True Loosely Coupled

Dependency Injection

About me

Sergio Romero

sromero@teksystems.com

Blog: http://sergeromerosoftware.wordpress.com

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Agenda

• Inversion of Control

• Dependency Injection

• Anti-patterns

• Patterns

• Composition Root

• Examples

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Inversion of Control

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Dependency Injection

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• Commonly used interchangeably as synonyms.

• Dependency Injection is a way to implement Inversion of Control.

Inversion of Control vs. Dependency

Injection

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• Late binding

• Extensibility

• Maintainability

• Parallel development

• Testability

Dependency Injection Benefits

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• Service Locator

• Bastard Injection

Dependency Injection Anti-Patterns

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• First described by Martin Fowler.

• It is basically a static Dictionary that contains all the definitions of

the objects to use:

Service Locator

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• Maintainability and reusability are hindered since:

> Dependencies are hidden and could throw unexpected and

hard to debug exceptions if the dependency is not properly

configured.

> All the assemblies now have a hard dependency on the

service locator’s one.

• Testability is hindered since configuration is needed for each

testing assembly.

• Not really necessary for DI to exist or work.

Why is it an anti-pattern?

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• Incorrectly confused with Poor Man’s DI.

• Typically the overloaded constructor is used for testing and the

default one for production code.

Bastard Injection

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• Use of foreign default: The class drags along the dependant

assembly which may not be needed in another module.

• The benefits of loose coupling are lost. The foreign default

hinders the ability to reuse or replace the class and even the

whole module.

• It makes parallel development more difficult since now the class

has a strong dependency.

• A DI Container will not know which constructor to use.

Why is it an anti-pattern?

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• Constructor Injection

• Property (Setter) Injection

• Method Injection

Dependency Injection Patterns

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• The Dependency is exposed in the class’ constructor.

• Viewed in isolation makes it seem that the instantiation of the

dependency is just pushed up one level.

Constructor Injection

Property Injection

• Should only be used when a good local default is available.

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Method Injection

• Best used when implementation varies between calls to the

method.

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• Where should we compose object graphs? As close as possible

to the application’s entry point.

• A (preferably) unique location in an application where modules

are composed together.

• Even a very modular application that uses loose coupling and

late binding to compose itself has a root that contains the entry

point into the application.

The Composition Root

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• Examples of composition roots:

> A console application is an executable (.exe) with a Main

method.

> An ASP.NET web application is a library (.dll) with an

Application_Start event handler in its Global.asax.

> A WPF application is an executable (.exe) with an App.xaml

file.

> A WCF service is a library (.dll) with a class that derives from a

service interface, although you can hook into a more low-level

entry point by creating a custom ServiceHostFactory.

The Composition Root

DI or IoC Containers

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• A DI Container is a software library that can automate many

of the tasks involved in composing objects and managing

their lifetimes.

• Object composition may be achieved via Poor Man’s DI, but

the challenges in managing the object’s lifetimes makes a DI

Container an invaluable tool.

• Most DI Containers will handle the most life cycles.

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• Some of the most common DI Containers are:

> Castle Windsor

> Structure Map

> Spring .Net

> Autofac

> Unity

> MEF

DI or IoC Containers

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Composition Root with Poor Man’s

DI Example

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Composition Root with IoC

Container Example

References http://en.wikipedia.org/wiki/Inversion_of_control

http://en.wikipedia.org/wiki/Dependency_Injection

http://martinfowler.com/articles/injection.html#InversionOfControl

http://martinfowler.com/bliki/InversionOfControl.html

http://blog.ploeh.dk/2010/02/03/ServiceLocatorisanAnti-Pattern/

http://blogs.clariusconsulting.net/kzu/what-is-all-the-fuzz-about-the-

new-common-iservicelocator/

http://blogs.msdn.com/b/nblumhardt/archive/2008/12/27/container-

managed-application-design-prelude-where-does-the-container-

belong.aspx

http://ayende.com/blog/4092/reviewing-nerddinner

http://blog.ploeh.dk/2011/07/28/CompositionRoot/

Dependency Injection in .Net, Mark Seeman, Manning Publications

Questions?