Hooked on MEF - Using MEF in ASP.NET MVC, and the Nerd Dinner MEF sample fix

I'm about to embark on a major project in Silverlight 4. In advance of that project, I've been exploring some of the newer technologies such as WCF RIA Services and the Microsoft Managed Extensibility Framework (MEF); both of which shipped in .NET 4.0. While my usage for MEF in Silverlight is more about plug-in and modular architectures, I also have a good bit of interest in MEF as a basic IoC mechanism too.

To get a handle on MEF, I decided to plug it into my TicketDesk 2.0 project, which is being written on the ASP.NET MVC platform. TicketDesk 2.0 uses a class library for all the business and entity framework bits. I generally avoid the IoC design pattern though, preferring instead to just use overloaded constructors; one that takes dependencies for unit testing, and the other that supplies default dependencies to be used by the application at runtime. This technique is common, and is sometimes called a poor-man's IoC. Insults aside, it is simple, easy to code, and works. Traditional IoC implementations on the other-hand tend to add complexity that doesn't do much to advance the application's core functionality.

But MEF is interesting because it offers functionality that can improve how the application actually works, and it has the by-product of being a decent, and simple, IoC container too.

Anyway, once I started using MEF for IoC in TicketDesk, I ran into a slight problem. See, MEF was designed with persistent applications like Silverlight in mind. But in an MVC web environment you have multiple user requests coming in, and many of the objects cannot easily be shared across multiple request threads. So using MEF in this environment requires that you deal with the fact that some objects have to be instantiated on a per-request basis, while others might be scoped to the entire application.

Fortunately, there is this genius person named Hamilton Verissimo de Oliveira (Hammett). Hammet is apparently one of the core devs on the MEF project, and he's done a good bit of writing about MEF in MVC environments. According to his blog, he's even working with the MVC team to get MEF officially supported in the ASP.NET MVC 3 platform.

His most recent sample code is a modified MEF enabled version of the Nerd-Dinner MVC sample application. Scott Hanselman blogged about the MEF version of Nerd Dinner and hosts the downloadable version of the code.

The code in the MEF version of Nerd Dinner is basically a beta of an extended version of MEF for use in MVC scenarios. It includes two class libraries that extend MEF and MVC. These extensions do two things:

1. Provide lazy MEF compositions on a per-request or per-application basis as appropriate. The per-request composition allows your application to handle compositions only for objects you need to service a request at runtime, while the per-application composition can be used for shared application scoped needs.
     
2. Provide convention driven MEF design pattern. MEF is normally attribute driven, where you explicitly declare exports and imports by decorating your code with MEF attributes. But MVC applications are convention driven, so this feature set allows MEF to auto-discover composable parts based on similar conventions... for example, the nerd dinner application treats controllers as composable exports without you having to decorate them with the MEF attributes.

Overall, the code is a mostly complete MVC compatible set of MEF extensions. But the nerd dinner sample only really uses MEF for an IoC design pattern... the models are declared as MEF exports and the extensions handle supplying the appropriate dependencies to the controllers at runtime. But nothing about the way this same is setup should, in theory, prevent you from using MEF in other ways such as a plug-in extensibility mechanism (which is MEF's strong suit anyway).

But... once I got to using the extensions in TicketDesk, I ran into a couple of unusual problems.

First, code in my referenced class library wasn't being passed to my controllers. If I moved the code into the MVC app itself though, it worked like a charm. Second, code in my MVC application that was marked as exports using attributes weren't being passed into constructors for objects in my class library. The nerd dinner example contains the models and controllers both directly in the MVC application assembly itself, so these issues didn't occur there. But once you split your code into two assemblies, things didn't work too smooth. Well... this is just sample software.

So... to track down and fix these problems.

One of the more confusing bits about the nerd-dinner code sample is how the MEF catalogs are built when the app starts up. In global.asax.cs the sample code looks like this:

protected override ComposablePartCatalog CreateRootCatalog()
{
    var agg = new AggregateCatalog();

    foreach (Assembly assembly in BuildManager.GetReferencedAssemblies())
    {
        agg.Catalogs.Add(new AssemblyDiscoveryCatalog(assembly));
        agg.Catalogs.Add(new AssemblyCatalog(assembly));
    }

    return agg;
}

Basically this just loops through all the assemblies in the application and builds an Aggregate catalog of all the MEF composable parts from each assembly. What's odd though is that, for each assembly, it builds two separate catalogs and adds both to the aggregate catalog. One is a standard AssemblyCatalog, and the other is a custom type of catalog called an AssemblyDiscoveryCatalog which is part of the extended MEF code shipped with the Nerd Dinner example.

