Dependency injection is a technique where dependencies are provided to classes through constructors, setters, or interfaces rather than having the class create the dependencies directly. This allows for looser coupling between classes and makes testing easier. Common forms of dependency injection include constructor injection, setter injection, and interface injection. While dependency injection improves testability, it can also leak implementation details and prevent deferred creation of dependencies, so it should be used carefully.

1. Dependency Injection
Chester Hartin
Code review 10/21/11

2. Introduction
 What is dependency injection?
 Constructor injection
 Setter injection
 Interface Injection
 DI Caution

3. What is Dependency Injection
 A type of loC (Inversion of Control)
where we move the creation and
binding of a dependency outside of
the class that depends on it

4. Packing a lunch

5. Lunch Will Be Provided

6. Where Do Dependencies Come From?
MyClass IDependency
Dependency

7. Where Do Dependencies Come From?
Injector
MyClass IDependency
Dependency

8. Constructor Injection
 Most Common
 Simple
 Passdependency into dependent class
via constructor

9. Example
public class Injector
{
ICreditCard creditCard = new MasterCard();
Shopper shopper = new Shopper(creditCard);
}
public class Shopper
{
private readonly ICreditCard creditCard;
public Shopper(ICreditCard creditCard)
{
this.creditCard = creditCard;
}
}

10. Setter Injection
 Create a setter on the dependent
class
 Use the setter to set the
dependency

11. Example
public class Injector
{
ICreditCard creditCard = new MasterCard();
Shopper shopper = new Shopper();
shopper.CreditCard = creditCard;
}
public class Shopper
{
public ICreditCard CreditCard { get; set; }
}

12. Interface Injection
 Dependent class implements an
interface
 Injector uses the interface to set
the dependency

13. Example
public class Injector
{
ICreditCard creditCard = new MasterCard();
Shopper shopper = new Shopper();
((IDependOnCreditCard)shopper).Inject(creditCard);
}
public class Shopper: IDependOnCreditCard
{
private ICreditCard creditCard;
public void Inject(ICreditCard creditCard)
{
this.creditCard = creditCard;
}
}
public interface IDependOnCreditCard
{
void Inject(ICreditCard creditCard);
}

14. DI Caution
 Leaks the internal implementation details
of a class
 Violates encapsulation
 Injecting “guts” into the class
 Prevents deferred creation
 Dependencies created before needed
 Numbs you from the pain
 Easier to unit test classes that should be
broken up
 Watch out for too many dependencies

15. DI Open-Source Libraries
 Unity
 Castle Windsor
 Structure Maps
 Ninject