SignalR in ASP.NET Core


SignalR is a Microsoft .NET library for implementing real-time web sites. It uses a number of techniques to achieve bi-directional communication between server and client; servers can push messages to connected clients anytime.

It was available in pre-Core ASP.NET and now a pre-release version was made available for ASP.NET Core. I already talked a few times about SignalR.


You will need to install the Microsoft.AspNetCore.SignalR.Client and Microsoft.AspNetCore.SignalR Nuget pre-release packages. Also, you will need NPM (Node Package Manager). After you install NPM, you need to get the @aspnet/signalr-client package, after which, you need to get the signalr-client-1.0.0-alpha1-final.js file (the version may be different) from the node_modules\@aspnet\signalr-client\dist\browser folder and place it somewhere underneath the wwwroot folder, so that you can reference it from your pages.

Next, we need to register the required services in ConfigureServices:, before Use


We will be implementing a simple chat client, so, we will register a chat hub, in the Configure method:

app.UseSignalR(routes =>

A note: UseSignalR must be called before UseMvc!

You can do this for any number of hubs. as long as you have different endpoints. More on this in a moment.

In your view or layout file, add a reference to the signalr-client-1.0.0-alpha1-final.js file:

<script src="libs/signalr-client/signalr-client-1.0.0-alpha1-final.js"></script>

Implementing a Hub

A hub is a class that inherits from (you guessed it) Hub. In it you add methods that may be called by JavaScript. Since we will be implementing a chat hub, we will have this:

public class ChatHub : Hub
public async Task Send(string message)
await this.Clients.All.InvokeAsync("Send", message);

As you can see, we have a single method, Send, which, for this example, takes a single parameter, message. You do not need to pass the same parameters on the broadcast call (InvokeAsync), you can send whatever you want.

Going back to the client side, add this code after the reference to the SignalR JavaScript file:

        var transportType = signalR.TransportType.WebSockets;
        //can also be ServerSentEvents or LongPolling
        var logger = new signalR.ConsoleLogger(signalR.LogLevel.Information);
        var chatHub = new signalR.HttpConnection(`http://${}/chat`, { transport: transportType, logger: logger });
        var chatConnection = new signalR.HubConnection(chatHub, logger);
        chatConnection.onClosed = e => {
            console.log('connection closed');
       chatConnection.on('Send', (message) => {
           console.log('received message');
       chatConnection.start().catch(err => {
           console.log('connection error');
       function send(message) {
           chatConnection.invoke('Send', message);

Notice this:

  1. A connection is created pointing to the current URL plus the chat suffix, which is the same that was registered in the MapHub call
  2. It is initialized with a specific transport, in this case, WebSockets, but this is not required, that is, you can let SignalR figure out for itself what works; for some operating systems, such as Windows 7, you may not be able to use WebSockets, so you have to pick either LongPolling or ServerSentEvents
  3. The connection needs to be initialized by calling start
  4. There is an handler for the Send method which takes the same single parameter (message) as the ChatHub’s Send method

So, whenever someone accesses this page and calls the JavaScript send function, it invokes the Send method on the ChatHub class. This class basically broadcasts this message to all connected clients (Clients.All). It is also possible to send messages to a specific group (we’ll see how to get there):

await this.Clients.Group("groupName").InvokeAsync("Send", message);
or to a specific client:
await this.Clients.Client("id").InvokeAsync("Send", message);
You can add a user, identified by a connection id and and a ClaimsPrincipal, if using authentication, as this:
public override Task OnConnectedAsync()
this.Groups.AddAsync(this.Context.ConnectionId, "groupName");

return base.OnConnectedAsync();
Yes, the OnConnectedAsync is called whenever a new user connects, and there is a similar method, OnDisconnectedAsync, for when someone disconnects:
public override Task OnDisconnectedAsync(Exception exception)
return base.OnDisconnectedAsync(exception);
The exception parameter is only non-null if there was some exception while disconnecting.
The Context property offers two properties, ConnectionId and User. User is only set if the current user is authenticated, but ConnectionId is always set, and does not change, for the same user.

Another example, imagine you wanted to send timer ticks into all connected clients, through a timer hub. You could do this in the Configure method:

TimerCallback callback = (x) => {
var hub = serviceProvider.GetService<IHubContext<TimerHub>>();
hub.Clients.All.InvokeAsync("Notify", DateTime.Now);

var timer = new Timer(callback);
timer.Change(TimeSpan.FromSeconds(0), TimeSpan.FromSeconds(10));
Here we are starting a Timer and, from there, we are getting a reference to the timer hub and calling its Notify method with the current timestamp. The TimerHub class is just this:
public class TimerHub : Hub
Notice that this class has no public method, because it is not meant to be callable by JavaScript, it merely is used to broadcast messages from the outside (the Timer callback).

