Section 13.5. Extending Interfaces

13.5 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 methods to keep track of the bytes saved.

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

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

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 13-4, 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). This is the same logic that says you can cast any object of a derived type to an object of a base type (that is, if Student derives from Human, then all Students are Human, even though not all Humans are Students).

Example 13-4. Extending interfaces

 Option Strict On Imports System Namespace InterfaceDemo          Interface IStorable         Sub Read( )         Sub Write(ByVal obj As Object)         Property Status( ) As Integer     End Interface 'IStorable          ' the Compressible interface is now the     ' base for ICompressible2     Interface ICompressible         Sub Compress( )         Sub Decompress( )     End Interface 'ICompressible          ' extend ICompressible to log the bytes saved     Interface ICompressible2         Inherits ICompressible     Sub LogSavedBytes( )    End Interface 'ICompressible2              ' Document implements both interfaces    Public Class Document         Implements ICompressible2, IStorable         ' the document constructor       Public Sub New(s As String)         Console.WriteLine("Creating document with: {0}", s)     End Sub 'New     ' implement IStorable         Public Sub Read( ) Implements IStorable.Read         Console.WriteLine("Implementing the Read Method for IStorable")         End Sub 'Read         Public Sub Write(ByVal o As Object) Implements IStorable.Write             Console.WriteLine( _                   "Implementing the Write Method for IStorable")         End Sub 'Write         Public Property Status( ) As Integer Implements IStorable.Status             Get                 Return myStatus             End Get             Set(ByVal Value As Integer)                 myStatus = Value             End Set         End Property         ' implement ICompressible         Public Sub Compress( ) Implements ICompressible.Compress             Console.WriteLine("Implementing Compress")         End Sub 'Compress         Public Sub Decompress( ) Implements ICompressible.Decompress             Console.WriteLine("Implementing Decompress")         End Sub 'Decompress         ' implement ICompressible2         Public Sub LogSavedBytes( ) Implements ICompressible2.LogSavedBytes             Console.WriteLine("Implementing LogSavedBytes")         End Sub 'LogSavedBytes         ' hold the data for IStorable's Status property         Private myStatus As Integer = 0     End Class 'Document     Class Tester         Public Sub Run( )             Dim doc As New Document("Test Document")             If TypeOf doc Is IStorable Then                 Dim isDoc As IStorable = doc                 isDoc.Read( )             Else                 Console.WriteLine("Could not cast to IStorable")             End If             If TypeOf doc Is ICompressible2 Then                 Dim ilDoc As ICompressible2 = doc                 Console.Write("Calling both ICompressible and ")                 Console.WriteLine("ICompressible2 methods...")                 ilDoc.Compress( )                 ilDoc.LogSavedBytes( )             Else                 Console.WriteLine("Could not cast to ICompressible2")             End If             If TypeOf doc Is ICompressible Then                 Dim icDoc As ICompressible = doc '                 Console.WriteLine( _                    "Treating the object as Compressible... ")                 icDoc.Compress( )             Else                 Console.WriteLine("Could not cast to ICompressible")             End If         End Sub 'Run         Shared Sub Main( )             Dim t As New Tester( )             t.Run( )         End Sub 'Main     End Class 'Tester End Namespace 'InterfaceDemo 

  Output:  Creating document with: Test Document Implementing the Read Method for IStorable Calling both ICompressible and ICompressible2 methods... Implementing Compress Implementing LogSavedBytes Treating the object as Compressible... Implementing Compress 

Example 13-4 starts by creating the ICompressible2 interface:

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

Notice that the syntax for extending an interface is the same as that for deriving from a class. This extended interface explicitly defines only one method, LogSavedBytes( ); but of course any class implementing this interface must also implement the base interface (ICompressible) and all its members.

You define the Document class to implement both IStorable and ICompressible2:

 Public Class Document     Implements ICompressible2, IStorable 

You are now free to cast the Document object to IStorable, ICompressible2, or to ICompressible:

 If TypeOf doc Is IStorable Then       Dim ilDoc 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 you take a look back at the output, you'll see that all three of these casts succeed.