Introduction to Dependency Injection pattern and a simple implementation in Microsoft Unity

1. IoC in Microsoft Unity
Francesco Garavaglia
03/2016

2. Agenda
 The Problem
 Composition by Examples
 Inversion of Control (IoC)
 Dependency Injection Microsoft
Unity
2

3. What problems
are we trying to
solve?
3

4. 4
The Problem
 We live in an age where writing software to a given set of requirements is no longer enough.
 We have to maintain and change existing code.
 Code quality ( What’s Bad ) R.Martin 1996
- Rigid (hard to modify)
- Fragile ( errors occur on almost every change)
- Immobile (not reusable)
The Problem

5. How?
Our solutions should be:
• Modular
• Testable
• Adaptive to change
• Whatever you want
5

6. How?
Our solutions should be:
• Modular
• Testable
• Adaptive to change
• Whatever you want
6

7. 7
We need a glossary
 Service —An object that performs a well-defined function when called upon
 Client —Any consumer of a service; an object that calls upon a service to perform a well-
understood function
 Dependency —A specific service that is required by another object to fulfill its function.
 Dependent —A client object that needs a dependency (or dependencies) in order to perform its
function.
The Problem

8. Composition:
OLD SCHOOL
8

9. 9
Ex1:Composition
Composition by Examples

10. 10
Ex1: Composition
public class SpellCheckerService{}
public class TextEditor
{
private SpellCheckerService _spellCheckerService;
public TextEditor()
{
_spellCheckerService = new SpellCheckerService();
}
}
class Program
{
static void Main(string[] args)
{
TextEditor textEditor = new TextEditor();
}
}
TextEditor
SpellChecker
Composition by Examples

11. 11
Ex1: Composition
•Simple
What is
Good
•It’s not testable
•It’s hard to
maintain/change
What is
Bad
Composition by Examples

12. Composition:
NEW SCHOOL
12

13. 13
Better approach
TextEditor
+ CheckSpelling() : bool
SpellCheckerService
+ CheckSpelling() : string
«interface»
ISpellCheckerService
+ CheckSpelling() : string
Dependency Inversion
A.High-level modules should not
depend on low-level modules. Both
should depend on abstractions.
B. Abstractions should not depend
upon details. Details should depend
upon abstractions.
Robert C. Martin 1996
Composition by Examples

14. 14
Context
Granny
+ Eat() : void
«interface»
IAppleProvider
+ GetApple() : IApple
RedAppleProvider
+ GetApple() : IApple
GoldenAppleProvider
+ GetApple() : IApple
«interface»
IPillProvider
ConcretePillProvider
Context
Composition by Examples

15. 15
Ex2:Loose Coupling
public class TextEditor
{
private readonly ISpellCheckerService _spellCheckerService;
public TextEditor(ISpellCheckerService spellCheckerService)
{
_spellCheckerService = spellCheckerService;
}
public string CheckSpelling()
{
return _spellCheckerService.CheckSpelling();
}
}
Composition by Examples

16. 16
Ex2: Unit Testing
// Mock
ISpellCheckerService mock = new SpellCheckerServiceMock();
// Instantiate
TextEditor textEditor = new TextEditor(mock);
// Check
Assert.AreEqual(“Mock”, textEditor.CheckSpelling());
Composition by Examples

17. 17
Ex2: Good vs Bad
• Dependencies are obvious.
• Dependency resolution is not
encapsulated.
• Unit Testing is applicable
• Architecture is much better
What is
Good
• We are resolving dependencies
manually while creating instances of
TextEditor.
What is
Bad
 TextEditor lost its “Sovereignty” and is not able to resolve dependencies by himself.
Composition by Examples

18. 18
Ex3: Using Factory
Composition by Examples

19. 19

20. 20

21. 21

22. 22

23. 23

24. 24

25. 25
Factory
TextEditorFactory
+ GetEnglishTextEditor() : TextEditor
+ GetFrenchTextEditor() : TextEditor
TextEditor
FrenchSpellCheckerServ ice
«interface»
ISpellCheckerServ ice
EnglishSpellCheckerServ ice
Composition by Examples

26. 26
What changed
 Any required combination of Text Editor and Spell Checking Service is created by object factory.
•It’s testable
•No manual wiring
What is
Good
•You have to maintain factory or service locator
•The more combinations the more methods we have in
factory.
•Your code knows about the specific factory or factory
interface. This means that infrastructure logic is mixed
with business logic.
•Factory may have states.
What is
Bad
Composition by Examples

27. 27
Ex4: Service Locator
Unfortunately, being a kind of Factory, Service Locators suffer from the same problems
of testability and shared state.
Composition by Examples

28. 28
What are we looking for?
Composition by Examples

29. Inversion of
Control
HOLLYWOOD PRINCIPLE:
DON’T CALL
ME, I’LL CALL
YOU
29

30. 30
Inversion of Control
IoC – is a common characteristic of
frameworks.
According to Martin Fowler the etymology of
the phrase dates back to 1988.
Inversion of Control

31. 31
Dependency Injection
 DI is a kind of IoC
 Inversion of Control is too generic a term
 DI pattern – describes the approach used to lookup a dependency.
 Dependency resolution is moved to Framework.
Inversion of Control

32. 32
SpellCheckerService
+ CheckSpelling() : bool
TextEditor
+ CheckSpelling() : bool
«interface»
ISpellCheckerService
We have already prepared basis
Loosely
coupled
structure
Inversion of Control