Sending Messages Into a Hub

Finally, it is also possible to send messages from the outside into a hub. When using a controller, you need to inject into it an instance of IHubContext<ChatHub>, from which you can send messages into the hub, which will then be broadcast to where appropriate:
private readonly IHubContext<ChatHub> _context;

public IActionResult Send(string message)
//for everyone
this._context.Clients.All.InvokeAsync("Send", message);
//for a single group
this._context.Clients.Group("groupName").InvokeAsync("Send", message);
//for a single client
this._context.Clients.Client("id").InvokeAsync("Send", message);

return this.Ok();

Note that this is not the same as accessing the ChatHub class, you cannot easily do that, but, rather, the chat hub’s connections.


SignalR has not been released yet, and it may still undergo some changes. For now, things appear to be working. On a future post I will talk more about SignalR, including its extensibility mechanisms and some more advanced scenarios. Stay tuned!

Persisting SignalR Connections Across Page Reloads

I recently had the need to keep a SignalR connection even if the page would reload. As far as I know, this cannot be done out of the box, either with hubs or persistent connections. I looked it up, but could find no solid solution, so here is my solution!

First, we need to create a “session id” that is to be stored at the browser side. Mind you, this is not an ASP.NET session, nor a SignalR connection id, it’s something that uniquely identifies a session. To maintain sessions we normally use cookies, but my solution uses instead HTML5 session storage. I had to generate a session id, and there were several solutions available, from pseudo-GUIDs, to the SignalR connection id, but I ultimately decided to use the timestamp; here is it:

function getSessionId()


    var sessionId = window.sessionStorage.sessionId;


    if (!sessionId)


        sessionId = window.sessionStorage.sessionId =;



    return sessionId;


As you can see, this function first checks to see if the session id was created, by inspecting the sessionStorage object, and, if not, sets it.

Next, we need to have SignalR pass this session id on every request to the server. For that, I used $.connection.hub.qs, the query string parameters object:

$.connection.hub.qs = { SessionId: getSessionId() };

$.connection.hub.start().done(function ()


    //connection started


Moving on to the server-side, I used a static collection to store, for each session id, each SignalR connection id associated with it – one for each page request. The reasoning is, each page reload generates a new SignalR connection id, but the session id is always kept:

public sealed class NotificationHub : Hub


    internal const string SessionId = "SessionId";


    public static readonly ConcurrentDictionary<string, HashSet<string>> sessions = new ConcurrentDictionary<string, HashSet<string>>();


    public static IEnumerable<string> GetAllConnectionIds(string connectionId)


        foreach (var session in sessions)


            if (session.Value.Contains(connectionId) == true)


                return session.Value;




        return Enumerable.Empty<string>();



    public override Task OnReconnected()




        return base.OnReconnected();



    public override Task OnConnected()




        return base.OnConnected();



    private void EnsureGroups()


        var connectionIds = null as HashSet<string>;

        var sessionId = this.Context.QueryString[SessionId];

        var connectionId = this.Context.ConnectionId;


        if (sessions.TryGetValue(sessionId, out connectionIds) == false)


            connectionIds = sessions[sessionId] = new HashSet<string>();






As you can see, both on OnConnected as in OnReconnected, I add the current connection id to the collection (ConcurrentDictionary<TKey, TValue> to allow multiple concurrent accesses) indexed by the session id that I sent in the SignalR query string. Then, I have a method that looks in the collection for all connection id entries that are siblings of a given connection id. If more than one exists, it means that the page has reloaded, otherwise, there will be a one-to-one match between connection ids and session ids.

The final step is to broadcast a message to all the sibling connection ids – a waste of time because only one is still possibly active, but since we have no way of knowing, it has to be this way:



public IHttpActionResult Notify(string connectionId, string message)


    var context = GlobalHost.ConnectionManager.GetHubContext<NotificationHub>();

    var connectionIds = NotificationHub.GetAllConnectionIds(connectionId).ToList();




    return this.Ok();


This Web API action method will get the context for our hub (NotificationHub), look up all of the sibling connection ids for the passed one, and then broadcast a message to all clients identified by these connection ids. It’s a way to send messages from outside of a page into a hub’s clients

Problems with this approach:

  • All tabs will get the same session id, but that also happens with cookies;
  • Although unlikely, it may be possible for two clients to get the same session id, which I implemented as the current timestamp; an easy fix would be, for example, to use a pseudo-GUID, the server-side session id, or even the SignalR connection id;
  • If the page reloads several times, there will be several connection id entries for the same session id – which will be kept throughout all reloads; no easy way to get around this, except possibly using some cache with expiration mechanism.

And that’s it. Enjoy!