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About Patterns
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Design patterns were introduced in Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides s seminal work Design Patterns: Elements of Reusable ObjectOriented Software (Addison-Wesley). The book specifies and describes 23 patterns that form the foundation of any study of the subject, which are still regarded as the essential core patterns today. These core patterns address issues in mainline object-oriented programming (OOP), and the original implementations were presented in C++ and Smalltalk (the primary OOP languages at the time they were developed). Since then, other books have
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1: C# Meets Design Patterns
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implemented the patterns in Java, Visual Basic, and C#. As the value of the pattern concept has become accepted, new patterns have been proposed to add to the original list. In addition, there are now patterns that are applicable to specific areas, such as software architecture, user interfaces, concurrency, and security. Although these patterns are extremely important in their areas, their adherents are fragmented, and the core set of universally accepted patterns has not been expanded. As outlined in the Preface, the discussion of each pattern in this book consists of a brief description of its role, an illustration, a look at its design and implementation, and an example, followed by a discussion of its uses and some exercises. New features of the C# language are introduced where patterns draw upon them; thus, you will learn more about the language as you learn about the patterns. The 23 patterns are divided into three groups: creational, structural, and behavioral. Within a group, though, there is no inherent ordering, and alphabetical ordering has traditionally been the default. In this book, we take an innovative approach by relating the patterns to the language features they require and introducing them in order of increasing language complexity. Several of the patterns in each group need only inheritance or interfaces, and it makes sense to deal with these first so that the focus can be on the patterns themselves and not on the language. The patterns that make use of more advanced language features (generics, indexers, and delegates) are then presented later. Novel features of C# can thus be introduced as we go along, rather than in a standalone introduction or appendix. A comprehensive index complements this approach. A key feature of any pattern handbook is the insight that it gives as to the use of patterns in real systems. Knowing that the Fa ade pattern is frequently used in compiler construction, or that adapters are prevalent in well-known graphical frameworks, reinforces their importance and helps to direct their use. However, in large systems patterns are seldom found in isolation, and often they work together. The Singleton pattern, for example, is often used in conjunction with other patterns when it is necessary to create only one version of a component. Thus, considerable attention is given at the end of each chapter to the comparative merits of the patterns explored.
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About UML
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An important part of each pattern s description is a Unified Modeling Language* (UML) class diagram. UML is a universally accepted way of describing software in diagrammatic form. The diagrams in the book make use of the UML features itemized in Table 1-1.
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* Defined by the Object Management Group (see http://www.uml.org).
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About UML
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Table 1-1. UML class diagram notation Program element Class
Class attribute +operation( )
Diagram element
Meaning Types and parameters specified when important; access indicated by + (public), (private), and # (protected).
Interface
<<interface>> IClass +operation( )
Name starts with I. Also used for abstract classes.
Note
descriptive text
Any descriptive text.
Package
Package
Grouping of classes and interfaces.
Inheritance
A
B inherits from A.
B Realization
A
B implements A.
B Association
A
B
A and B call and access each other s elements.
Association (one way)
A
B
A can call and access B s elements, but not vice versa.
Aggregation
A
B
A has a B, and B can outlive A.
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1: C# Meets Design Patterns
Table 1-1. UML class diagram notation (continued) Program element Composition Diagram element Meaning A has a B, and B depends on A.
A
B
There are three kinds of blocks, for classes, interfaces/abstract classes, and packages. The class is the most common diagram element and contains details of some of its corresponding C# class s more important attributes (or fields) and operations (or methods). A UML diagram is not meant to be an exact copy of a program, and thus only the elements that are important to the pattern under consideration are shown. The accessibility of all attributes and operations (private, public, or protected) is indicated. The default for attributes is private and for operations is public. Deviations from the defaults will be highlighted as they occur. The types associated with attributes and operations are not usually given. However, when these are important, they can be inserted after the identifier, separated by a colon. The same relaxed approach applies to parameters of methods, which are not usually shown in a diagram. Notes are very useful for explaining relationships, such as when a method in one class calls a particular method in another, when this information is directly relevant to the pattern. In most cases, though, six types of lines give enough information. The Decorator pattern, which we will consider first, has a reasonably rich diagram, and it will be used to explain the lines in more detail.
About C# 3.0
An objective of this book is to present C# in its very best style. The version we are using is C# 3.0 on the .NET 3.5 Beta 2 Framework (August 2007). C# 1.0 came out in December 2002, embodying much of the research in OOP that had taken place since Java was launched seven years previously. C# 2.0 was released in final form in September 2005, and the ECMA standard was made available in June 2006. C# 2.0 added five significant features to C# 1.0, most of which are used in the patterns in this book: Generics that allow classes, structs, interfaces, delegates, and methods to be parameterized by the types of data they store and manipulate Anonymous methods that allow code blocks to be written inline where delegate values are expected Iterators, which are methods to incrementally compute and yield sequences of values
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