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Do Application Design Patterns Make Sense in ASP.NET?

Alex Homeralex@stonebroom.com

You may like to write these down now...• http://www.daveandal.net/articles/• http://www.daveandal.net/download/

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Agenda

What are Design Patterns? Basic Design Patterns for ASP.NET Basic Pattern Support within ASP.NET Controller Patterns for ASP.NET Advanced Patterns for ASP.NET Conclusion and Summary

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Design Patterns are Scary...!

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You Use Patterns Every Day...

try { something }catch (SqlException qx){ handle SQL error }catch (SecurityException sx){ handle security violation }catch (Exception ex){ handle all other exceptions }finally{ clean up resources }

Using xr As XmlReader = XmlReader.Create("file.xml") ... read some XML ...End Using

StructuredExceptionHandling

Factory

Lifetime

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What are Design Patterns?

Informal Design Patterns• Code constructs, best practice, well-

structured, common sense, the accepted approach, evolved over time

Formal Design Patterns• Documented as "Context", "Problem",

"Solution", and a UML diagram

• Have specific aims

• Solve specific issues

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Why Do We Need Design Patterns?

Engineering disciplines require patterns • Structural Engineering

• Electronic Engineering

• ... even Social Engineering!

Creating software is also Engineering• Structural design and architecture

• Componentization and interconnections

• User interface design and implementation

• Deployment, testing, and management

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Formal Design Patterns"Fundamental to any science or engineering

discipline is a common vocabulary for expressing its concepts, and a language for relating them together."

"... a body of literature to help software developers resolve recurring problems encountered throughout all of software development."

"... a shared language for communicating insight and experience about these problems and their solutions."

from http://www.scmpatterns.com/

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The Gang of Four (GOF)

Erich Gamma, Richard Helm, Ralph Johnson, and John Vlissides

"Design Patterns: Elements of Reusable Object-Oriented Software" (1995)

The Hillside Group http://hillside.net/patterns/DPBook/GOF.html

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Microsoft .NET Design Patterns

"Enterprise Solution Patterns Using Microsoft .NET"

From the patterns and practices (p&p) Group

Printed book (ISBN: 0735618399) PDF Download

• http://www.microsoft.com/downloads/details.aspx?familyid=3C81C38E-ABFC-484F-A076-CF99B3485754

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.NET Design Pattern Frameworks

"data & object factory"• Design Pattern Framework 2.0• http://www.dofactory.com/Framework/

Framework.aspx

Ingenious MVC for .NET 2.0 Web and WinForms• http://sourceforge.net/projects/ingeniousmvc

Microsoft "patterns & practices" Group• CAB and ObjectBuilder

• Enterprise Library

• Software Factories (Smart Client, Mobile, Web Service)

• http://msdn.microsoft.com/practices/

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Pattern Types and Families

These are just a few of the common ones ...PatternShare.org lists more than 250 !

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Common Patterns in ASP.NET Model-View-Controller (MVC) Model-View-Presenter (MVP) Use Case Controller Command Publish-Subscribe / Observer Plug-in / Module / Intercepting Filter Service Agent / Proxy / Broker Provider / Adapter Factory / Builder / Injection Singleton Repository

Presentation Logic

Host or Behavioral

Creational

Structural

Persistence

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Agenda

What are Design Patterns? Basic Design Patterns for ASP.NET Basic Pattern Support within ASP.NET Controller Patterns for ASP.NET Advanced Patterns for ASP.NET Conclusion and Summary

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MVC & MVP Presentation Patterns

Both improve reusability of business logic MVC has dependency between model and view MVP improves testability but adds complexity

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Provider / Adapter Patterns

Used by ASP.NET itself, and other frameworks such as Enterprise Library

Allows behavioral changes without prior knowledge of requirements

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Service Agent / Proxy / Broker

Removes dependencies between client and service through intermediate brokers

Range of different implementations, with or without service agent logic component

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Repository Persistence Pattern

Virtualizes storage of an entity in a persistent medium, such as a database or as XML

Hides storage implementation from the application code

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Singleton Creation Pattern Provides for creation of a class for

which only a single instance can exist Useful for exposing read-only data Useful for exposing static methods

that do not rely on instance data Include private default constructor to

prevent client instantiation Provide a static GetInstance method

that returns the current instance

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Agenda

What are Design Patterns? Basic Design Patterns for ASP.NET Basic Pattern Support within ASP.NET Controller Patterns for ASP.NET Advanced Patterns for ASP.NET Conclusion and Summary

