如何在wp7中查找用户发送推送通知的设备状态？

Question

我已经在我的WP7应用程序中实现了推送通知，并获得了用户发送通知的通道uris列表。我的问题出现在我开始发送通知时，如果任何设备/ uri断开连接/不活动，通知根本不会发送给其他活动用户。那么，我该如何处理这个问题呢？ 提前致谢。

public Mainpage()
    {
        InitializeComponent();
        if (IsolatedStorageSettings.ApplicationSettings.Contains("DeviceId"))
        {
            _deviceId = (Guid)IsolatedStorageSettings.ApplicationSettings["DeviceId"];
        }
        else
        {
            _deviceId = Guid.NewGuid();
            IsolatedStorageSettings.ApplicationSettings["DeviceId"] = _deviceId;
        }
        SetupNotificationChannel();
    }
private void SetupNotificationChannel()
    {
        _channel = HttpNotificationChannel.Find(ChannelName);

        if (_channel == null)
        {
            _channel = new HttpNotificationChannel(ChannelName);
            _channel.ChannelUriUpdated += ChannelUriUpdated;
            _channel.ErrorOccurred += (s, e) => Deployment.Current.Dispatcher.BeginInvoke(() => ErrorOccurred(e));
            _channel.Open();

        }
        else
        {
            RegisterForNotifications();
        }
    }
private void ErrorOccurred(NotificationChannelErrorEventArgs e)
    {
        MessageBox.Show("ERROR:" + e.Message);
    }
    void ChannelUriUpdated(object sender, NotificationChannelUriEventArgs e)
    {
        _channel = HttpNotificationChannel.Find(ChannelName);

        if (!_channel.IsShellTileBound)
        {
            Collection<Uri> ListOfAllowedDomains = new Collection<Uri> { new Uri("http://www.abcdef.com") };

            _channel.BindToShellTile(ListOfAllowedDomains);
        }

        if (!_channel.IsShellToastBound)
        {
            _channel.BindToShellToast();
        }

        RegisterForNotifications();
    }

    private void RegisterForNotifications()
    {

        RegisterWithNotificationService();
        _channel.ShellToastNotificationReceived += (s, e) => Deployment.Current.Dispatcher.BeginInvoke(() => ToastReceived(e));
        //_channel.HttpNotificationReceived += (s, e) => Deployment.Current.Dispatcher.BeginInvoke(() => HttpNotificationReceived(e));
        _channel.ErrorOccurred += (s, e) => Deployment.Current.Dispatcher.BeginInvoke(() => ErrorOccurred(e));
    }
private void RegisterWithNotificationService()
    {
        ServiceProxy.WCFServiceClient svc = new ServiceProxy.WCFServiceClient();
        svc.SubscribeCompleted += new EventHandler<System.ComponentModel.AsyncCompletedEventArgs>(svc_SubscribeCompleted);
        svc.SubscribeAsync(IsolatedStorageSettings.ApplicationSettings["DeviceId"].ToString(), _channel.ChannelUri.ToString());
    }

private void ToastReceived(NotificationEventArgs e)
    {
        MessageBox.Show(string.Format(e.Collection["wp:Text1"] + "\n" + e.Collection["wp:Text2"]));
    }