private void E_ReceivedMsg(object sender, string e)
 {
     if (e.StartsWith("@quit"))
     {
         var client = sender as ClientHandler;
         ConnectedUsers.Remove(client.Username); // remove username
     }
     else
     {
         App.Current.Dispatcher.Invoke(() =>
         {
             var client = sender as ClientHandler;
             ReceivedMsg.Add(client.Username + ":" + e);
             RaisePropertyChanged(nameof(CountMsg));
             if (!ConnectedUsers.Contains(client.Username))
             {
                 ConnectedUsers.Add(client.Username);
             }
         });
     }
 }