Dependency Resolution in Onion Architecture

.net, asp.net-mvc, dependency-injection, onion-architecture

Solution

Your question is

"how to integrate a different DI tool (like SimpleInjector, Unity or Autofac) into an Onion Project?"

I’m using StructureMap instead of Ninject, the way it’s integrated should be ok for any other DI framework.

As you said, only the Dependency Resolution Layer should reference all the other layers, it’s the outermost layer of your Onion Architecture. Well, to do so, I’ve created a project called BootStrapper. This is the only project where I reference StructureMap assemblies. In the App_Start folder of this project, I have a file named StructureMapMvc.cs which looks like this:

[assembly: WebActivator.PreApplicationStartMethod(typeof(XXXX.BootStrapper.App_Start.StructuremapMvc), "Start")]

namespace XXXX.BootStrapper.App_Start
{
    public static class StructuremapMvc
    {
        public static void Start()
        {
            IContainer container = IoC.Initialize();
            System.Web.Mvc.DependencyResolver.SetResolver(new StructureMapDependencyResolver(container));
            GlobalConfiguration.Configuration.DependencyResolver = new StructureMapHttpDependencyResolver(container);
            ControllerBuilder.Current.SetControllerFactory(new StructureMapControllerFactory());
        }
    }
}

The interesting line is:

[assembly: WebActivator.PreApplicationStartMethod(typeof(XXXX.BootStrapper.App_Start.StructuremapMvc), "Start")]

According to the nugget package’s description:

WebActivator is a NuGet package that allows other packages to execute some startup code in web apps.

Pretty cool, huh? The last thing you have to put in place is to be sure that the BootStrapper project assembly will be pushed to the /bin folder of your web application (easy to set up using a post build action or OutputTo nugget package). This will avoid you to reference the BootStrapper project in your MVC project and break the Onion Architecture principle.

So, with all this in place, it adheres totally with the Composition Root Pattern and when your app will start, modules will be composed together.

Hope this helps!

Problem

The Onion Architecture is a way of structuring applications to maintain separation of concern and loose coupling (example project at: http://onionarch.codeplex.com/). Dependency Injection/Resolution is a key aspect of this architecture, since it is used to tie all the layers together. The above link contains an example application on how to structure an ASP.NET MVC using the Onion layering. I really like it, but most of these examples use Ninject (which we all know is pretty slow). I was wondering if someone could perhaps eloborate on how to integrate a different DI tool (like SimpleInjector, Unity or Autofac) into an Onion Project. It is key that all layers only have 1 dependency (including the MVC project), namely the Core layer. Except for the Dependency Resolution layer, this layer can reference all the layers. I'm having a hard time setting the MVC project as a startup project, using DI, and not including a reference to the DI tool in the MVC layer.

Original source