问在ASP.NET MVC中异步使用WebClient？

EN

string downloadString = null;

ActionResult MyAction()
{
    //get the download location
    WebClient client = StartDownload(uri);
    //do other stuff
    CheckAndFinalizeDownload(client);
    client.Dispose();
}

WebClient StartDownload(Uri uri)
{
    WebClient client = new WebClient();
    client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(Download_Completed);
    client.DownloadStringAsync(uri);
    return client;
}

void CheckAndFinalizeDownload(WebClient client)
{
    if(this.downloadString == null)
    {
        Thread.Sleep(3000);
    }
    if(this.downloadString == null)
    {
        client.CancelAsync();
        this.downloadString = string.Empty;
    }
}

void Download_Completed(object sender, DownloadStringCompletedEventArgs e)
{
    if(!e.Cancelled && e.Error == null)
    {
        this.downloadString = (string)e.Result;
    }
}