Autofac.Core.DependencyResolutionException using InstancePerRequest() in WEB API - asp.net-web-api

Question

I'm using Autofac for DI with the builder.RegisterApiControllers(Assembly.GetExecutingAssembly()).InstancePerRequest(); but sometimes it gives an error on all my controllers (System.InvalidOperationException: An error occurred when trying to create a controller of type 'UserController'. Make sure that the controller has a parameterless public constructor. ---> Autofac.Core.DependencyResolutionException)
and when I give a rebuild of WEB API and it starts working fine.
Here is my code in startup.cs
private void ConfigureAutofac(HttpConfiguration config, IAppBuilder app)
{
var builder = new ContainerBuilder();
//Register HttpRequestMessage
builder.RegisterType<CurrentRequest>().InstancePerRequest();
builder.Register(c => new UrlHelper(c.Resolve<CurrentRequest>().Value));
//Register Web API controllers.
builder.RegisterApiControllers(Assembly.GetExecutingAssembly()).InstancePerRequest();
//Register the Autofac filter provider.
builder.RegisterWebApiFilterProvider(config);
//Register the Autofac model binder provider.
builder.RegisterWebApiModelBinderProvider();
#region Register managers
var businessasm = BuildManager.GetReferencedAssemblies()
.Cast<Assembly>()
.Where(n => n.FullName.Contains("Business"))
.FirstOrDefault();
builder.RegisterAssemblyTypes(businessasm)
.Where(t => t.Name.EndsWith("Manager") && t.Name != "DocumentManager")
.AsImplementedInterfaces()
.InstancePerRequest();
builder.RegisterGeneric(typeof(DocumentManager<>))
.As(typeof(IDocumentManager<>))
.InstancePerRequest();
#endregion
//Set the dependency resolver to be Autofac.
var container = builder.Build();
config.DependencyResolver = new AutofacWebApiDependencyResolver(container);
config.MessageHandlers.Insert(0, new ApiDelegatingHandler());
config.MessageHandlers.Insert(1, new ActivityLogHandler());
app.UseAutofacMiddleware(container);
app.UseAutofacWebApi(config);
}

Answer 1

The error indicates that UserController (which you didn't show above) takes a parameter that isn't registered with Autofac.
Since it works sometimes and not others, my guess is that the problem is this line:
var businessasm = BuildManager.GetReferencedAssemblies()
.Cast<Assembly>()
.Where(n => n.FullName.Contains("Business"))
.FirstOrDefault();
FirstOrDefault will return null if no assembly is found; and later on when you register all the things that end with Manager it means nothing will get registered.
My guess is that the UserController needs one of these managers, Autofac doesn't have it registered, and instantiation fails. If you have a breakpoint on the next line, you can see if businessasm is null; or, alternatively, switch to .First() instead of .FirstOrDefault() if you always expect the registrations to work.
Oh, and if you happen to have two assemblies with Business in them, you might want to make sure you're getting the right one. Assembly load order isn't guaranteed to be consistent.
That said I see the exception message you posted seems to indicate there are some nested exceptions in the stack. Don't stop reading at the first exception. Unfortunately with DI and other layers in the stack, you get exceptions that have inner exceptions that may, themselves, have inner exceptions... and the full set of messages may actually have more information that can help you troubleshoot.
Autofac has a really good troubleshooting page and Autofac v6 has some pretty detailed diagnostics that might help figure out where the missing things are if you can't figure it out from the exceptions.