Dependency injection is the pattern of inverting the access to services, rather passing them to a class that depends on it.

1. Dependency injection
THIS TALK IS LOOSELY COUPLED

2. Lets face it…

3. The OOP Family
OOP
Objects
More…
IoC
Event loops
Schedulers
More..
Dependency inversion
Dependency injection More..

4. What?
Dependency Object
Injection
Object to
Client

5. What?
public class ExampleClass
{
private Logger logger;
public ExampleClass()
{
this.logger = new Logger();
this.logger.Log(“Constructor call”);
}
}
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”);
}
}
Original What we aim for

6. Why?
Problem
Constraints
Updating ServiceA ,need to update ClassA.
Need implementation in compile time.
Not easily testable
Repetitive code for creation.
Tightly coupled
ClassA
SerivceA
ServiceB
Uses

7. Benefits
Benefit Description
Late Binding Services can be swapped depending on needs.
Extensibility Code can be extended and reused.
Parallel development Code can be developed in parallel.
Maintainability Clearly defined classes are easy to maintain.
Testability Unit testing is possible.

8. Dependency inversion principle (SOLID)
A. High-level modules should not depend on low-level modules. Both should depend on abstractions.
B. Abstractions should not depend on details. Details should depend on abstractions.

9. Example Dependency inversion

10. Dependency Injection implementations
Constructor injection
Method injection
Property injection

11. Constructor injection
This is the most common DI.
Injection DEPENDENCY through the class’s constructor when instantiating that class.
Should be used when the injected dependency is required for the class to function.
Solves most INJECTION goals

12. Method injection
Inject the dependency into a single method.
Whole class does not
Generally uncommon, usually used for edge cases.
public class ContactManager : IContactManager
{
….
public bool Save(IContactPersistence contactDatabaseService,
IContact contact)
{
if (logger == null)
{
throw new ArgumentNullException(“logger”);
}
…. // Additional business logic executed before calling the save
return contactDatabaseService.Save(contact);
}
}

13. Property injection
Also called Setter injection.
Used when a class has optional dependencies(Swappable).
Requires NULL checking.
Does not require adding or modifying constructors.

14. Why Not?
Harder to read
More entities to manage
You need to ensure dependencies can be resolved.
Certain implementations can have performance drawbacks.