C# 3.0 Design Patterns

Pattern Comparison

Pattern Comparison

There are considerable similarities between the Strategy and State patterns, but the main difference is one of intent:

• A Strategy object encapsulates an algorithm

• A State object encapsulates a state-dependent behavior (and possibly state transitions)

Both patterns are concerned with polymorphism. Both patterns define a parent interface or abstract class and a series of subclasses that implement the methods therein. And both patterns have a context that they maintain and use to decide on state transitions. So, the biggest difference between the patterns is that we encapsulate an algorithm into strategy classes in the Strategy pattern, but we encapsulate state into state classes in the State pattern.

The Template Method pattern is more like the Strategy pattern in that it is algorithm based. The steps of the algorithm are specified in the Template Method and some are deferred to domain classes.

These similarities and differences are illustrated in the simple theory code in Example 7-1, Example 7-3, and Example 7-5. Table 7-1 summarizes them.

Table 7-1. Comparison of Strategy, State, and Template Method patterns

Â	Strategy	State	Template Method
Action achieved by	Context invokes Strategy	Client invokes Context	Algorithm invoking Operations
Switching modes based on	Client inspecting Context	State inspecting Context	Instantiation of classes in Client
Context/IPrimitives contains	Current Strategy	Current State	Operational interface