Question

I’m trying to get my head around IoC w. dependency injection and I have followed this blogpost from Joel Abrahamsson: http://joelabrahamsson.com/entry/inversion-of-control-introduction-with-examples-in-dotnet

I have set up my projects like this:

• Model
• Interfaces
• Controllers
• DAL

And my classes is as follows:

Car

public class Car
{
    public int Id { get; set; }
    public string Brand { get; set; }
    public string Year { get; set; }
}

CarController

public class CarController
 {
     private readonly ICar _carInterface;

     public CarController(ICar car)
     {
         _carInterface = car;
     }

     public void SaveCar(Car car)
     {
          _carInterface.SaveCar(car);
     }
}

ICar interface

public interface ICar
{
    void SaveCar(Car car);
}

DbCar

public class DbCar: ICar
{
    public void SaveCar(Car car)
    {
        throw new NotImplementedException();
    }
}

Now, on the UI, I’m not sure what to do with this 😉 I can sure enough make the Car object that I need, but when new’ing up a CarController, it (of course) expects an ICar interface which I can’t give it.

I’ve got a feeling that I have misunderstood something along the way of reading Joels (great) article 🙂 And I was hoping that maybe someone could shed a light on what I have missed/misunderstood.

Any help/hint is greatly appreciated!

Thanks a lot in advance.

All the best,

Bo

Answer 1

It looks like ICar is not an interface to a car. Rather it’s an interface to a repository for saving cars in. So it should be called ICarRepository (or possibly IGarage.)

You say:

but when new’ing up a CarController, it (of course) expects an ICar
interface which I can’t give it.

Why not? You have an implementation, DbCar. Why can’t you give it one of those?

You asked in a comment about this expression:

new CarController(new DbCar())

Specifically that by writing that line of code, you’ve tied the controller to a specific implementation of the car repository.

That’s true, but only in that case. Somewhere else in a unit test you could write:

new CarController(new FakeCarRepository())

The CarController class is an independent module, and its dependency on other things is clear from its constructor’s parameter list.

IoC or “dependency injection” frameworks are libraries that provide a standard way to construct classes such as CarController, so that the parameters it needs will be specified in a config file. But that’s probably overkill for a simple app.