描述：
我想使用 WebClient 的 DownloadStringAsync 下载多个 URL（大约 300 个），其中一些 URL 下载得很好，但对于其他一些 URL，下载只会永远挂起。
问题：
- 如何捕获无法下载的有问题的网址？
- 根据附加的代码，如何实现“DownloadStringAsync”方法的超时时间以跳过无法下载的 URL？
我已经用谷歌搜索了解决方案，但人手不足。然后尝试在此处的数据库中搜索具有相同结果的解决方案。任何帮助表示赞赏。
这是我正在使用的代码：
public class ParallelDownloading
{
private int _totalDownloadedURLs = 0;
private ConcurrentQueue<DownloadFile> _queueToDownlaod;
private IList<Task> _downloadingTasks;
private Timer _downloadTimer;
public event EventHandler<OnFileDownloadCompletedEventArgs> OnDownloadURLCompleted;
private int _parallelDownloads;
public ParallelDownloading(int parallelDownloads)
{
_totalDownloadedURLs = 0;
_queueToDownlaod = new ConcurrentQueue<DownloadFile>();
_downloadingTasks = new List<Task>();
_downloadTimer = new Timer();
_parallelDownloads = parallelDownloads;
ServicePointManager.DefaultConnectionLimit = parallelDownloads;
}
public void StartParallelDownloading()
{
_downloadTimer.Elapsed += new ElapsedEventHandler(DownloadTimer_Elapsed);
_downloadTimer.Interval = 800;
_downloadTimer.Start();
}
public void EnqueueFileToDownload(DownloadFile file)
{
_queueToDownlaod.Enqueue(file);
}
void DownloadTimer_Elapsed(object sender, ElapsedEventArgs e)
{
StartDownload();
}
private void StartDownload()
{
lock (_downloadingTasks)
{
if (_downloadingTasks.Count < _parallelDownloads && _queueToDownlaod.Count > 0)
{
DownloadFile fileToDownload;
if (_queueToDownlaod.TryDequeue(out fileToDownload))
{
var task = new Task(() =>
{
var client = new WebClient();
client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(client_DownloadStringCompleted);
client.DownloadStringAsync(new Uri(fileToDownload.Link), fileToDownload.Link);
}, TaskCreationOptions.LongRunning);
task.ContinueWith(DownloadOverCallback, TaskContinuationOptions.None);
_downloadingTasks.Add(task);
task.Start();
}
}
}
}
void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
_totalDownloadedURLs ++;
if (e.Error == null && e.Cancelled == false)
{
//do something
}
OnFileDownloadCompletedEventArgs evtArgs = new OnFileDownloadCompletedEventArgs();
evtArgs.DownloadedFiles = _totalDownloadedURLs;
OnDownloadURLCompleted(this, evtArgs);
}
public void DownloadOverCallback(Task downloadingTask)
{
lock (_downloadingTasks)
{
_downloadingTasks.Remove(downloadingTask);
}
}
}