Autofac with Web API 2 - Parameter-less constructor error

Question

I am trying to get Autofac setup on my Web API 2 project. I have only used it a couple of times before. I used Nuget an installed both Autofac and Autofac.WebApi2. Then in my Startup class I did this:

public partial class Startup
{
    public void Configuration(IAppBuilder app)
    {

        // Get our http configuration
        var config = new HttpConfiguration();

        // Register the Autofac middleware FIRST. This also adds
        // Autofac-injected middleware registered with the container.
        var container = ConfigureInversionOfControl(app, config);

        // Register all areas
        AreaRegistration.RegisterAllAreas();  
        GlobalConfiguration.Configure(WebApiConfig.Register);

        // Use our web api
        app.UseWebApi(config);
    }

    /// <summary>
    /// Configures Autofac DI/IoC
    /// </summary>
    /// <param name="app"></param>
    /// <param name="config"></param>
    private IContainer ConfigureInversionOfControl(IAppBuilder app, HttpConfiguration config)
    {

        // Create our container
        var builder = new ContainerBuilder();

        // You can register controllers all at once using assembly scanning...
        builder.RegisterApiControllers(Assembly.GetExecutingAssembly());

        // Register our module
        builder.RegisterModule(new AutofacModule());

        // Build
        var container = builder.Build();

        // Lets Web API know it should locate services using the AutofacWebApiDependencyResolver
        config.DependencyResolver = new AutofacWebApiDependencyResolver(container);

        // Return our container
        return container;
    }
}

The module I created looks like this:

public class AutofacModule : Module
{
    protected override void Load(ContainerBuilder builder)
    {

        // Create our Singletons
        builder.RegisterType<DatabaseContext>().As<DbContext>().InstancePerRequest();

        // Create our Services
        builder.RegisterType<UnitOfWork>().As<IUnitOfWork>().InstancePerRequest();
        builder.RegisterType<AnswerService>().As<IAnswerService>().InstancePerRequest();
        builder.RegisterType<CategoryService>().As<ICategoryService>().InstancePerRequest();
        builder.RegisterType<FeedService>().As<IFeedService>().InstancePerRequest();
        builder.RegisterType<FilterService>().As<IFilterService>().InstancePerRequest();
        builder.RegisterType<QuestionGroupService>().As<IQuestionGroupService>().InstancePerRequest();
        builder.RegisterType<StateService>().As<IStateService>().InstancePerRequest();

        // Create our providers
        //builder.RegisterType<AnswerProvider>().As<IAnswerProvider>().InstancePerLifetimeScope();
        builder.RegisterType<CategoryProvider>().As<ICategoryProvider>().InstancePerRequest();
        builder.RegisterType<FeedProvider>().As<IFeedProvider>().InstancePerRequest();
        builder.RegisterType<FilterProvider>().As<IFilterProvider>().InstancePerRequest();
        builder.RegisterType<QuestionGroupProvider>().As<IQuestionGroupProvider>().InstancePerRequest();
        builder.RegisterType<StateProvider>().As<IStateProvider>().InstancePerRequest();
    }
}

And I added a provider as a parameter for my API constructor like this:

// Readonly properties
private readonly ICategoryProvider _provider;

/// <summary>
/// Default constructor
/// </summary>
public CategoriesController(ICategoryProvider provider)
{
    _provider = provider;
}

But, when I try to call the API (using postman) I get this error:

{
    "message": "An error has occurred.",
    "exceptionMessage": "An error occurred when trying to create a controller of type 'CategoriesController'. Make sure that the controller has a parameterless public constructor.",
    "exceptionType": "System.InvalidOperationException",
    "stackTrace": "   at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)\r\n   at System.Web.Http.Controllers.HttpControllerDescriptor.CreateController(HttpRequestMessage request)\r\n   at System.Web.Http.Dispatcher.HttpControllerDispatcher.<SendAsync>d__1.MoveNext()",
    "innerException": {
        "message": "An error has occurred.",
        "exceptionMessage": "Type 'Piiick.Api.Controllers.CategoriesController' does not have a default constructor",
        "exceptionType": "System.ArgumentException",
        "stackTrace": "   at System.Linq.Expressions.Expression.New(Type type)\r\n   at System.Web.Http.Internal.TypeActivator.Create[TBase](Type instanceType)\r\n   at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.GetInstanceOrActivator(HttpRequestMessage request, Type controllerType, Func`1& activator)\r\n   at System.Web.Http.Dispatcher.DefaultHttpControllerActivator.Create(HttpRequestMessage request, HttpControllerDescriptor controllerDescriptor, Type controllerType)"
    }
}

I am now stuck, I can't see where the issue is at all. The category provider has a constructor like this:

/// <summary>
/// Default constructor
/// </summary>
public CategoryProvider(IUnitOfWork unitOfWork, ICategoryService service)
{

    // Map our properties
    _unitOfWork = unitOfWork;
    _service = service;
}

and the CategoryService has a constructor like this:

/// <summary>
/// The default constructor
/// </summary>
/// <param name="unitOfWork"></param>
public CategoryService(IUnitOfWork unitOfWork)
    : base(unitOfWork)
{
}

and finally, the UnitOfWork has a constructor like this:

/// <summary>
/// Default constructor
/// </summary>
/// <param name="context">The database context</param>
public UnitOfWork(DbContext context)
{
    _context = context;
    _repositories = new Dictionary<Type, object>();
}

From what I can tell, everything is set up correctly. Can anyone spot anything I have forgotten to do?

Answer 1

I found the issue, I was using HttpConfiguration to set the dependency resolver, it should have been GlobalConfiguration.Configuration.

So when I changed my line from:

config.DependencyResolver = new AutofacWebApiDependencyResolver(container);

to

GlobalConfiguration.Configuration.DependencyResolver = new AutofacWebApiDependencyResolver(container);

Everything started working.