Now, the aggregate catalog is supposed to merge multiple catalogs, eliminating duplicates and all that... but why build two separate catalogs from each assembly in the first place? Shouldn't AssemblyDiscoveryCatalog alone contain all the parts from any one assembly?

Now... I have been unable to understand exactly what it is that causes the problems I was seeing. When I examine the catalogs in the debugger, and the way they are used, the aggregate catalog appears to contain all the composable parts it should. And when the controller factory is invoked, it finds the right controller, and uses a container that has all the right parts in it.

What I did find that was that by removing the duplicate catalog being created in global.asax.cs did fix the problem where my controllers weren't being given the imports when those parts were coming from the referenced class library.

I chose to remove the AssemblyCatalog from the aggregate, leaving just the AssemblyDiscoveryCatalog being added for each reference assembly.

protected override ComposablePartCatalog CreateRootCatalog()
{
    var agg = new AggregateCatalog();

    foreach (Assembly assembly in BuildManager.GetReferencedAssemblies())
    {
        agg.Catalogs.Add(new AssemblyDiscoveryCatalog(assembly));
        //agg.Catalogs.Add(new AssemblyCatalog(assembly)); 
    }

    return agg;
}

This worked fine, but then I ran into the second problem... if my class lib needed imports from the MVC application, they weren't getting them... basically the opposite problem.
Digging deeper into the second problem, I was able to find a cause. The AssemblyDiscoveryCatalog type has a minor bug:

private IEnumerable InspectAssembliesAndBuildPartDefinitions()
{
    var parts = new List();

    foreach(var assembly in this.Assemblies)
    {
        var attributes = assembly.GetCustomAttributes(typeof(DiscoveryAttribute), true);

        if (attributes.Length == 0)
        {
             parts.AddRange(new AssemblyCatalog(assembly).Parts);
        }
        else
        {
            foreach (DiscoveryAttribute discoveryAtt in attributes)
            {
                var discovery = discoveryAtt.DiscoveryMethod.New();
                var discoveredParts = discovery.BuildPartDefinitions(assembly.GetTypes());

                parts.AddRange(discoveredParts);
            }
        }
    }

    return parts;
}

First this code checks the assembly for a "DiscoveryAttribute". This attribute marks the entire assembly as one that contains classes for which the modified MEF system should "infer" composable parts based on conventions rather than by looking for the explicit MEF attributes. In the nerd dinner example, this attribute is declared at the top of the Conventions.cs class.

Notice how the code immediately afterwards works though. If the assembly isn't marked with the DiscoveryAttribute, it uses the standard MEF mechanisms to add all the parts to the catalog; those mechanisms do so by looking for the explicit MEF attributes in the code.

But when the assembly is marked with the DiscoveryAttribute, then the code does something quite different; it goes through the assembly looking for dynamically discoverable parts based on any defined conventions. It then adds those dynamically discovered parts to the catalog.

And that's the bug! The branch handling DiscoveryAttribute assemblies doesn't have any code that looks for traditional MEF parts declared by attributes!

So... all I had to do was modify this code so it adds both discoverable and inferred attributes both:

private IEnumerable InspectAssembliesAndBuildPartDefinitions()
{
    var parts = new List();

    foreach (var assembly in this.Assemblies)
    {
        var attributes = assembly.GetCustomAttributes(typeof(DiscoveryAttribute), true);

        parts.AddRange(new AssemblyCatalog(assembly).Parts); // add the standard MEF locatable parts
        if (attributes.Length > 0)
        {
            //add any convention inferred parts 
            foreach (DiscoveryAttribute discoveryAtt in attributes)
            {
                var discovery = discoveryAtt.DiscoveryMethod.New();
                var discoveredParts = discovery.BuildPartDefinitions(assembly.GetTypes());

                parts.AddRange(discoveredParts);
            }
        }
    }

    return parts;
}

Now we have a single kind of catalog that can locate both inferred and explicitly declared MEF parts in any assembly. And after this change, my MVC controllers are getting imports from the class lib, and the class lib is getting imports from the MVC app.

Everyone is happy...

Except that it still bugs me why having the two kinds of catalog added to the aggregate catalog keeps the controllers from getting imports from my class library. I suspect strongly though that this may be some kind of bug in the core MEF implementation, perhaps with how Lazy composition actually works.

But either way... fixing the bug in AssemblyDiscoveryCatalog allows you to handle everything in one kind of catalog and works around both problems.