Programming Visual Basic .NET, Second Edition

Extending Interfaces

It is possible to extend an existing interface to add new methods or members, or to modify how existing members work. For example, you might extend ICompressible with a new interface, ICompressible2, which extends the original interface with a method to keep track of the bytes saved.

The following code creates a new interface named ICompressible2 that is identical to ICompressible except that it adds the method LogSavedBytes( ):

Interface ICompressible2
    Inherits ICompressible
    Sub LogSavedBytes( )
End Interface 'ICompressible2

Tip

Notice that your new interface (ICompressible2) inherits from the base interface (ICompressible). Classes can inherit only from a single class, but interfaces can inherit from more than one interface, as shown later in this chapter.

Classes are now free to implement either ICompressible or ICompressible2, depending on whether they need the additional functionality. If a class does implement ICompressible2, it must implement all the methods of both ICompressible2 and also ICompressible. Objects of that type can be cast either to ICompressible2 or to ICompressible.

In Example 8-4, you’ll extend ICompressible to create ICompressible2. You’ll then cast the Document first to be of type IStorable, then to be of type ICompressible2. Finally, you’ll cast the Document object to ICompressible. This last cast is safe because any object that implements ICompressible2 must also have implemented ICompressible (the former is a superset of the latter). ...