SignalR 2

public class ChatHub : Hub
{
    public void Send(string user, string message)
        => Clients.All.receiveMessage(user, message);   // calls a client-side "receiveMessage" function
}

[assembly: OwinStartup(typeof(MyApp.Startup))]
namespace MyApp
{
    public partial class Startup
    {
        public void Configuration(IAppBuilder app)
        {
            app.MapSignalR();                    // maps hubs at /signalr by default
        }
    }
}

public class ChatHub : Hub
{
    public override Task OnConnected()
    {
        Clients.Caller.welcome("Connected!");
        return base.OnConnected();
    }

    public Task JoinRoom(string room)
        => Groups.Add(Context.ConnectionId, room);

    public Task SendToRoom(string room, string message)
        => Clients.Group(room).receiveMessage(Context.User.Identity.Name, message);

    public Task SendToUser(string userId, string message)
        => Clients.User(userId).receiveMessage(message);   // requires a configured IUserIdProvider
}

// generated hub proxies (/signalr/hubs, auto-generated from server-side Hub classes)
var chat = $.connection.chatHub;
chat.client.receiveMessage = function (user, message) { appendMessage(user, message); };

$.connection.hub.start().done(function () {
    chat.server.send("Alice", "Hello!");
});

// .NET client (Microsoft.AspNet.SignalR.Client) -- e.g. a WPF app or a service talking to a SignalR 2 server
var connection = new HubConnection("https://example.com/");
var chat = connection.CreateHubProxy("ChatHub");
chat.On<string, string>("receiveMessage", (user, message) => Console.WriteLine($"{user}: {message}"));
await connection.Start();
await chat.Invoke("Send", "Alice", "Hello!");

[Authorize]
public class ChatHub : Hub
{
    [Authorize(Roles = "Support")]
    public void BroadcastAlert(string message) => Clients.All.alert(message);
}

// NuGet: Microsoft.AspNet.SignalR.SqlServer / .Redis / .ServiceBus
GlobalHost.DependencyResolver.UseSqlServer("Data Source=...;Initial Catalog=SignalR;Integrated Security=True");
// or: GlobalHost.DependencyResolver.UseRedis("server", 6379, "password", "signalr-app");
// or: GlobalHost.DependencyResolver.UseServiceBus(connectionString, "signalr-app");