Dependency injection in MVC 3 using unity framework

Question

I have implemented dependency injection in MVC 3 unity framework and following the instructions.

It worked , but I have a few questions regarding this:

Here is my implementation :

public interface ID
{

    string MReturn();
}

And the classes which implement this interface are:

public class D:ID
{
    public string MReturn()
    {
        return "Hi";
    }
}
public class E : ID
{
    public string MReturn()
    {
        return "HiE";
    }
}

public class F : ID
{
    public string MReturn()
    {
        return "Hif";
    }
}

In the bootstrapper class ,

    private static IUnityContainer BuildUnityContainer()
    {

        var container = new UnityContainer();
        container.RegisterType<ID, D>();

        container.RegisterType<IController, HomeController>("feedbackRepo");
        container.RegisterType<ID, E>();
        container.RegisterType<ID, F>();
      // register all your components with the container here
        // it is NOT necessary to register your controllers

        // e.g. container.RegisterType<ITestService, TestService>();            

        return container;
    }

Now my question is

"I want to set the service class D in the Homecontroller constructor, but according to the above code, it is setting "class F" in the constructor.

Is there any way to do this?any modification to the above code?

Answer 1

The reason why F is injected is because it is the last registered implementation of ID. It basically overwrites the previous registrations.

If you have different implementations of some interface/base class and you want to inject specific implementations in different controllers you could register them as named instances:

container.RegisterType<ID, D>("d");
container.RegisterType<ID, E>("e");
container.RegisterType<ID, F>("f");

and then register the controller in the container and inject the desired named instance of ID:

container.RegisterType<HomeController>(
    new PerRequestLifetimeManager(),
    new InjectionConstructor(new ResolvedParameter<ID>("d"))
);

Note that the controller is registered with PerRequestLifetimeManager to ensure that new instances are created for each HTTP request.