Dependency Injection What, Why and How

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Dependency InjectionWhat, Why and How

• Peter Provost
• peter.provost_at_microsoft.com
• http//www.peterprovost.org/
• Slides will be here

Brad Wilson bradwils_at_microsoft.com http//agilepro
grammer.com/dotnetguy
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What is Dependency Injection?

• DI is all about wiring up objects
• Come on, that isnt hard. Weve been doing that
  for years!

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A Real World Example?
Web App
Stock Quotes
Authenticator
Error Handler
Logger
Database
This example was originally created by Jim
Weirich in Ruby on his blog. See his original
article at http//onestepback.org/index.cgi/Tech/R
uby/DependencyInjectionInRuby.rdoc
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The Old Way

• public class WebApp
• public WebApp()
• quotes new StockQuotes()
• authenticator new Authenticator()
• database new Database()
• logger new Logger()
• errorHandler new ErrorHandler()
• // More code here...

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What about the child objects?

• How does the StockQuotes find the Logger?
• How does the Authenticator find the database?
• Etc.?
• Suppose you want to use a TestingLogger instead?
  Or a MockDatabase?

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Service Locator Interface

• public interface ILocator
• TObject GetltTObjectgt()

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Service Locator Example

• public class MyLocator ILocator
• protected DictionaryltType, objectgt dict new
  DictionaryltType,objectgt()
• public MyLocator()
• dict.Add(typeof(ILogger), new Logger())
• dict.Add(typeof(IErrorHandler), new
  ErrorHandler(this))
• dict.Add(typeof(IQuotes), new StockQuotes(this))
  
• dict.Add(typeof(IDatabase), new Database(this))
• dict.Add(typeof(IAuthenticator), new
  Authenticator(this))
• dict.Add(typeof(WebApp), new WebApp(this))

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StockQuotes with Locator

• public class StockQuotes
• public StockQuotes(ILocator locator)
• errorHandler locator.GetltIErrorHandlergt()
• logger locator.GetltILoggergt()
• // More code here...

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Good things

• Classes are decoupled from explicit imlementation
  types
• Easy to externalize the config

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Bad things

• Everyone takes a dependency on the ILocator
• Hard to store constants and other useful
  primitives
• Creation order is still a problem

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Dependency Injection Containers

• Gets rid of the dependency on the ILocator
• Object is no longer responsible for finding its
  dependencies
• The container does it for you

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Then what?

• Write your objects the way you want
• Setup the container
• Ask the container for objects
• The container creates objects for you and
  fulfills dependencies

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Setting Up the Container (Code)

• DIContainer container new DIContainer()
• container.RegisterltILogger, Loggergt()
• container.RegisterltIDatabase, Databasegt()
• container.RegisterltIErrorHandler,
  ErrorHandlergt()
• container.RegisterltIQuotes, StockQuotesgt()
• container.RegisterltIAuthenticator,
  Authenticatorgt()
• container.RegisterltIWebApp, WebAppgt()

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Setting Up the Container (XML)

• ltDIContainergt
• ltTypeMap fromILogger toLogger /gt
• ltTypeMap fromIDatabase toDatabase /gt
• ltTypeMap fromIErrorHandler toErrorHandler
  /gt
• ltTypeMap fromIQuotes toStockQuotes /gt
• ltTypeMap fromIAuthenticator toAuthenticator
  /gt
• lt/DIContainergt

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Creating Objects

• Test
• public void WebAppTest()
• DIContainer container GetContainer()
• IStockQuotes quotes container.GetltIStockQuotes
  gt()
• IAuthenticator auth container.GetltIAuthenticat
  orgt()
• Assert.IsNotNull( quotes.Logger )
• Assert.IsNotNull( auth.Logger )
• Assert.AreSame( quotes.Logger, auth.Logger )

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The Possibilities are Endless?

• To Singleton or Not to Singleton?
• Nested Containers
• Property Setter
• Object Lifetime
• Method Invocation
• Event Wire-up
• Instrumentation
• Method Interception via Dynamic Proxies

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Existing Frameworks

• Java
• Pico Container
• Spring Framework
• HiveMind
• Ruby
• Rico
• Copland

• Python
• PyContainer
• .NET
• Pico.NET
• Spring.NET
• pp Object Builder

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Demo

• patterns practices Object Builder

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Looking for a Job?

• patterns practices is hiring
• See one of us after the talk or e-mailDarrel
  Snow dsnow_at_microsoft.com