Hub.Clients.User(userId).methodCall always throws Object reference not set to an instance of an object

Question

I have a NotificationHub class which inherits from the Hub class.

public class NotificationHub : Hub
    {
        public void Send(string userId, Notification notification)
        {
            Clients.User(userId)
                .notificationReceived(notification);
        }
    }

This always fails with

[NullReferenceException: Object reference not set to an instance of an object.]
   Microsoft.AspNet.SignalR.Hubs.SignalProxy.Invoke(String method, Object[] args) +88
   Microsoft.AspNet.SignalR.Hubs.SignalProxy.TryInvokeMember(InvokeMemberBinder binder, Object[] args, Object& result) +12
   CallSite.Target(Closure , CallSite , Object , <>f__AnonymousType0`4 ) +351

However if I do this:

public class NotificationHub : Hub
    {
        public void Send(string userId, Notification notification)
        {
            var context = GlobalHost.ConnectionManager.GetHubContext<NotificationHub>();

            context.Clients.User(userId)
                .notificationReceived(notification);
        }
    }

It works .... what gives here? Most of the examples I've seen do not require getting the context explicitly, should it not already be available from Hub? I would rather not have to grab it explicitly every time.

Here is my IoC Setup:

GlobalHost.DependencyResolver.Register(typeof(IHubActivator), () => new SimpleInjectorHubActivator(container));
GlobalHost.DependencyResolver.Register(typeof(IUserIdProvider), () => new SignalRHubUserIdProvider());

Activator:

public class SimpleInjectorHubActivator : IHubActivator
    {
        private readonly Container _container;

        public SimpleInjectorHubActivator(Container container)
        {
            _container = container;
        }

        public IHub Create(HubDescriptor descriptor)
        {
            return (IHub) _container.GetInstance(descriptor.HubType);
        }
    }

Answer 1

If you want to send something to clients from outside of hub handler methods (ie not during handling message on server), you have to use GlobalHost.ConnectionManager.GetHubContext<NotificationHub>();

Reason is that when the method is called to handle some client side message, hub instance is created by SignalR and Clients property is correctly initialized. This is not the case when you calling method yourself from server code (and probably creating hub instance yourself).

Imho error message is not very clear and this use case should be handled better by SignalR. Anyway for the same reason i suggest to separate all the methods sending messages to clients which are intended to be called from server code to different class (not derived from Hub).