Section 13.6. Combining Interfaces

13.6 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 

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 following code invokes four methods on iscDoc (the IStorableCompressible object). Only one of these methods is defined in IStorableCompressible, but all four are methods defined by interfaces that IStorableCompressible extends or combines.

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