C# 3.0 Design Patterns

Pattern Comparison

The Adapter pattern has much in common with the patterns discussed in Chapter 2. The differences are in the intents of the patterns. A bridge, for example, separates an interface and its implementation so that they can vary independently, whereas an adapter changes the interface of an existing object. The Adapter pattern is more useful for one-off changes, whereas the Bridge pattern anticipates that change might happen continuously.

A decorator enhances an object without changing its interface and should be transparent to the application. An adapter is not transparent, as it is the named implementation of the interface the client sees. The Proxy pattern does not change any interfaces; it defines substitute objects for other objects.

From a certain point of view, the Façade pattern is also adapting requests: it transforms high-level requests into a sequence of lower-level requests. The Façade's intent is to hide complexity, and the Façade subsystems are not intended to be accessible by the client.

To complete the picture, we can classify the Adapter and Façade patterns according to the mechanisms shown in Table 4-2.

Table 4-2. Comparison of Adapter and Façade patterns

Mechanism	Adapter	Façade
Original	Adaptee	SubsystemA, B, and C
Interface	ITarget	Façade
New	Adapter	Operation1 and 2
Client	Aggregates ITarget	Accesses Façade
Client activates	New	New
Original changed by	No change	No change
New classes/subsystems	Adapter provides adaptations to their methods	Façade supplies high-level operations
Operation routed	From new to original	From new to original