Dependency Injection

San Diego, July 27th, 2013

SolCalCode Camp

Clean Code II

Theo Jungeblut• Engineering manager & lead by day

at AppDynamics in San Francisco

• Coder & software craftsman by night, house builder and soon to be dad

• Architects decoupled solutions & crafts maintainable code to last

• Worked in healthcare and factory automation, building mission critical applications, framework & platforms

• Degree in Software Engineeringand Network Communications

• Enjoys cycling, running and eatingtheo@designitright.net

www.designitright.net

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Overview

• What is the issue?• What is Dependency Injection?• What are Dependencies?• What is the IoC-Container doing for you?• What, how, why?• Q & A

UI

Service

ProcessorProcessor

Service

ResourcesResources

UI

Processor

A cleanly layered Architecture

What is the problem?

What is Clean Code?

Clean Code is maintainable

Source code must be:• readable & well structured• extensible• testable

Code Maintainability *

Principles Patterns Containers

Why? How? What?

Extensibility Clean Code Tool reuse

* from: Mark Seemann’s “Dependency Injection in .NET” presentation Bay.NET 05/2011

What is Dependency Injection?

Without Dependency Injectionpublic class ExampleClass{

private Logger logger;

public ExampleClass(){

this.logger = new Logger();

this.logger.Log(“Constructor call”);}

}

public class ExampleClass{

private Logger logger;

public ExampleClass(){

this.logger = new Logger();

this.logger.Log(“Constructor call”);}

}

Without Dependency Injection

Avoid

Inversion of Control –

Constructor Injectionhttp://www.martinfowler.com/articles/injection.html

public class ExampleClass{

private ILogger logger;

public ExampleClass(ILogger logger){

this.logger = logger;if (logger == null){

throw new ArgumentNullException(“logger”);

}

this.logger.Log(“Constructor call”);}

}

Why is Dependency Injection beneficial?

Benefits of Dependency InjectionBenefit Description

Late binding Services can be swapped withother services.

Extensibility Code can be extended and reusedin ways not explicitly planned for.

Paralleldevelopment

Code can be developed in parallel.

Maintainability Classes with clearly definedresponsibilities are easier to maintain.

TESTABILITY Classes can be unit tested.

* from Mark Seemann’s “Dependency Injection in .NET”, page 16

The Adapter Patternfrom Gang of Four, “Design Patterns”

What are

Dependencies ?

Stable Dependency

“A DEPENDENCY that can be referenced without any detrimental effects.

The opposite of a VOLATILE DEPENDENCY. “

* From Glossary: Mark Seemann’s “Dependency Injection in .NET”

Volatile Dependency“A DEPENDENCY that involves side effects that

may be undesirable at times.

This may include modules that don’t yet exist, or that have adverse requirements on its

runtime environment.

These are the DEPENDENCIES that are addressed by DI.“

* From Glossary: Mark Seemann’s “Dependency Injection in .NET”

Lifetime a Job

for the Container

public class ExampleClass{

private Logger logger;

public ExampleClass(){

this.logger = new Logger();

this.logger.Log(“Constructor call”);}

}

Without Dependency Injection

Avoid

“Register, Resolve, Release”“Three Calls Pattern by Krzysztof Koźmic: http://kozmic.pl/

1. Register 2. Resolve

Build up You code

Execute ReleaseClean

up

What the IoC-Container will do for you

1. Register 2. Resolve

Build up

ReleaseClean up

Separation of Concern (SoC)probably by Edsger W. Dijkstra in 1974

You codeExecu

te

• Focus on purpose of your code• Know only the contracts of the

dependencies• No need to know

implementations• No need to handle lifetime of

the dependencies

The 3 Dimensions of DI

1.Object Composition2.Object Lifetime3.Interception

Register - Composition Root

• XML based Configuration• Code based Configuration• Convention based (Discovery)

Resolve

Resolve a single object request for example by Constructor Injection by resolving the needed object graph for this object.

Release

Release objects from Container when not needed anymore.

