How to catch 404 WebException for WebClient.DownloadFileAsync

问题

This code:

try
{
  _wcl.DownloadFile(url, currentFileName);
}
catch (WebException ex)
{
  if (ex.Status == WebExceptionStatus.ProtocolError && ex.Response != null)
    if ((ex.Response as HttpWebResponse).StatusCode == HttpStatusCode.NotFound)
      Console.WriteLine("\r{0} not found.     ", currentFileName);
}

downloads file and informs if 404 error occured.

I decided to download files asynchronously:

try
{
  _wcl.DownloadFileAsync(new Uri(url), currentFileName);
}
catch (WebException ex)
{
  if (ex.Status == WebExceptionStatus.ProtocolError && ex.Response != null)
    if ((ex.Response as HttpWebResponse).StatusCode == HttpStatusCode.NotFound)
      Console.WriteLine("\r{0} not found.     ", currentFileName);
}

Now this catch block does not fire if server returns a 404 error and WebClient produces an empty file.

回答1:

You need to handle the DownloadFileCompleted event and check the Error property of the AsyncCompletedEventArgs.

There are good examples in the links.

回答2:

You can try this code:

WebClient wcl;

void Test()
{
    Uri sUri = new Uri("http://google.com/unknown/folder");
    wcl = new WebClient();
    wcl.OpenReadCompleted += onOpenReadCompleted;
    wcl.OpenReadAsync(sUri);
}

void onOpenReadCompleted(object sender, OpenReadCompletedEventArgs e)
{
    if (e.Error != null)
    {
        HttpStatusCode httpStatusCode = GetHttpStatusCode(e.Error);
        if (httpStatusCode == HttpStatusCode.NotFound)
        {
            // 404 found
        }
    }
    else if (!e.Cancelled)
    {
        // Downloaded OK
    }
}

HttpStatusCode GetHttpStatusCode(System.Exception err)
{
    if (err is WebException)
    {
        WebException we = (WebException)err;
        if (we.Response is HttpWebResponse)
        {
            HttpWebResponse response = (HttpWebResponse)we.Response;
            return response.StatusCode;
        }
    }
    return 0;
}

来源：https://stackoverflow.com/questions/16387904/how-to-catch-404-webexception-for-webclient-downloadfileasync