Proper use of [Import] attribute in MEF

Question

I'm learning MEF and I wanted to create a simple example (application) to see how it works in action. Thus I thought of a simple translator. I created a solution with four projects (DLL files):

Contracts
Web
BingTranslator
GoogleTranslator

Contracts contains the ITranslate interface. As the name applies, it would only contain contracts (interfaces), thus exporters and importers can use it.

public interface ITranslator
{
    string Translate(string text);
}

BingTranslator and GoogleTranslator are both exporters of this contract. They both implement this contract and provide (export) different translation services (one from Bing, another from Google).

[Export(typeof(ITranslator))]
public class GoogleTranslator: ITranslator
{
    public string Translate(string text)
    {
        // Here, I would connect to Google translate and do the work.
        return "Translated by Google Translator";
    }
}

and the BingTranslator is:

[Export(typeof(ITranslator))]
public class BingTranslator : ITranslator
{
    public string Translate(string text)
    {
        return "Translated by Bing";
    }
}

Now, in my Web project, I simply want to get the text from the user, translate it with one of those translators (Bing and Google), and return the result back to the user. Thus in my Web application, I'm dependent upon a translator. Therefore, I've created a controller this way:

public class GeneralController : Controller
{
    [Import]
    public ITranslator Translator { get; set; }

    public JsonResult Translate(string text)
    {
        return Json(new
        {
            source = text,
            translation = Translator.Translate(text)
        });
    }
}

and the last piece of the puzzle should be to glue these components (parts) together (to compose the overall song from smaller pieces). So, in Application_Start of the Web project, I have:

        var parts = new AggregateCatalog
            (
                new DirectoryCatalog(Server.MapPath("/parts")), 
                new DirectoryCatalog(Server.MapPath("/bin"))
            );
        var composer = new CompositionContainer(parts);
        composer.ComposeParts();

in which /parts is the folder where I drop GoogleTranslator.dll and BingTranslator.dll files (exporters are located in these files), and in the /bin folder I simply have my Web.dll file which contains importer. However, my problem is that, MEF doesn't populate Translator property of the GeneralController with the required translator. I read almost every question related to MEF on this site, but I couldn't figure out what's wrong with my example. Can anyone please tell me what I've missed here?

Answer 1

OK what you need to do is (without prescribing for performance, this is just to see it working)

public class GeneralController : Controller
{
    [Import]
    public ITranslator Translator { get; set; }

    public JsonResult Translate(string text)
    {
        var container = new CompositionContainer(
        new DirectoryCatalog(Path.Combine(HttpRuntime.BinDirectory, "Plugins")));
        CompositionBatch compositionBatch = new CompositionBatch();
        compositionBatch.AddPart(this);
        Container.Compose(compositionBatch);

        return Json(new
        {
            source = text,
            translation = Translator.Translate(text)
        });
    }
}

---

I am no expert in MEF, and to be frank for what I use it for, it does not do much for me since I only use it to load DLLs and then I have an entry point to dependency inject and from then on I use DI containers and not MEF.

MEF is imperative - as far as I have seen. In your case, you need to pro-actively compose what you need to be MEFed, i.e. your controller. So your controller factory need to compose your controller instance.

Since I rarely use MEFed components in my MVC app, I have a filter for those actions requiring MEF (instead of MEFing all my controllers in my controller facrory):

public class InitialisePluginsAttribute : ActionFilterAttribute
{
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        CompositionBatch compositionBatch = new CompositionBatch();
        compositionBatch.AddPart(filterContext.Controller);
        UniversalCompositionContainer.Current.Container.Compose(
            compositionBatch);
        base.OnActionExecuting(filterContext);
    }
}

Here UniversalCompositionContainer.Current.Container is a singleton container initialised with my directory catalogs.

---

My personal view on MEF

MEF, while not a DI framework, it does a lot of that. As such, there is a big overlap with DI and if you already use DI framework, they are bound to collide.

MEF is powerful in loading DLLs in runtime especially when you have WPF app where you might be loading/unloading plugins and expect everything else to work as it was, adding/removing features.

For a web app, this does not make a lot of sense, since you are really not supposed to drop a DLL in a working web application. Hence, its uses are very limited.

I am going to write a post on plugins in ASP.NET MVC and will update this post with a link.