VB.NET Language in a Nutshell, Second Edition

Interfaces, Abstract Members, and Classes

We have alluded to the fact that a class may implement all, some, or none of the members of the interfaces that it defines. Any interface member that does not have an implementation is referred to as an abstract member. The purpose of an abstract member is to provide a member signature (a template, if you will) that can be implemented by one or more derived classes, generally in different ways.

Let us clarify this with an example. Recall from our discussion of inheritance that the CEmployee class defines and implements an IncSalary method that increments the salary of an employee. Recall also that the CExecutive and CSecretary derived classes override the implementation of the IncSalary method in the base class CEmployee.

Suppose that, in a more complete employee model, there is a derived class for every type of employee. Moreover, each of these derived classes overrides the implementation of the IncSalary method in the base class CEmployee. In this case, the implementation of IncSalary in the base class will never need to be called! So why bother to give the member an implementation that will never be used?

Instead, we can simply provide an empty IncSalary method, as shown here:

' Employee class
Public Class CEmployee

    . . .

    Public Overridable Sub IncSalary(ByVal sngPercent As Single)
    End Sub

End Class

Alternatively, if we want to require that all derived classes implement the IncSalary method, we can use the MustOverride keyword, as ...