How can we support modular and testable patterns with ASP.NET MVC 4 and MEF 2? - asp.net-mvc

Question

We're trying to use MEF 2 with ASP.NET MVC 4 to support an extensible application. There are really 2 parts to this question (hope that's okay SO gods):
How do we use Microsoft.Composition and the MVC container code (MEF/MVC demo source) to replace Ninject as our DI for ICoreService, ICoreRepository, IUnitOfWork, and IDbContext?
It looks like we can't use both Ninject and the MVC container at the same time (I'm sure many are saying "duh"), so we'd like to go with MEF, if possible. I tried removing Ninject and setting [Export] attributes on each of the relevant implementations, spanning two assemblies in addition to the web project, but Save() failed to persist with no errors. I interpreted that as a singleton issue, but could not figure out how to sort it out (incl. [Shared]).
How do we load multiple assemblies dynamically at runtime?
I understand how to use CompositionContainer.AddAssemblies() to load specific DLLs, but for our application to be properly extensible, we require something more akin to how I (vaguely) understand catalogs in "full" MEF, which have been stripped out from the Microsoft.Composition package (I think?); to allow us to load all IPluggable (or whatever) assemblies, which will include their own UI, service, and repository layers and tie in to the Core service/repo too.
EDIT 1
A little more reading solved the first problem which was, indeed, a singleton issue. Attaching [Shared(Boundaries.HttpRequest)] to the CoreDbContext solved the persistence problem. When I tried simply [Shared], it expanded the 'singletonization' to the Application level (cross-request) and threw an exception saying that the edited object was already in the EF cache.
EDIT 2
I used the iterative assembly loading "meat" from Nick Blumhardt's answer below to update my Global.asax.cs code. The standard MEF 2 container from his code did not work in mine, probably because I'm using the MEF 2(?) MVC container. Summary: the code listed below now works as desired.
CoreDbContext.cs (Data.csproj)
[Export(typeof(IDbContext))]
[Shared(Boundaries.HttpRequest)]
public class CoreDbContext : IDbContext { ... }
CoreRepository.cs (Data.csproj)
[Export(typeof(IUnitOfWork))]
[Export(typeof(ICoreRepository))]
public class CoreRepository : ICoreRepository, IUnitOfWork
{
[ImportingConstructor]
public CoreRepository(IInsightDbContext context)
{
_context = context;
}
...
}
CoreService.cs (Services.csproj)
[Export(typeof(ICoreService))]
public class CoreService : ICoreService
{
[ImportingConstructor]
public CoreService(ICoreRepository repository, IUnitOfWork unitOfWork)
{
_repository = repository;
_unitOfWork = unitOfWork;
}
...
}
UserController.cs (Web.csproj)
public class UsersController : Controller
{
[ImportingConstructor]
public UsersController(ICoreService service)
{
_service = service;
}
...
}
Global.asax.cs (Web.csproj)
public class MvcApplication : System.Web.HttpApplication
{
protected void Application_Start()
{
CompositionProvider.AddAssemblies(
typeof(ICoreRepository).Assembly,
typeof(ICoreService).Assembly,
);
// EDIT 2 --
// updated code to answer my 2nd question based on Nick Blumhardt's answer
foreach (var file in System.IO.Directory.GetFiles(Server.MapPath("Plugins"), "*.dll"))
{
try
{
var name = System.Reflection.AssemblyName.GetAssemblyName(file);
var assembly = System.Reflection.Assembly.Load(name);
CompositionProvider.AddAssembly(assembly);
}
catch
{
// You'll need to craft exception handling to
// your specific scenario.
}
}
}
}

Answer 1

If I understand you correctly, you're looking for code that will load all assemblies from a directory and load them into the container; here's a skeleton for doing that:
var config = new ContainerConfiguration();
foreach (var file in Directory.GetFiles(#".\Plugins", "*.dll"))
{
try
{
var name = AssemblyName.GetAssemblyName(file);
var assembly = Assembly.Load(name);
config.WithAssembly(assembly);
}
catch
{
// You'll need to craft exception handling to
// your specific scenario.
}
}
var container = config.CreateContainer();
// ...
Hammett discusses this scenario and shows a more complete version in F# here: http://hammett.castleproject.org/index.php/2011/12/a-decent-directorycatalog-implementation/
Note, this won't detect assemblies added to the directory after the application launches - Microsoft.Composition isn't intended for that kind of use, so if the set of plug-ins changes your best bet is to detect that with a directory watcher and prompt the user to restart the app. HTH!

