How can I ignore https certificate warnings in the c# signalr client?

You should register a method for ServerCertificateValidationCallback event.

This code just registers an anonymous method which returns true when the event is fired.

ServicePointManager.ServerCertificateValidationCallback +=
                  (sender, certificate, chain, sslPolicyErrors) => true;

Be careful, this is a global setting. So all ssl/tls request signalr or http will use this setting.

I believe that I have found a way that seems to work but isn't global like the ServicePointManager.ServerCertificateValidationCallback approach that is typically recommended. I started by making a subclass of the SignalR "DefaultHttpClient" class as follows:

class CustomHttpClient : DefaultHttpClient
    {
        private readonly System.Net.Security.RemoteCertificateValidationCallback _serverCertificateValidationCallback;

        public CustomHttpClient (System.Net.Security.RemoteCertificateValidationCallback serverCertificateValidationCallback) : base()
        {
            this._serverCertificateValidationCallback = serverCertificateValidationCallback;
        }

        protected override HttpMessageHandler CreateHandler()
        {
            var rv = base.CreateHandler() as WebRequestHandler;
            if (this._serverCertificateValidationCallback != null)
                rv.ServerCertificateValidationCallback = this._serverCertificateValidationCallback;
            return rv;
        }
    }

Now I can use my custom HttpClient implementation when I call "Start" on my HubConnection instance as follows:

var hubConnection = new HubConnection("my server url");
var myHub = hubConnection.CreateHubProxy("my hub name");
hubConnection.Start(new CustomHttpClient((sender, certificate, chain, sslPolicyErrors) =>
                {
                    //put some validation logic here if you want to.
                    return true;
                }));

This should allow you to validate the server certificate as you see fit, but keep the scope to the current HubConnection instead of affecting all HTTP traffic from your app.