`MustInherit`

In VB.NET the MustInherit keyword changes a Class. It makes the Class only be used when Classes inherit from it. It is like an abstract class in C#.

A MustInherit Class cannot be instantiated directly. Instead it serves as a template. A separate class that uses the MustInherit class is always needed.

Example

In this VB.NET program, we introduce a Class called Page that is a MustInherit class. We then show a Class called TextPage that inherits from Page.

Detail In the Main Sub, we create a new instance of TextPage. We then call Number() from the Page base Class.

Note You cannot create a new instance of Page with New Page. A MustInherit class is not used this way.

Tip The MustInherit Page class is part of TextPage, but we do not instantiate it directly.

MustInherit Class Page
    Public Function Number() As Integer
        Return 1
    End Function
End Class

Class TextPage
    Inherits Page
End Class

Module Module1
    Sub Main()
        ' Create new TextPage.
        Dim t As TextPage = New TextPage

        ' Write result of Number Function.
        Console.WriteLine(t.Number())
    End Sub
End Module2
100

Intermediate language

In .NET IL the MustInherit keyword is changed to the abstract modifier. So in essence a MustInherit class is an abstract class.

Tip In the C# language, this terminology (the keyword abstract) is used directly.

.class private abstract auto ansi ConsoleApplication1.Page
       extends [mscorlib]System.Object
{
} // end of class ConsoleApplication1.Page
.class private auto ansi ConsoleApplication1.TextPage
       extends ConsoleApplication1.Page
{
} // end of class ConsoleApplication1.TextPage

Overloads, Shadows

A complex interaction exists between a MustInherit Class and its derived Class. In the derived Class (TextPage), we can provide an Overloads Function.

Then The base implementation (of Number) is ignored when a TextPage instance is used.

Detail The Overloads implementation is used. Similarly we can use a Shadows field on a derived class.

Tip This means that when the derived class is used, the Shadows field is always used. No confusion exists.

Note This program does not use the _value field on Page or the Number() Function on Page. Instead it uses only those members on TextPage.

MustInherit Class Page
    ''' <summary>
    ''' Field.
    ''' </summary>
    Public _value As Integer

    ''' <summary>
    ''' A base implementation.
    ''' </summary>
    Public Function Number() As Integer
        Return 1
    End Function
End Class

Class TextPage
    Inherits Page
    ''' <summary>
    ''' Shadows the _value field.
    ''' </summary>
    Public Shadows _value As Integer

    ''' <summary>
    ''' Overloads the Base Function.
    ''' </summary>
    Public Overloads Function Number() As Integer
        Return 2
    End Function
End Class

Module Module1
    Sub Main()
        ' Overloads Number Function is used.
        Dim t As TextPage = New TextPage
        Console.WriteLine(t.Number())

        ' Use Shadows field.
        t._value = 100
        Console.WriteLine(t._value)
    End Sub
End Module2
100

`NotInheritable`

A NotInheritable class cannot be derived from by another class. This may impart some performance advantages. Common classes, like String, are NotInheritable.

NotInheritable Class Test

End Class

Class Test2
    Inherits Test
    ' This does not compile.
End Class

Module Module1
    Sub Main()
    End Sub
End Module
Error 1
'Test2' cannot inherit from class 'Test' because 'Test'
is declared 'NotInheritable'.

Discussion

The MustInherit keyword is useful in many programs. We can a MustInherit Class type as the value in a Dictionary. Then use derived types as the elements in the Dictionary.

Tip With this pattern, we can access any key and call a Function on the Value from the MustInherit Class.

And All values can act differently but be used in the same way. This is a simple program design.

The MustInherit keyword specifies that a class is to be used only as a template. It cannot be used directly. We create new classes, with the Inherits keyword, that use this template.