Anti Patterns

Control Freak

http://www.freakingnews.com/The-Puppet-Master-will-play-Pics-102728.asp

// UNITY Exampleinternal static class Program{ private static UnityContainer unityContainer; private static SingleContactManagerForm singleContactManagerForm;

private static void InitializeMainForm() { singleContactManagerForm = unityContainer.Resolve<SingleContactManagerForm>(); }}

Inversion of Control –

Service Locatorhttp://www.martinfowler.com/articles/injection.html

// UNITY Exampleinternal static class Program{ private static UnityContainer unityContainer; private static SingleContactManagerForm singleContactManagerForm;

private static void InitializeMainForm() { singleContactManagerForm = unityContainer.Resolve<SingleContactManagerForm>(); }}

Inversion of Control –

Service Locatorhttp://www.martinfowler.com/articles/injection.html

Anti-Pattern

based on Mark Seemann

Inversion of Control –

Setter (Property) Injection

// UNITY Examplepublic class ContactManager : IContactManager{ [Dependency] public IContactPersistence ContactPersistence { get { return this.contactPersistence; }

set { this.contactPersistence = value; } }}

http://www.martinfowler.com/articles/injection.html

Property Injection

+ Easy to understand

- Hard to implement robust

* Take if an good default exists

- Limited in application otherwise

Method Injectionpublic class ContactManager : IContactManager{ …. public bool Save (IContactPersistencecontactDatabaseService, IContact contact) { if (logger == null) {

throw new ArgumentNullException(“logger”); } …. // Additional business logic executed before calling the save

return contactDatabaseService.Save(contact); }}

http://www.martinfowler.com/articles/injection.html

Method Injection

• Needed for handling changing dependencies in method calls

Ambient Contextpublic class ContactManager : IContactManager{ …. public bool Save (….) { …. IUser currentUser = ApplicationContext.CurrentUser; …. }}

* The Ambient Context object needs to have a default value if not assigned yet.

Ambient Context

• Avoids polluting an API with Cross Cutting Concerns

• Only for Cross Cutting Concerns

• Limited in application otherwise

InterceptionPublic class LoggingInterceptor : IContactManager{ public bool Save(IContact contact) { bool success;

this. logger.Log(“Starting saving’);

success = this.contactManager.Save(contact);

this. logger.Log(“Starting saving’);

return success; }}

Public class ContactManager : IContactManager{ public bool Save(IContact contact) { ….

return Result }}

* Note: strong simplification of what logically happens through interception.

Dependency Injection Container & more• Typically support all types of Inversion of Control mechanisms• Constructor Injection• Property (Setter) Injection• Method (Interface) Injection• Service Locator

•.NET based DI-Container• Unity• Castle Windsor • StructureMap• Spring.NET• Autofac• Puzzle.Nfactory• Ninject• PicoContainer.NET• and more

Related Technology:• Managed Extensibility Framework (MEF)

The “Must Read”-Book(s)

http://www.manning.com/seemann/

by Mark Seemann

Dependency Injection is a set of software design principles and patterns that enable us to develop loosely coupled code.

Summary Clean Code - DIMaintainability is achieved through:

• Simplification, Specialization Decoupling (KISS, SoC, IoC, DI)

• Dependency InjectionConstructor Injection as default, Property and Method Injection as needed, Ambient Context for Dependencies with a default,Service Locator never

• Registration Configuration by Convention if possible, exception in Code as neededConfiguration by XML for explicit extensibility and post compile setup

• Quality through Testability (all of them!)

Graphic by Nathan Sawaya

courtesy of brickartist.com

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Q & A

Graphic by Nathan Sawaya courtesy of brickartist.com

theo@designitright.netwww.designitright.net www.speakerrate.com/theoj

References… http://www.manning.com/seemann/http://en.wikipedia.org/wiki/Keep_it_simple_stupidhttp://picocontainer.org/patterns.htmlhttp://en.wikipedia.org/wiki/Dependency_inversion_principlehttp://en.wikipedia.org/wiki/Don't_repeat_yourselfhttp://en.wikipedia.org/wiki/Component-oriented_programminghttp://en.wikipedia.org/wiki/Service-oriented_architecturehttp://www.martinfowler.com/articles/injection.htmlhttp://www.codeproject.com/KB/aspnet/IOCDI.aspxhttp://msdn.microsoft.com/en-us/magazine/cc163739.aspxhttp://msdn.microsoft.com/en-us/library/ff650320.aspxhttp://msdn.microsoft.com/en-us/library/aa973811.aspxhttp://msdn.microsoft.com/en-us/library/ff647976.aspxhttp://msdn.microsoft.com/en-us/library/cc707845.aspxhttp://msdn.microsoft.com/en-us/library/bb833022.aspxhttp://unity.codeplex.com/

Lego (trademarked in capitals as LEGO)

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