The document discusses dependency injection and clean code principles. It defines dependency injection as a method of implementing inversion of control by passing dependencies (such as logger objects) into a class from outside rather than instantiating them inside the class. This allows for loose coupling, extensibility, and testability. The document outlines benefits of dependency injection like late binding, maintainability, and parallel development. It discusses the roles of an IoC container in composing object graphs via registration and resolving dependencies, as well as managing object lifetimes. Anti-patterns like service locators and ambient contexts are also covered.

1. Dependency
Injection
San Diego, July 27th, 2013
SolCal
Code Camp
Clean Code II

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
• Enjoys cycling, running and eating
theo@designitright.net
www.designitright.net

3. Rate Session & Win a Shirt
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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. UI
Service
ProcessorProcessor
Service
ResourcesResources
UI
Processor
A cleanly layered Architecture

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

15. 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”);
}
}

16. Why is Dependency
Injection beneficial?

17. 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
development
Code can be developed in parallel.
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

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

19. What
are
Dependencies ?

20. 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”

21. 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”

22. Lifetime
a Job
for the Container

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

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

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

26. Separation of Concern (SoC)
probably by Edsger W. Dijkstra in 1974
You code
Execu
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

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

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

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

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

31. Anti
Patterns

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

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

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

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

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

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

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

39. 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.

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

41. Interception
Public 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.

42. 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)

43. 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.

44. 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
• 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!)
Graphic by Nathan Sawaya
courtesy of brickartist.com

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

46. 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)

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