Design Patterns

Design Patterns

Design patterns are recognized solutions to common problems defined originally by the Gang of Four programmers. Design patterns are used throughout the ASP.NET Framework.  The various patterns are commonly divided into several different groups depending on the nature of the design problem they intend to solve.

Creational Patterns

• Factory - This pattern is used to create concrete class instances without specifying the exact class type.  
• Abstract Factory - This pattern is used to create concrete class instances without specifying the exact class type. The Abstract Factory Pattern provides a way to encapsulate a group of individual factories that have a common theme.
  
• Flyweight - A pattern used to maximize the sharing of objects resulting in reduced memory consumption.
• Singleton - This pattern insures that only a single instance of a given object can exist.
• Builder - This pattern separate the construction of a complex object from its representation so that the same construction process can create different representations..

Structural Patterns

• Adapter - Convert the interface of a class into another interface clients expect. Adapter lets the classes work together that couldn't otherwise because of incompatible interfaces
• Bridge - Decouples an abstraction from its implementation so that the two can vary independantly.
• Composite - Compose objects into tree structures to represent part-whole hierarchies. Composite lets clients treat individual objects and compositions of objects uniformly.
• Decorator - Allows an objects behavior to be altered at runtime.
• Facade - Used to provide a simpler interface into a more complicated portion of code. 
• Proxy - Provides a Placeholder for another object to control access to it.

Behavioral Patterns

• Chain of Responsibility - The chain of responsibility pattern is a way of communicating between objects.
• Command - Encapsulates a request as an object, thereby letting you parameterize clients with different requests, queue or log requests, and support undoable operations.   
• Iterator - Provides a way to sequentially access aggregate objects without exposing the structure of the aggregate.
• Mediator - The mediator pattern encapsulate the interaction between a set of objects.
• Memento - Allows you to save the state of an object externally from that object.
• Observer - Allows a single object to notify many dependent objects that its state has changed.
• State - Allows an object to change its behaviour when its internal state changes.
• Strategy - Allows multiple algorithms to be used interchangeably at runtime.
• Visitor - The visitor design pattern enables us to create new operations to be performed on an existing structure.
• Template Method -  Defines the skeleton of an algorithm then lets subclasses implement the behaviour that can vary.

Antipatterns

Antipatterns are misapplied design patterns. Common Antipatterns include:

• The Blob/God Object - When one class contains all of the methods, operations and logic of your application
• Re-coupling - building an unnecessary dependency between objects
• Poltergeists - object whose main purpose in life is to pass messages to another object
• Sequential Coupling - a class that imposes a particular order on its method calls. 

Other patterns

• Aggregator Provider Pattern
• Lazy loading

Videos

• MSDN Webcast: Design Patterns in .NET (Level 200)

Articles

• Design Patterns in C# and VB.NET - Gang of Four (GOF)
• Design pattern in simple examples
• Discover the Design Patterns You're Already Using in the .NET Framework - MSDN Magazine

Books

• Design Patterns: Elements of Reusable Object-Oriented Software
• Head First Design Patterns
• C# 3.0 Design Patterns
  

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