Question

Recently I’ve switched to Ninject 2.0 release and started getting the following error:

Error occured: Error activating SomeController
More than one matching bindings are available.
Activation path:
  1) Request for SomeController

Suggestions:
  1) Ensure that you have defined a binding for SomeController only once.

However, I’m unable to find certain reproduction path. Sometimes it occurs, sometimes it does not.
I’m using NinjectHttpApplication for automatic controllers injection. Controllers are defined in separate assembly:

public class App : NinjectHttpApplication
{
    protected override IKernel CreateKernel()
    {
        INinjectModule[] modules = new INinjectModule[] {
            new MiscModule(),
            new ProvidersModule(),
            new RepositoryModule(),
            new ServiceModule()
        };

        return new StandardKernel(modules);
    }

    protected override void OnApplicationStarted()
    {
        RegisterRoutes(RouteTable.Routes);
        RegisterAllControllersIn("Sample.Mvc");
        base.OnApplicationStarted();
    }

    /* ............. */

}

Maybe someone is familiar with this error.

Any advice?

Answer 1

I finally figured this issue out recently. Apparently, the NinjectHttpApplication.RegisterAllControllersIn() function doesn’t do all of the proper bindings needed. It binds your concrete controller implementations to IController requests. For example, if you have a controller class called SampleMvcController, which inherits from System.Web.Mvc.Controller. It would do the following named binding during application start:

kernel.Bind<IController>().To(SampleMvcController).InTransientScope().Named("SampleMvc");

But when debugging the NinjectControllerFactory, I find that request are being made for the Ninject Kernel to return an object for the class “SampleMvcController”, not for a concrete implementation of IController, using the named binding of “SampleMvc”.

Because of this, when the first web request that involves the SampleMvcController is made, it creates a binding of SampleMvcController to itself. This is not thread safe though. So if you have several web requests being made at once, the bindings can potentially happen more than once, and now you are left with this error for having multiple bindings for the SampleMvcController.

You can verify this by quickly refreshing an MVC URL, right after causing your web application to restart.

The fix:

The simplest way to fix this issue is to create a new NinjectModule for your controller bindings, and to load this module during application start. Within this module, you self bind each of your defined controllers, like so:

class ControllerModule : StandardModule {
      public override Load() {
        Bind<SampleMvcController>().ToSelf();
        Bind<AnotherMvcController>().ToSelf();
      }
    }

But if you don’t mind changing the Ninject source code, you can modify the RegisterAllControllersIn() function to self bind each controller it comes across.