public class Startup
{
    /// <summary>
    /// Configure the signalR program
    /// </summary>
    public void Configuration(IAppBuilder app)
    {
        // Maps signalR hubs to the app builder pipeline at "/signalr"
        app.MapSignalR();
    }
}

public class GameHub : Hub
{
    // Players list that save the players connection id and group id
    private static Dictionary<string, string> players = new Dictionary<string, string>();

    /// <summary>
    /// When player join in the game
    /// </summary>
    public override Task OnConnected()
    {
        // Add player to the players list and set the group id to -2
        players.Add(Context.ConnectionId, "-2");

        return base.OnConnected();
    }

    /// <summary>
    /// When player disconnected
    /// </summary>
    public override Task OnDisconnected(bool stopCalled)
    {
        string groupName = players[Context.ConnectionId];

        // Remove player from the players list and send message to other player to tell him the opponent left
        players.Remove(Context.ConnectionId);
        Clients.Group(groupName).OpponentLeft();

        return base.OnDisconnected(stopCalled);
    }
      
    // And other methods...
    ...
}

public class SignalRClient : MonoBehaviour 
{
    // SignalR variables
    private static Uri uri = new Uri("http://localhost:5000/signalr/");
    private static GameHub gameHub;
    private static Connection signalRConnection;
    ...
    public void Connect()
    {
        // Initialize the connection
        gameHub = new GameHub();
        signalRConnection = new Connection(uri, gameHub);
        signalRConnection.Open();
        
        signalRConnection.OnConnected += (conn) => 
        {
            Debug.Log("Connect Successfully!");
        };
        ...
    }
    ...
    /// <summary>
    /// A class for connection with server hub
    /// </summary>
    public class GameHub : Hub
    {
        public GameHub() : base("GameHub")
        {
            // Register callback functions that received from the server
            base.On("JoinToOpponent", Joined);
            ...
        }

        /// <summary>
        /// Return the join state from server - -1: Opponent not found, Otherwise: Opponent found
        /// </summary>
        private void Joined(Hub hub, MethodCallMessage msg)
        {
            int playerId = int.Parse(msg.Arguments[0].ToString());

            if (playerId == -1)
                Debug.Log("Waiting for an opponent...");
            else
                Debug.Log("Player joined!");
        }
        ...
    }
}