Most of the examples quoted for usage of Dependency Injection, we can solve using the factory pattern as well. Looks like when it comes to usage/design the difference between dependency injection and factory is blurred or thin.
Once someone told me that its how you use it that makes a difference!
I once used StructureMap a DI container to solve a problem, later on I redesigned it to work with a simple factory and removed references to StructureMap.
Can anyone tell me what is the difference between them and where to use what, whats the best practice here?
Dependency Injection
Instead of instantiating the parts itself a car asks for the parts it needs to function.
class Car
{
private Engine engine;
private SteeringWheel wheel;
private Tires tires;
public Car(Engine engine, SteeringWheel wheel, Tires tires)
{
this.engine = engine;
this.wheel = wheel;
this.tires = tires;
}
}
Factory
Puts the pieces together to make a complete object and hides the concrete type from the caller.
static class CarFactory
{
public ICar BuildCar()
{
Engine engine = new Engine();
SteeringWheel steeringWheel = new SteeringWheel();
Tires tires = new Tires();
ICar car = new RaceCar(engine, steeringWheel, tires);
return car;
}
}
Result
As you can see, Factories and DI complement each other.
static void Main()
{
ICar car = CarFactory.BuildCar();
// use car
}
Do you remember goldilocks and the three bears? Well, dependency injection is kind of like that. Here are three ways to do the same thing.
void RaceCar() // example #1
{
ICar car = CarFactory.BuildCar();
car.Race();
}
void RaceCar(ICarFactory carFactory) // example #2
{
ICar car = carFactory.BuildCar();
car.Race();
}
void RaceCar(ICar car) // example #3
{
car.Race();
}
Example #1 – This is the worst because it completely hides the dependency. If you looked at the method as a black box you would have no idea it required a car.
Example #2 – This is a little better because now we know we need a car since we pass in a car factory. But this time we are passing too much since all the method actually needs is a car. We are passing in a factory just to build the car when the car could be built outside the method and passed in.
Example #3 – This is ideal because the method asks for exactly what it needs. Not too much or too little. I don’t have to write a MockCarFactory just to create MockCars, I can pass the mock straight in. It is direct and the interface doesn’t lie.
This Google Tech Talk by Misko Hevery is amazing and is the basis of what I derived my example from. http://www.youtube.com/watch?v=XcT4yYu_TTs
