Dependency injection is a design patter with the potential to write cleaner code. Over the lifetime of a product, maintaining the product is actuall one - if the the most - expensive areas of the overall product costs. Writing clean code can significantly lower these costs. Writing clean code also makes you more efficient during the initial development time and results in a more stable code base. In this talk, we will dive into the basics of Inversion of Control (IoC) and Dependency Injection (DI) to review different ways of achieving decoupling. We will explorer best practices, design, and anti-patterns.

1. East Bay.NET
Cut your Dependencies
Dependency
Injection
Berkeley, February 14th 2013

2. 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 Engineering
and Network Communications
theo@designitright.net
• Enjoys cycling, running and eating
www.designitright.net

3. Rate Session & Win a Shirt
http://www.speakerrate.com/theoj

4. Where to get the Slides
http://www.slideshare.net/theojungeblut

5. 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

6. A cleanly layered Architecture
UI UI
Service Service
Processor Processor Processor
Resources Resources

7. What is the problem?

9. What is Clean Code?

10. Clean Code is maintainable
Source code must be:
• readable & well structured
• extensible
• testable

11. 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

12. What is
Dependency Injection?

13. Without Dependency Injection
public class ExampleClass
{
private logger logger;
public ExampleClass()
{
this.logger = new Logger();
this.logger.Log(“Constructor call”);
}
}

14. Without Dependency Injection
public class ExampleClass
{
private logger logger;
public ExampleClass()
{
this.logger = new Logger();
this.logger.Log(“Constructor call”);
}
}

15. Inversion of Control –
Constructor Injection
http://www.martinfowler.com/articles/injection.html
public class ExampleClass
{
private logger logger;
public ExampleClass(ILogger logger)
{
this.logger = logger;
if (logger == null)
{
throw new ArgumentNullException(“logger”);
}
this.logger.Log(“Constructor call”);
}
}

16. Benefits of Dependency Injection
Benefit Description
Late binding Services can be swapped with
other services.
Extensibility Code can be extended and reused
in ways not explicitly planned for.
Parallel Code can be developed in parallel.
development
Maintainability Classes with clearly defined
responsibilities are easier to
maintain.
TESTABILITY Classes can be unit tested.
* from Mark Seemann’s “Dependency Injection in .NET”, page 16

17. The Adapter Pattern
from Gang of Four, “Design Patterns”

18. What
are
Dependencies ?

19. 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”

20. 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”

21. Lifetime
a Job
for the Container

22. “Register, Resolve, Release”
“Three Calls Pattern by Krzysztof Koźmic: http://kozmic.pl/
Build 1. Register Execu Clean
You code Release
up 2. Resolve te up

23. What the IoC-Container will do for you
Build 1. Register Clean
Release
up 2. Resolve up

24. Separation of Concern (SoC)
probably by Edsger W. Dijkstra in 1974
• Focus on purpose of your code
• Know only the contracts of the
Execu
You code
dependencies
te
• No need to know
implementations
• No need to handle lifetime of
the dependencies

25. The 3 Dimensions of DI
1.Object Composition
2.Object Lifetime
3.Interception

26. Register - Composition Root
• XML based Configuration
• Code based Configuration
• Convention based (Discovery)

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

28. Release
Release objects from Container
when not needed anymore.

29. Anti
Patterns

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

31. Inversion of Control –
Service Locator
http://www.martinfowler.com/articles/injection.html
// UNITY Example
internal static class Program
{
private static UnityContainer unityContainer;
private static SingleContactManagerForm singleContactManagerForm;
private static void InitializeMainForm()
{
singleContactManagerForm =
unityContainer.Resolve<SingleContactManagerForm>();
}
}

32. Inversion of Control –
Service Locator
http://www.martinfowler.com/articles/injection.html
// UNITY Example
internal static class Program
{
private static UnityContainer unityContainer;
private static SingleContactManagerForm singleContactManagerForm;
private static void InitializeMainForm()
{
singleContactManagerForm =
unityContainer.Resolve<SingleContactManagerForm>();
}
}

