Adapter Pattern in VB.NET

The Gang Of Four (GoF) defined the Adaptor pattern as follows in their most famous book "Design Patterns" Gamma et al., Addison-Wesley, ISBN:0-201-63361-2" - "Convert the interface of a class into another interface clients expect. Adapter lets classes work together that couldn't otherwise because of incompatible interfaces."

During object-oriented developments, some times we have to use an unrelated class along with our existing class hierarchy. The simplest solution is to make a wrapper or adaptor around the foreign class, which is acceptable by the existing class hierarchy. This is what known as the ADAPTOR PATTERN or WRAPPER PATTERN. 

Let us see the following class hierarchy.   



How we can fit a foreign class or interface say ADAPTEE to this class hierarchy. It is possible by introducing an ADAPTOR class as shown below.



In the above case the ADAPTOR class inherits from both BASE CLASS & ADAPTEE class. This type of mechanism based on the multiple inheritances is known as class adaptors. Remember that this may be introducing the violation of Open Closed Principle (OCP) or Lisknov's Substitution Principle (LSP). But if we are using class adaptors, better to use a private inheritance with respect to the ADAPTEE class.
 
Instead of inheriting from the ADAPTEE class, the ADAPTOR class can contain an instance of ADAPTEE class and use that instance. This mechanism employs the object adaptors. The UML representation of Object adaptor is shown below.



In class adaptor, it is possible to override some of the behaviors of the ADAPTEE class in ADAPTOR class. But class adaptor will not work when we want to adapt ADAPTEE class and all its derived classes. 

But an object adaptor can work with the ADAPTEE class and all its subclasses. 

Non-software Example 

Socket wrenches provide an example of the Adapter. A socket attaches to a ratchet, provided that the size of the drive is the same. Typical drive sizes in the United States are 1/2" and 1/4". Obviously, a 1/2" drive ratchet will not fit into a 1/4" drive socket unless an adapter is used. A 1/2" to 1/4" adapter has a 1/2" female connection to fit on the 1/2" drive ratchet, and a 1/4" male connection to fit in the 1/4" drive socket. [Michael Duell, "Non-software examples of software design patterns", Object Magazine, Jul 97, p54].

Known Uses

Distributed object databases like CORBA use Adaptors to integrate native language objects into the database. Adaptors are also used to make non-objects, like text files, look like objects.

VB.NET Implementation

' This is an example for Object Adaptor
' Structural Pattern: ADAPTOR
' Author: rajeshvs@msn.com 
Imports System
'the class which is acceptable by the client
Interface Existing
Sub Method()
'thrird class which we have to be accepted by the client
Class Adaptee
Public Sub ForeignMethod()
Console.WriteLine("Foreign Adapted Method")
End Sub 'ForeignMethod
End Class 'Adaptee
'Object Adaptor
'Adaptor class which makes the Adaptee class accessible by the client
'inherit from Existing class
Class Adaptor
Inherits Existing
Public Sub Method()
ad.ForeignMethod()
End Sub 'Method
Private ad As New Adaptee
End Class 'Adaptor
'Client class
Class Client
Public Sub ClientMethod(ByVal e1 As Existing)
e1.Method()
End Sub 'ClientMethod
End Class 'Client
Class MyMain
Public Shared Sub Main()
Dim gc As New Client
Dim e1 = New Adaptor
gc.ClientMethod(e1)
End Sub 'Main
End Class 'MyMain
End Interface 'Existing