8.6 Combining Interfaces

You can also create new interfaces by combining existing interfaces and optionally adding new methods or properties. For example, you might decide to combine the definitions of IStorable and ICompressible2 into a new interface called IStorableCompressible. This interface would combine the methods of each of the other two interfaces, but would also add a new method, LogOriginalSize( ), to store the original size of the pre-compressed item:

Interface IStorableCompressible 
Inherits IStorable, ICompressible2
     Sub LogOriginalSize( )
End Interface

Having created this interface, you can now modify Document to implement IStorableCompressible:

Public Class Document
    Implements IStorableCompressible

You are now free to cast the Document object to any of the four interfaces you've created so far:

If TypeOf doc Is IStorable Then
      Dim isDoc As IStorable = doc

If TypeOf doc Is ICompressible Then
    Dim icDoc As ICompressible = doc

If TypeOf doc Is ICompressible2 Then
    Dim ic2Doc As ICompressible2 = doc

If TypeOf doc Is IStorableCompressible Then
   Dim iscDoc As IStorableCompressible = doc

You can then use the four variables to invoke the appropriate methods from the various interfaces:

isDoc.Read( )
icDoc.Compress( )
ic2Doc.LogSavedBytes( )
iscDoc.LogOriginalSize( )

Remember that when you cast to the new, combined interface, you can invoke any of the methods of any of the interfaces it extends or combines. The preceding code invokes four methods of iscDoc (the IStorableCompressible object). Only one of the preceding methods is defined in IStorableCompressible, but all four are methods defined by interfaces that IStorableCompressible extends or combines.