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Pattern Support within ASP.NET

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Pattern Support within ASP.NET

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Pattern Support within ASP.NET

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Pattern Support within ASP.NET

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Pattern Support within ASP.NET

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How To... MVP: use ASP.NET code-behind model or

extend code file with custom classes

Provider: use built-in providers or create custom providers from a base class or interface

Repository: typed DataSet, third-party tools (such as CodeSmith), or custom code

Adapter: use built-in (such as CSS control adapters) or create custom adapters

Service Agent and Proxy: Web References in Visual Studio, maybe wrap in custom class

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Basic Patterns in ASP.NET Example

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Agenda

What are Design Patterns? Basic Design Patterns for ASP.NET Basic Pattern Support within ASP.NET Controller Patterns for ASP.NET Advanced Patterns for ASP.NET Conclusion and Summary

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Use Case Controller

Coordinates and sequences interaction between the system and the users to carry out a process

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Page Controller & Front Controller

Select content to display in a page using different partial views, or...

Select which page (view) to display

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Plug-in/Module/Intercepting Filter

Allows the features or behavior of an application to change when it loads separate components that implement extra functionality

Plug-ins and Modules may be custom assemblies (modules), or general-purpose software components such as ActiveX controls or Java applets

Intercepting Filters reside in an application pipeline, such as the ASP.NET HTTP pipeline

Commonly, the components extend the features of the host application

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Implementation in ASP.NET

HTTP Module performs redirection at front Page Controller selects view elements

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How To... Use Case: custom branching code in

presenter class - case statements using Server.Transfer, or displaying partial Views as User Controls

Page Controller: custom base class for Page that handles Init or Load event to perform common tasks then calls a method in the actual page to create page-specific content

Front Controller: an HTTP Module that adds a custom handler to a pipeline event that performs the appropriate Server.Transfer

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Page & Front Controller Example

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Agenda

What are Design Patterns? Basic Design Patterns for ASP.NET Basic Pattern Support within ASP.NET Controller Patterns for ASP.NET Advanced Patterns for ASP.NET Conclusion and Summary

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Factory / Builder / Injection

Separates the construction of a complex object from its representation

Allows use of the same construction process to create different representations

In the Factory pattern, the subclasses of the object generator determine the object type

In the Builder and Injection patterns, the client passes instructions or hints to the object generator to specify the required object type.

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Command Design Pattern

Separates command invoker and receiver

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Observer and Publish/Subscribe

Separates creator and receiver(s) of events

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Command-Observer Implementation

Client creates and subscribes objects

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Command-Observer Implementation

Introduces a dependency between Observer and Subject, but removes the requirement for events

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Command-Observer Example

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Observer and Publish/Subscribe

Separates creator and receiver(s) of events

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Publish-Subscribe Example

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Conclusions - 1

MVP and the various Controller patterns are useful, but firing update events from Model usually is not.

Page Controller and Front Controller allow for custom use case behavior by showing different views or activating different presenters. Front Controller can make use of the Intercepting Filter pattern.

Repository is useful for virtualization of source data.

Singleton is useful for reducing the need for multiple instances.

Service Agent and Proxy are ideal for interacting with remote services.

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Conclusions - 2

Provider is useful for accessing various types of data sources.

Adapter is useful for extending ASP.NET controls.

Factory, Builder, Injection, Observer, and Command are probably less useful, more complex, and can be difficult to implement.

Event-driven patterns like Publish-Subscribe are generally useful only if all subscribers are instantiated within the page lifetime.

The composite Command-Observer event-free approach is useful for removing dependencies between classes.

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References Information about Design Patterns:

• http://msdn.microsoft.com/library/en-us/dnpag/html/intpatt.asp

• http://www.patternshare.org/• http://msdn.microsoft.com/architecture/• http://msdn.microsoft.com/practices/• http://www.dofactory.com/Patterns/Patterns.aspx• http://hillside.net/

Contact: alex@stonebroom.com Slides & code: http://www.daveandal.net/download/ Article: http://www.daveandal.net/articles/