如何在 WebClient.DownloadFileAsync 上实现超时

Question

所以我认为 Webclient.DownloadFileAysnc 会有一个默认超时，但是查看文档我无法在任何地方找到任何关于它的信息，所以我猜它没有。

我正在尝试从 Internet 下载文件，如下所示：

 using (WebClient wc = new WebClient())
 {
      wc.DownloadProgressChanged += ((sender, args) =>
      {
            IndividualProgress = args.ProgressPercentage;
       });
       wc.DownloadFileCompleted += ((sender, args) =>
       {
             if (args.Error == null)
             {
                  if (!args.Cancelled)
                  {
                       File.Move(filePath, Path.ChangeExtension(filePath, ".jpg"));
                  }
                  mr.Set();
              }
              else
              {
                    ex = args.Error;
                    mr.Set();
              }
        });
        wc.DownloadFileAsync(new Uri("MyInternetFile", filePath);
        mr.WaitOne();
        if (ex != null)
        {
             throw ex;
        }
  }

但是如果我关闭我的 WiFi（模拟互联网连接断开），我的应用程序就会暂停并且下载会停止，但它永远不会通过 DownloadFileCompleted 方法报告。

出于这个原因，我想在我的 WebClient.DownloadFileAsync 方法上实现超时。这可能吗？

顺便说一句，我使用的是 .Net 4，不想添加对第三方库的引用，所以不能使用 Async/Await 关键字

Answer 1

您可以使用 WebClient.DownloadFileAsync()。现在在计时器中，您可以像这样调用 CancelAsync()：

System.Timers.Timer aTimer = new System.Timers.Timer();
System.Timers.ElapsedEventHandler handler = null;
handler = ((sender, args)
      =>
     {
         aTimer.Elapsed -= handler;
         wc.CancelAsync();
      });
aTimer.Elapsed += handler;
aTimer.Interval = 100000;
aTimer.Enabled = true;

否则创建你自己的微客户端

  public class NewWebClient : WebClient
    {
        protected override WebRequest GetWebRequest(Uri address)
        {
            var req = base.GetWebRequest(address);
            req.Timeout = 18000;
            return req;
        }
    }

Answer 2

创建一个 WebClientAsync class 在构造函数中实现计时器。这样您就不会将计时器代码复制并粘贴到每个实现中。

public class WebClientAsync : WebClient
{
    private int _timeoutMilliseconds;

    public EdmapWebClientAsync(int timeoutSeconds)
    {
        _timeoutMilliseconds = timeoutSeconds * 1000;

        Timer timer = new Timer(_timeoutMilliseconds);
        ElapsedEventHandler handler = null;

        handler = ((sender, args) =>
        {
            timer.Elapsed -= handler;
            this.CancelAsync();
        });

        timer.Elapsed += handler;
        timer.Enabled = true;
    }

    protected override WebRequest GetWebRequest(Uri address)
    {
        WebRequest request = base.GetWebRequest(address);
        request.Timeout = _timeoutMilliseconds;
        ((HttpWebRequest)request).ReadWriteTimeout = _timeoutMilliseconds;

        return request;
    }



    protected override voidOnDownloadProgressChanged(DownloadProgressChangedEventArgs e)
    {
        base.OnDownloadProgressChanged(e);
        timer.Reset(); //If this does not work try below
        timer.Start();
    }
}

如果您在下载文件时失去互联网连接，这将允许您超时。

Answer 3

这是另一个实现，我试图避免任何共享 class/object 变量以避免多次调用的麻烦：

 public Task<string> DownloadFile(Uri url)
        {
            var tcs = new TaskCompletionSource<string>();
            Task.Run(async () =>
            {
                bool hasProgresChanged = false;
                var timer = new Timer(new TimeSpan(0, 0, 20).TotalMilliseconds);
                var client = new WebClient();

                void downloadHandler(object s, DownloadProgressChangedEventArgs e) => hasProgresChanged = true;
                void timerHandler(object s, ElapsedEventArgs e)
                {
                    timer.Stop();
                    if (hasProgresChanged)
                    {
                        timer.Start();
                        hasProgresChanged = false;
                    }
                    else
                    {
                        CleanResources();
                        tcs.TrySetException(new TimeoutException("Download timedout"));
                    }
                }
                void CleanResources()
                {
                    client.DownloadProgressChanged -= downloadHandler;
                    client.Dispose();
                    timer.Elapsed -= timerHandler;
                    timer.Dispose();
                }

                string filePath = Path.Combine(Path.GetTempPath(), Path.GetFileName(url.ToString()));
                try
                {
                    client.DownloadProgressChanged += downloadHandler;
                    timer.Elapsed += timerHandler;
                    timer.Start();
                    await client.DownloadFileTaskAsync(url, filePath);
                }
                catch (Exception e)
                {
                    tcs.TrySetException(e);
                }
                finally
                {
                    CleanResources();
                }

                return tcs.TrySetResult(filePath);
            });

            return tcs.Task;
        }

如何在 WebClient.DownloadFileAsync 上实现超时

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