

The keyword Shadows here 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 will be invoked, not Window.DrawWindow( ). Note, however, that ListBox.DrawWindow( ) can invoke the DrawWindow( ) method of its base class with the code:

 MyBase.DrawWindow( ) 'invoke the base method 

11.2.4 Controlling Access

The visibility of a class and its members can be restricted 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, someClass, as follows:

 Public Class someClass {    '...    Protected myValue As Integer } 

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.