33. It’s time to
introduce new
role: Injector
Injector
(sometimes referred to as
a provider or container)
33

34. Unity
34

35. 35
Ex5: Unity
using Microsoft.Practices.Unity;
UnityContainer container = new UnityContainer();
container.RegisterType<ISpellCheckerService, SpellCheckingService>();
TextEditor textEditor = container.Resolve<TextEditor>();
DI with Unity

36. 36
What changed
 Unity container now resolves dependencies
•Automated dependency
resolution
•Business logic and
infrastructure are decoupled.
What is
Good
What is
Bad
DI with Unity

37. 37
Injection Types
 Interface injection (by Martin Fowler)
 Constructor Injection (by Martin Fowler)
 Setter injection (by Martin Fowler)
 Method call injection (Unity)
 Method decorator injection (Guice)
DI with Unity

38. 38
Unity Configuration
<configSections>
<section name="unity" type="Microsoft.Practices.Unity.Configuration.UnityConfigurationSection, Microsoft.Pr
actices.Unity.Configuration"/>
</configSections>
<unity xmlns="http://schemas.microsoft.com/practices/2010/unity">
<alias alias="ISpellCheckerService" type="Unity.Config.ISpellCheckerService, Unity.Config" />
<alias alias="SpellCheckingService" type="Unity.Config.SpellCheckingService, Unity.Config" />
<namespace name="Unity.Config" />
<assembly name="Unity.Config" />
<container>
<register type="ISpellCheckerService" mapTo="SpellCheckingService" />
</container>
</unity>
DI with Unity

39. 39
Unity Configuration
using Microsoft.Practices.Unity;
using Microsoft.Practices.Unity.Configuration;
UnityContainer container = new UnityContainer();
container.LoadConfiguration();
TextEditor textEditor = container.Resolve<TextEditor>();
DI with Unity

40. 40
Dependency tree
DI with Unity

41. 41
Dependency tree
public interface IAdditionalDependency{}
public class AdditionalDependency : IAdditionalDependency{}
public class SpellCheckingService: ISpellCheckerService {
public SpellCheckingService( IAdditionalDependency dependency){}
}
UnityContainer container = new UnityContainer();
container.RegisterType<ISpellCheckerService, SpellCheckingService>();
container.RegisterType<IAdditionalDependency, AdditionalDependency>();
TextEditor textEditor = container.Resolve<TextEditor>();
DI with Unity

42. 42
Dependency tree
SpellCheckerService
+ CheckSpelling() : bool
TextEditor
+ CheckSpelling() : bool
«interface»
ISpellCheckerService
AdditionalDependency
+ CheckSpelling() : bool
«interface»
IAditionalDependenct
DI with Unity

43. 43
Defining Injection Constructor
public class TextEditor
{
private readonly ISpellCheckerService _spellCheckerService;
[InjectionConstructor]
public TextEditor(ISpellCheckerService spellCheckerService)
{
_spellCheckerService = spellCheckerService;
}
public TextEditor(ISpellCheckerService spellCheckerService,string name)
{
_spellCheckerService = spellCheckerService;
}
}
DI with Unity

44. 44
Property Injection
public class TextEditor
{
public ISpellCheckerService SpellCheckerService {get; set;}
[Dependency]
public ISpellCheckerService YetAnotherSpellcheckerService{get;set;}
}
UnityContainer container = new UnityContainer();
container.RegisterType<TextEditor>(new InjectionProperty("SpellCheckerService"));
container.RegisterType<ISpellCheckerService, SpellCheckingService>();
TextEditor textEditor = container.Resolve<TextEditor>();
DI with Unity

45. 45
Method call injection
public class TextEditor
{
public ISpellCheckerService SpellcheckerService {get; set;}
public void Initialize (ISpellCheckerService spellcheckerService)
{
_spellCheckerService = spellcheckerService;
}
}
UnityContainer container = new UnityContainer();
container.RegisterType<ISpellCheckerService, SpellCheckingService>();
TextEditor textEditor = container.Resolve<TextEditor>();
DI with Unity

46. 46
Lifetime Managers
 TransientLifetimeManager
Returns a new instance of the requested type for each call. (default behavior)
 ContainerControlledLifetimeManager
Implements a singleton behavior for objects. The object is disposed of when you dispose of the
container.
 ExternallyControlledLifetimeManager
Implements a singleton behavior but the container doesn't hold a reference to object which will
be disposed of when out of scope.
 HierarchicalifetimeManager
Implements a singleton behavior for objects. However, child containers don't share instances
with parents.
 PerResolveLifetimeManager
Implements a behavior similar to the transient lifetime manager except that instances are reused
across build-ups of the object graph.
 PerThreadLifetimeManager
Implements a singleton behavior for objects but limited to the current thread.
DI with Unity

47. 47
Unity Singleton
UnityContainer container = new UnityContainer();
container.RegisterType<ISpellCheckerService, SpellCheckingService>(new
ContainerControlledLifetimeManager());
TextEditor textEditor = container.Resolve<TextEditor>();
DI with Unity

48. 48
Container Hierarchy
DI with Unity

49. 49
Unity Limitations
• When your objects and classes have no dependencies on other objects or classes.
• When your dependencies are very simple and do not require abstraction.
DI with Unity

50. 50
References
 Martin Fowler – Dependency Injection
http://martinfowler.com/articles/injection.html
 R. Martin - Dependency Inversion principle
http://www.objectmentor.com/resources/articles/dip.pdf
 Developer's Guide to Dependency Injection Using Unity
https://msdn.microsoft.com/en-us/library/dn223671(v=pandp.30).aspx

51. Thanks
FRANCESCO.GARAVAGLIA@GMAIL.COM