WebClient DownloadFile 为重定向的 URI 下载损坏的 jpeg

Question

我在使用 WebClient 类下载 jpeg 图像时遇到了一些问题。我正在尝试通过以下代码下载 jpeg 图片：

public bool DownloadUri(string strUri, string destPath)
{
    bool result = false;
    try
    {
        Uri uri = new Uri(strUri, UriKind.RelativeOrAbsolute);
        WebClient client = new WebClient();
        client.DownloadFile(uri, destPath);
        result = true;
    }
    catch (Exception ex)
    {
        result = false;
    }
    return result;
}       

大多数工作正常，但当 jpeg 不存在时，我尝试下载的站点会重定向并且下载不会引发异常，它会下载大小仅为 8k 的损坏 jpeg。这是我提供的一个违规网址：

http://axisproperty.net.au/Upload/listing/500748870/500748870_1_500266205%2c1336368187%2cImageA.jpg

我已经使用 Uri.TryCreate 和 Uri.IsWellFormedUriString(urlAttribute.Value, UriKind.Absolute) 验证了 uri。我尝试执行 File.Exists(uri.ToString()) 来验证 uri 是否具有与其关联的有效文件，尽管即使对于有效的 url 也会失败。

有没有其他方法可以验证文件确实存在，这样我就不会下载所有这些不必要的损坏 jpeg 文件？

Answer 1

webclient.downloadfile 方法不处理 http 重定向

你应该为此使用 webrequest

public static int DownloadFile(String remoteFilename, String localFilename)
    {
      // Function will return the number of bytes processed
      // to the caller. Initialize to 0 here.
      int bytesProcessed = 0;

      // Assign values to these objects here so that they can
      // be referenced in the finally block
      Stream remoteStream = null;
      Stream localStream = null;
      WebResponse response = null;

      // Use a try/catch/finally block as both the WebRequest and Stream
      // classes throw exceptions upon error
      try
      {
        // Create a request for the specified remote file name
        WebRequest request = WebRequest.Create(remoteFilename);
        if (request != null)
        {
          // Send the request to the server and retrieve the
          // WebResponse object 
          response = request.GetResponse();
          if (response != null)
          {
            // Once the WebResponse object has been retrieved,
            // get the stream object associated with the response's data
            remoteStream = response.GetResponseStream();

            // Create the local file
            localStream = File.Create(localFilename);

            // Allocate a 1k buffer
            byte[] buffer = new byte[1024];
            int bytesRead;

            // Simple do/while loop to read from stream until
            // no bytes are returned
            do
            {
              // Read data (up to 1k) from the stream
              bytesRead = remoteStream.Read(buffer, 0, buffer.Length);

              // Write the data to the local file
              localStream.Write(buffer, 0, bytesRead);

              // Increment total bytes processed
              bytesProcessed += bytesRead;
            } while (bytesRead > 0);
          }
        }
      }
      catch (Exception e)
      {
        Console.WriteLine(e.Message);
      }
      finally
      {
        // Close the response and streams objects here 
        // to make sure they're closed even if an exception
        // is thrown at some point
        if (response != null) response.Close();
        if (remoteStream != null) remoteStream.Close();
        if (localStream != null) localStream.Close();
      }

      // Return total bytes processed to caller.
      return bytesProcessed;
    }

Answer 2

对不起，我应该更清楚。我提供的 url 无效，因此重定向到未找到页面或类似页面，因此在这种情况下，我不想下载，因为该文件中没有有效的 jpeg 图像。我找到了以下解决方案：

public static bool CheckURL(string URL)
{
    try
    {
        HttpWebRequest req = (HttpWebRequest)WebRequest.Create(URL);
        req.UserAgent = "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)";
        using (HttpWebResponse rsp = (HttpWebResponse)req.GetResponse())
        {
            if ((rsp.StatusCode == HttpStatusCode.OK) && rsp.ContentType.ToLower() == "image/jpeg")
            {
                return true;
            }
        }
    }
    catch
    {
        return false;
    }
    return false;
}