Defining Design Patterns

· “Design patterns constitute a set of rules describing how to accomplish

certain tasks in the realm of software development.” (Pree 1994)

“Design patterns focus more on reuse of recurring architectural design

themes, while frameworks focus on detailed design and

implementation.” (Coplien and Schmidt 1995)

“A pattern addresses a recurring design problem that arises in specific

design situations and presents a solution to it.” (Buschmann et al.

“Patterns identify and specify abstractions that are above the level of

single classes and instances, or of components.” (Gamma et al., 1993)

But while it is helpful to draw analogies to architecture, cabinet making,

and logic, design patterns are not just about the design of objects but about

the interaction between objects. One possible view of some of these

patterns is to consider them as communication patterns.

Some other patterns deal not just with object communication but with

strategies for object inheritance and containment. It is the design of

simple, but elegant, methods of interaction that makes many design

patterns so important.

Design patterns can exist at many levels from very low-level specific

solutions to broadly generalized system issues. There are now hundreds of

patterns in the literature. They have been discussed in articles and at

conferences of all levels of granularity. Some are examples that apply

widely, and a few writers have ascribed pattern behavior to class

groupings that apply to just a single problem (Kurata 1998).

It has become apparent that you don’t just write a design pattern off the

top of your head. In fact, most such patterns are discovered rather than

written. The process of looking for these patterns is called “pattern

mining,” and it is worthy of a book of its own.

The 23 design patterns selected for inclusion in the original Design

book were those that had several known applications and that

Creational patterns create objects for you rather than having you

instantiate objects directly. This gives your program more flexibility in

deciding which objects need to be created for a given case.

Structural patterns help you compose groups of objects into larger

structures, such as complex user interfaces or accounting data.

Behavioral patterns help you define the communication between

objects in your system and how the flow is controlled in a complex

program.

We’ll be looking at C# versions of these patterns in the chapters that

follow, and we will provide at least one complete C# program for each of

the 23 patterns. This way yo u can examine the code snippets we provide

and also run, edit, and modify the complete working programs on the

accompanying CD-ROM. You’ll find a list of all the programs on the CDROM

at the end of each pattern description.