Web 客户端的 DownloadStringDone 事件处理程序未触发

我正在创建一个Silverlight仪表板，显示多个设备和网站（向上，向下等）的状态。我正在尝试使用 WebClient 类连接到一个网站，看看它是否启动。但是 DownloadStringCompleted 事件处理程序永远不会被触发。这是一个与这篇文章非常相似的问题。

public void LoadPortalStatus(Action<IEnumerable<ChartModel>> success, Action<Exception> fail)
{
    List<NetworkPortalStatusModel> pingedItems = new List<NetworkPortalStatusModel>();
    // Add the status for the portal
    BitmapImage bi = IsPortalActive() 
            ? (new BitmapImage(new Uri("led_green_black-100x100.png", UriKind.Relative))) 
            : (new BitmapImage(new Uri("led_red_black-100x100.png", UriKind.Relative)));
    NetworkPortalStatusModel nsm = new NetworkPortalStatusModel
    {
        Unit = "Portal",
        StatusIndicator = new Image { Width = 100, Height = 100, Source = bi }
    };
    pingedItems.Add(nsm);
    // Send back to the UI thread
    System.Windows.Deployment.Current.Dispatcher.BeginInvoke(_delagateSuccess, new object[] { pingedItems });
}
private bool IsPortalActive()
{
    bool IsActive = false;
    WebClient wc = new WebClient();
    wc.DownloadStringCompleted += (s, e) =>
        {
            if (e.Cancelled) 
            {
                _delagateFail(new Exception("WebClient page download cancelled"));
            }
            else if (e.Error != null)
            {
                _delagateFail(e.Error);
            }
            else
            {
                _portalHtmlResponse = e.Result;
                if (_portalHtmlResponse.Contains("Somerville, Ma"))
                {
                    IsActive = true;
                }
            }
        };
    wc.DownloadStringAsync(new Uri("https://portal.nbic.com/monitor.aspx"));
    return IsActive;
}

有人看到这里的问题吗？