Using ASP.NET MVC with MEF

I wrote this post almost a year ago, but never published it for some reason. Anyway, here is a little MVC/MEF magic.

By default a controller in MVC must have a parameterless constructor. When using MEF a good practice is to inject the services via constructor parameters. These two in combination obviously creates an issue where the following scenario will not work out of the box, since there is no parameterless constructor for MVC to use.

Note that the PartCreationPolicy is set to NonShared since a new controller have to be initialized for each request.

[Export]
[PartCreationPolicy(CreationPolicy.NonShared)]
public class HomeController : Controller
{
    private readonly IServiceClient _service;

    [ImportingConstructor]
    public HomeController(IServiceClient service)
    {
        _service = service;
    }

    public ActionResult Index()
    {
        return View();
    }
}

For this scenario to work we need to override the default controller factory with a custom implementation. This class basically just gets initialized with our MEF container and the overridden GetControllerInstance uses this container to get an appropriate controller for the current request.

public class MefControllerFactory : DefaultControllerFactory
{
    private readonly CompositionContainer _compositionContainer;

    public MefControllerFactory(CompositionContainer compositionContainer)
    {
        _compositionContainer = compositionContainer;
    }

    protected override IController GetControllerInstance(RequestContext requestContext, Type controllerType)
    {
        var export = _compositionContainer.GetExports(controllerType, null, null).SingleOrDefault();

        IController result;

        if (export != null)
        {
            result = export.Value as IController;
        }
        else
        {
            result = base.GetControllerInstance(requestContext, controllerType);
            _compositionContainer.ComposeParts(result);
        }

        return result;
    }
}

The only thing left to do now is to tell MVC to use our custom controller factory with the SetControllerFactory method in Global.asax with our container as an argument to the MefControllerFactory.

protected void Application_Start()
{
    ControllerBuilder.Current.SetControllerFactory(new MefControllerFactory(_container));
}

When this is set up an empty constructor is no longer necessary in our controllers since MEF will be responsible for instantiation of the controllers.