33. Inversion of Control –
Setter (Property) Injection
http://www.martinfowler.com/articles/injection.html
// UNITY Example
public class ContactManager : IContactManager
{
[Dependency]
public IContactPersistence ContactPersistence
{
get { return this.contactPersistence; }
set { this.contactPersistence = value; }
}
}

34. Property Injection
+ Easy to understand
- Hard to implement robust
* Take if an good default exists
- Limited in application otherwise

35. Method Injection
http://www.martinfowler.com/articles/injection.html
public 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);
}
}

36. Method Injection
• Needed for handling changing
dependencies in method calls

37. Ambient Context
public class ContactManager : IContactManager
{
….
public bool Save (….)
{
….
IUser currentUser = ApplicationContext.CurrentUser;
….
}
}
* The Ambient Context object needs to have a default value if not assigned yet.

38. Ambient Context
• Avoids polluting an API with Cross
Cutting Concerns
• Only for Cross Cutting Concerns
• Limited in application otherwise

39. Interception
Public class LoggingInterceptor : IContactManager
{
public bool Save(IContact contact)
{
bool success; Public class ContactManager :
IContactManager
this. logger.Log(“Starting saving’); {
public bool Save(IContact contact)
success = {
this.contactManager.Save(contact); ….
this. logger.Log(“Starting saving’); return Result
}
return success; }
}
}
* Note: strong simplification of what logically happens through interception.

40. 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
Related Technology:
• Spring.NET • Managed Extensibility Framework (MEF)
• Autofac
• Puzzle.Nfactory
• Ninject
• PicoContainer.NET
• and more

41. The “Must Read”-Book(s)
by Mark Seemann
Dependency
Injection is a set of
software design
principles and
patterns that
enable us to
develop loosely
coupled code.
http://www.manning.com/seemann/

42. Summary Clean Code - DI
Maintainability is achieved through:
• Simplification, Specialization Decoupling
(KISS, SoC, IoC, DI)
• Dependency Injection
Constructor Injection as default,
Property and Method Injection as needed,
Ambient Context for Dependencies with a default,
Service Locator never
Graphic by Nathan Sawaya
courtesy of brickartist.com
• Registration
Configuration by Convention if possible, exception in Code as needed
Configuration by XML for explicit extensibility and post compile setup
• Quality through Testability
(all of them!)

43. Q&A
Downloads,
Feedback & Comments:
theo@designitright.net
www.designitright.net
www.speakerrate.com/theoj
Graphic by Nathan Sawaya courtesy of brickartist.com

44. References…
http://www.manning.com/seemann/
http://en.wikipedia.org/wiki/Keep_it_simple_stupid
http://picocontainer.org/patterns.html
http://en.wikipedia.org/wiki/Dependency_inversion_principle
http://en.wikipedia.org/wiki/Don't_repeat_yourself
http://en.wikipedia.org/wiki/Component-oriented_programming
http://en.wikipedia.org/wiki/Service-oriented_architecture
http://www.martinfowler.com/articles/injection.html
http://www.codeproject.com/KB/aspnet/IOCDI.aspx
http://msdn.microsoft.com/en-us/magazine/cc163739.aspx
http://msdn.microsoft.com/en-us/library/ff650320.aspx
http://msdn.microsoft.com/en-us/library/aa973811.aspx
http://msdn.microsoft.com/en-us/library/ff647976.aspx
http://msdn.microsoft.com/en-us/library/cc707845.aspx
http://msdn.microsoft.com/en-us/library/bb833022.aspx
http://unity.codeplex.com/
Lego (trademarked in capitals as LEGO)

45. Blog, Rating, Slides
www.speakerrate.com/theoj
www.slideshare.net/theojungeblut
http://www.DesignItRight.net

46. Please fill out the
feedback, and…
… thanks for you attention!