Visual Basic 6.0

Visual Basic 2008

When a new class is created through implementation inheritance, the new class automatically gains all the members and implementation of the base class. The existing class is called the "base class." The new class is the "derived class." Additional members can be added to the class. The behavior of the base class can be changed by writing code in the new class. This technique is called overriding.

You can create classes by inheriting from the classes you create, from classes in references you add to your project, or from objects in the .NET Framework. Many classes of the .NET Framework are related by inheritance. For example, the TextBox class inherits from the System.Windows.Forms..::.TextBoxBase class.

Visual Basic 6.0

Visual Basic 2008

Inheritance is declared in Visual Basic 2008 by the Inherits statement. In this example, the derived class Oak inherits from a base class Tree.

Visual Basic

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Public Class Oak
    Inherits Tree
    ' Add code here to extend or 
    ' modify the behavior of the Tree class.
End Class

Visual Basic 6.0

Visual Basic 2008

Along with implementation inheritance, Visual Basic 2008 defines two class modifiers, MustInherit and NotInheritable. These modifiers let the programmer control the inheritance relationships in an application.

The MustInherit modifier on a class declaration indicates a class that cannot be instantiated:

Visual Basic

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Public MustInherit Class BaseClass
End Class

This means that BaseClass cannot appear after the New keyword. Only classes that inherit from BaseClass and that do not also have the MustInherit modifier can be instantiated. In object-oriented literature and in other object-oriented languages, a MustInherit class is called an abstract class. A class that is not a MustInherit class, one that can be instantiated, is called a concrete class.

A related concept is the sealed class, a class that cannot serve as a base class. The NotInheritable keyword on a class definition indicates this status:

Visual Basic

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Public NotInheritable Class DerivedClass
End Class

A NotInheritable class marks the edge of an inheritance hierarchy.

For more information, see Inheritance in Visual Basic.

Classes acting as interfaces in Visual Basic 6.0 are upgraded to interfaces in Visual Basic 2008. Consider this example of a base class and derived class from Visual Basic 6.0:

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' Contents of class BaseClass
Public Sub BaseMethod()
End Sub

' Contents of class DerivedClass
Implements BaseClass
Private Sub BaseClass_BaseMethod()
End Sub

The Upgrade Wizard produces this upgraded code:

Visual Basic

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Option Strict Off
Option Explicit On
Interface _BaseClass
    Sub BaseMethod()
End Interface

Friend Class BaseClass
    Implements _BaseClass
    Public Sub BaseMethod() Implements _BaseClass.BaseMethod
    End Sub
End Class

Friend Class DerivedClass
    Implements _BaseClass
    Private Sub BaseClass_BaseMethod() Implements _BaseClass.BaseMethod
    End Sub
End Class

Using inheritance instead of interfaces, the upgraded code can be modified to:

Visual Basic

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Friend Class BaseClass
    Public Sub BaseMethod()
        ' Add code here to define BaseMethod.
    End Sub
End Class

Friend Class DerivedClass
    Inherits BaseClass
End Class

This eliminates one level of indirection, _BaseClass. Also, it eliminates one method definition, BaseClass_BaseMethod, because the method is inherited from the base class and does not need to be coded again. If the programmer wanted a different behavior for the derived class, the BaseMethod can be overridden as shown:

Visual Basic

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Friend Class BaseClass
    Public Overridable Sub BaseMethod()
        ' Add code here to define BaseMethod.
    End Sub
End Class

Friend Class DerivedClass
    Inherits BaseClass
    Public Overrides Sub BaseMethod()
        ' Add code here to define behavior for DerivedClass.
    End Sub
End Class

Some techniques to consider in upgraded interfaces include:

• Replace Interface statements with Inherits statements.

• Implement the base class as an interface, rather than as a class that implements an interface. For more information see Interface Changes for Visual Basic 6.0 Users.

• Implement base classes as MustInherit classes.

• Implement derived classes as NotInheritable.