Composable services with ASP.NET WebApi…sort of

A while ago I had blogged about Service Composition using ServiceStack and MEF. With ServiceStack no longer free, I figured I’d try the same thing using ASP.NET Web Api from Big Mike. The result was…not as good.

To make it work, I had to rely heavily on Autofac and their Web Api Integration packages.

First I defined the IComposableApiMetadata interface which is to be implemented by my clients. It’s 2 methods are pretty simple: one takes a ContainerBuilder to allow them to inject anything extra as per their needs, while the other returns the specific Assembly from which the ApiControllers would be loaded.

	public interface IComposableApiMetadata
	{
	void Compose(ContainerBuilder builder);
	Assembly ApiAssembly();
	}

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IComposableApiMetadata.cs

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Here’s an example of what it would look like

	[Export(typeof(IComposableApiMetadata))]
	public class ExternalApiMetadata : IComposableApiMetadata {
	public void Compose(ContainerBuilder builder) {
	builder.Register(e => new BookRepository()).As<IBookRepository>().InstancePerApiRequest();
	}
	public Assembly ApiAssembly() {
	return typeof(BooksController).Assembly;
	}
	}

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ExternalApiMetadata.cs

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With that done, all that should be left is calling it from the Application Start (i.e. Global.asax) right?

	public class WebApiApplication : System.Web.HttpApplication {
	[ImportMany]
	List<IComposableApiMetadata> m_composers;
	private CompositionContainer m_container;
	protected void Application_Start() {
	var catalog = new AggregateCatalog();
	var servicesFolder = @"C:\dllFolder";
	catalog.Catalogs.Add(new DirectoryCatalog(servicesFolder));
	m_container = new CompositionContainer(catalog);
	m_container.ComposeParts(this);
	var builder = new ContainerBuilder();
	foreach (var composableApiMetadata in m_composers) {
	composableApiMetadata.Compose(builder);
	builder.RegisterApiControllers(composableApiMetadata.ApiAssembly());
	}
	var container = builder.Build();
	var resolver = new AutofacWebApiDependencyResolver(container);
	GlobalConfiguration.Configuration.DependencyResolver = resolver;
	AreaRegistration.RegisterAllAreas();
	WebApiConfig.Register(GlobalConfiguration.Configuration);
	FilterConfig.RegisterGlobalFilters(GlobalFilters.Filters);
	RouteConfig.RegisterRoutes(RouteTable.Routes);
	BundleConfig.RegisterBundles(BundleTable.Bundles);
	}
	}

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Global.asax.cs

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Sadly, this isn’t enough. I had to actually reference the client dlls in my ASP.NET MVC project, which somewhat defeats the purpose of using such a Composer. Sure I could have a process automatically update the csproj with the adequate references but that feels silly.

I am sure there is a better way and once I find it I will make sure to blog about it. Until then I hope the hour or so I spent on this will help one of you guys out there.