如何捕获 WebClient.DownloadFileAsync 的 404 WebException 错误

Question

以下是代码：

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);
}

下载文件并提示是否发生404错误。

我决定异步下载文件：

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);
}

现在，如果服务器返回404错误且WebClient生成一个空文件，则此catch块不会触发。

- Paul

Answer 1

您需要处理 DownloadFileCompleted 事件，并检查 AsyncCompletedEventArgs 的 Error 属性。
这些链接中有很好的示例。

- Jim Mischel

---

每次删除一个空文件？ - Paul

@Paul：是的，如果出现错误。显然 DownloadFileAsync 打开一个文件以准备写入。完成后无论是否有错误，都会关闭该文件。如果你不想要这个文件，就删除它。 - Jim Mischel

Answer 2

你可以尝试这段代码：

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;
}

- Sergey