Answer 2

MEF is not intended to be used as DI framework. Which means that you should separate your "plugins" (whatever they are) composition from your infrastructure dependencies, and implement the former via MEF and the latter via whatever DI framework you prefer.

Answer 3

I think there are a little misunderstandings on what MEF can and can't do.
Originally MEF was conceived as purely an extensibility architecture, but as the framework evolved up to its first release, it can be fully supported as a DI container also. MEF will handle dependency injection for you, and does so through it's ExportProvider architecture. It is also entirely possible to use other DI frameworks with MEF. So in reality there are a number of ways things could be achieved:
Build a NinjectExportProvider that you can plug into MEF, so when MEF is searching for available exports, it will be able to interrogate your Ninject container.
Use an implementation of the Common Services Locator pattern to bridge between MEF and Ninject or vice versa.
Because you are using MEF for the extensibility, you'll probably want to use the former, as this exposes your Ninject components to MEF, which in turn exposes them to your plugins.
The other thing to consider, which is a bit disappointing, is in reality there isn't a lot of room for automagically plugging in of features ala Wordpress on ASP.NET. ASP.NET is a compiled and managed environment, and because of that you either resort to late-binding by loading assemblies manually at runtime, or you restart the application to pick up the new plugins, which sort of defeats the object of being able to plug new extensions in through the application.
My advice, is plan your architecture to pick up any extensibility points as startup and assume that any core changes will require a deployment and application restart.
In terms of the direct questions asked:
The CompositionProvider accepts in instance of ContainerConfiguration which is used internally to create the CompositionContainer used by the provider. So you could use this as the point by which you customise how you want your container to be instantiated. The ContainerConfiguration supports a WithProvider method:
var configuration = new ContainerConfiguration().WithProvider(new NinjectExportDescriptorProvider(kernel));
CompositionProvider.SetConfiguration(configuration);
Where NinjectExportDescriptorProvider might be:
public class NinjectExportDescriptorProvider: ExportDescriptorProvider
{
private readonly IKernel _kernel;
public NinjectExportDescriptorProvider(IKernel kernel)
{
if (kernel == null) throw new ArgumentNullException("kernel");
_kernel = kernel;
}
public override IEnumerable<ExportDescriptorPromise> GetExportDescriptors(
CompositionContract contract, DependencyAccessor dependencyAccessor)
{
var type = contract.ContractType;
if (!_kernel.GetBindings(type).Any())
return NoExportDescriptors;
return new[] {
new ExportDescriptorPromise(
contract,
"Ninject Kernel",
true, // Hmmm... need to consider this, setting it to true will create it as a shared part, false as new instance each time,
NoDependencies,
_ => ExportDescriptor.Create((c, o) => _kernel.Get(type), NoMetadata)) };
}
}
}
Note: I have not tested this, this is all theory, and is based on the example AppSettingsExportDescriptorProvider at: http://mef.codeplex.com/wikipage?title=ProgrammingModelExtensions
It's different from using the standard ExportProvider, because using the CompostionProvider is built around lightweight composition. But essentially you're wrapping up access to your Ninject kernel and making it available to your CompositionContainer.
As with adding a specific new provider (see above), you can use the ContainerConfiguration to read the available assemblies, probably something like:
var configuration = new ContainerConfiguration().WithAssemblies(AppDomain.GetAssemblies())
Again, I haven't tested all of this, but I hope it at least points you in the right direction.