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Introduction to Design Patterns Raju Golla …………………………………… .…….

What are Design Patterns General and reusable solutions to common problems in software design. Solutions to problems that have already been resolved Structure or interfaces how the source code should be organised. Interaction between Classes i.e., collaboration .NET Framework BCL already makes extensive use of patterns. Deal with higher level application and software design (Enterprise architecture) as well as low level abstractions on top of the source code. It is a dialect for programmers, use templates rather than reinventing the wheel, improves system and application design.

What they are not? Reusable components such as assemblies, algorithms or class specifications

History Success architect Christopher Alexander book 'A Pattern Language: Towns, builds, construction, 1977' (Non IT) Design patterns: Elements of Reusable Object Oriented Software Gang of Four: Erich Gamma, Richard Helm, Ralph Johnson and John Vlissides (1995). First time documented successful design patterns.

Classifications Design patterns for Object oriented programming. Functional programming. Creational Structural Behavioral Security Concurrency Sql User interface Relational Social Distributed

Singleton pattern Member of Creational family of patterns The intention of this pattern is to ensure that a class has only one instance Make the class itself is responsible for keeping track of its sole instance The class must be accessible to clients There can be only one

Singleton implementation classic

Outcome The default implementation of Singleton pattern is not thread-safe, it should not be used in multi threaded applications i.e., ASP.NET web apps. Introduces tight coupling among collaborating classes. Difficult to test It is feasible to use IOC (Inversion of Control) to avoid tight coupling and testability issues.

Strategy Encapsulate related algorithms Lets the algorithm vary and evolve from the class using it Allows the class to maintain a separate purpose Separates the implementation from the delivery of its results.

Strategy – when to be used? Switch statements. Decouple algorithm implementation from a class. Adding a new method (Calculation) requires class modification.

Strategy implementation Interface Implementing classes

Implementation continued.. Client

Decorator Structural design pattern Allows to add functionality to objects at run time. Alternative to sub classing. Design flexibility Allows adding behaviour to objects at run time. Supports open closed principle.

When to consider? Legacy systems Add functionality to UI controls (Controls, buttons etc) Sealed classes (it is feasible to add

Implementation Classic implementation -Base class Derived classes

Implementation continued… Client Output

Implementation continued… Concrete decorator

State Member of behavioral pattern Allows object to change methods behaviour based on state of the object, not the configuration. Encapsulate the logic of each state into a single object Allows for dynamic state discovery Makes unit testing easier and efficient

Benefits Separation of concerns Transition between sates is explicit and clear. Reuse of the state object. Simplify the program Easier maintainability.

References Microsoft Patterns and practices at http://msdn.microsoft.com/en-us/practices/default Microsoft Patterns and practices team: Application architecture guide http://apparchguide.codeplex.com/ Head First design patterns C# in depth

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Introduction To Design Patterns

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