通过Http范围请求快速检测GIF尺寸（本地部署，在Azure上失败）

Question

Something more or less like the routine below works in couple of different environments for a given URL (eg http://covers.oreilly.com/images/0636920022886/bkt.gif ) but fails on Azure with the following exception:

System.ArgumentException: Parameter is not valid. at System.Drawing.Image.FromStream(Stream stream, Boolean useEmbeddedColorManagement, Boolean validateImageData) at System.Drawing.Image.FromStream(Stream stream)

In all cases, the assembly in quetion is System.Drawing, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a, so I'm assuming the data received is different somehow on Azure.

    public static DimensionResult Is404(string url)
    {
        DimensionResult result = null;

        HttpWebRequest request = Http.PrepareGetRequest(new Uri(url), false, false, "Mozilla/5.0 (compatible; MSIE 9.0; Windows NT 6.1; WOW64; Trident/5.0)");
        request.Timeout = 2500;
        request.Method = "GET";
        request.AddRange(0, 2048);
        request.KeepAlive = false;
        request.AllowAutoRedirect = true;

        try
        {
            result = new DimensionResult();

            using (HttpWebResponse response = (HttpWebResponse)request.GetResponse())
            {
                result.ContentEncoding = response.ContentEncoding;
                result.Url = response.ResponseUri.ToString();
                result.Is404 = (response.StatusCode != HttpStatusCode.PartialContent && response.StatusCode != HttpStatusCode.OK) || System.Text.RegularExpressions.Regex.IsMatch(response.ContentType, "text|html", System.Text.RegularExpressions.RegexOptions.IgnoreCase);

                if (!result.Is404)
                        using (System.Drawing.Image image = System.Drawing.Image.FromStream(response.GetResponseStream()))
                        {
                            result.Width = image.Width;
                            result.Height = image.Height;
                        }
            }
        }
        catch (Exception ex)
        {
            result.Exception = ex;
            result.Is404 = true;
        }

        return result;
    }

Please don't focus on the number of bytes requested (this is a simplified version) but on what settings in the networking stack of .NET could account for a difference response from server to server?

In both cases, I've logged the response headers so far and they're identical, no network traces yet:

Date:Fri, 20 Apr 2012 11:47:05 GMT,Server:Apache,Accept-Ranges:bytes,Last-Modified:Fri, 24 Feb 2012 17:21:00 GMT,Content-Range:bytes 0-2048/3556,Content-Length:2049,Content-Type:image/gif,Cache-Control:max-age=2592000,Expires:Sun, 20 May 2012 11:47:05 GMT,Connection:close

UPDATE : I've logged the bytes received in both environments and they happen to be identical! So same response headers, same response length, same response content, same assembly, different behavior.

Answer 1

According to http://en.wikipedia.org/wiki/Graphics_Interchange_Format , a gif file begins with the character “GIF87a” or “GIF89a”, followed by its width/height. So if all you need is the width/height information, you can read bytes 6-9. The GDI+ (System.Drawing.dll) is not needed. From my experience, using System.Drawing.dll in server environments and in particular Windows Azure can lead to unpredictable results. If you need advanced bitmap processing, you can use WIC or WPF (which uses WIC under the hook).

Best Regards,

Ming Xu.