MVC2 Using Unity IoC - How are controllers resolved?

Question

I have a question concerning how the Unity container I have set up is resolving controller dependencies. I've been searching around for an explanation of this but haven't found anything that is real clear on the subject. And maybe the answer is staring me in the face... Take a look at the following code that I'm sure many MVC guys have seen:

protected override IController GetControllerInstance(System.Web.Routing.RequestContext requestContext, Type controllerType)
{
    IController controller;
    try
    {
        controller = container.Resolve(controllerType) as IController;
    }
    catch (Exception ex)
    {
        throw new InvalidOperationException(string.Format("Error resolving controller {0}", controllerType.Name), ex);
    }
    return controller;
}

So that is my override of GetControllerInstance in my controller factory that I have set up. Obviously, the controller types are being resolved out of the Unity container but how are they getting registered in the container in the first place? How does the MVC framework know to register types in the container? I realize that the types are being resolved out of that container but I don't like not knowing how it is resolving the controller types.

Darin's answer is correct, however you should be aware that not all IOC containers support resolving a type that has not been explicitly registered with it. — Ryan
– Ryan, Commented Aug 31, 2010 at 18:48
Ah, I think this finally makes sense. Between Darin's answer and your comment my curiosity has been satisfied. Thanks for the help! — Anthony
– Anthony, Commented Aug 31, 2010 at 18:58

Darin Dimitrov · Accepted Answer · 2010-08-31 18:50:58Z

1

It is the DefaultControllerFactory which is responsible for this. Here's how it works:

ASP.NET MVC uses reflection to list all types that inherit from Controller in all assemblies and caches them. If you really want to dig further look at the source code or use reflector and checkout this method on the DefaultControllerFactory type:

protected internal virtual Type GetControllerType(
    RequestContext requestContext, string controllerName)

When a request comes it uses the routing table to determine which is the current controller and tries to find it in the list of cached controller types. If it finds one it calls the GetControllerInstance method passing the given type so that the DI framework could provide the controller instance given its type.

edited Aug 31, 2010 at 18:50

answered Aug 31, 2010 at 18:45

Darin Dimitrov

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3 Comments

Anthony Over a year ago

I think I understand up to that point but I'm a little fuzzy on the framework caching the controller types. I still feel like there's a disconnect in my understanding. So you're saying that the MVC framework caches the controller types in the DI framework that I'm using? It just seems like the DI framework is magically registering controller types behind the scenes without me knowing about it. Or, does the Unity container already have the ability to scan the current assembly for types that might be requested? Maybe that's where I'm getting confused...

Darin Dimitrov Over a year ago

Yes it's caching them. How do you imagine reflecting over all the types over all the referenced assemblies on each request? This will be slooooooow.

Anthony Over a year ago

Yes, I understand that would be incredibly slow. However, as I understand it now, the Unity container itself is not registering the cached types. According to Ryan's comment, it's the container's ability to resolve a type that hasn't been explicitly registered that allows this to work at all.

Jared314 · Accepted Answer · 2010-08-31 18:53:27Z

0

MVC does not register the types with Unity. The Unity container inspects what you ask for to see if it can instantiated it with what it knows about.

answered Aug 31, 2010 at 18:53

Jared314

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