Using HttpWebRequest with dynamic URI causes "parameter is not valid" in Image.FromStream

I'm trying to obtain an image to encode to a WordML document. The original version of this function used files, but I needed to change it to get images created on the fly with an aspx page. I've adapted the code to use HttpWebRequest instead of a WebClient. The problem is that I don't think the page request is getting resolved and so the image stream is invalid, generating the error "parameter is not valid" when I invoke Image.FromStream.

    public string RenderCitationTableImage(string citation_table_id)
{
    string image_content = "";
    string _strBaseURL = String.Format("http://{0}",
        HttpContext.Current.Request.Url.GetComponents(UriComponents.HostAndPort, UriFormat.Unescaped));
    string _strPageURL = String.Format("{0}{1}", _strBaseURL,
        ResolveUrl("~/Publication/render_citation_chart.aspx"));

    string _staticURL = String.Format("{0}{1}", _strBaseURL,
        ResolveUrl("~/Images/table.gif"));

    string _fullURL = String.Format("{0}?publication_id={1}&citation_table_layout_id={2}",
                                        _strPageURL, publication_id, citation_table_id);


    try
    {
        HttpWebRequest request = (HttpWebRequest)WebRequest.Create(_fullURL);
        HttpWebResponse response = (HttpWebResponse)request.GetResponse();
        Stream image_stream = response.GetResponseStream();
        // Read the image data
        MemoryStream ms = new MemoryStream();
        int num_read;
        byte[] crlf = System.Text.Encoding.Default.GetBytes("\r\n");
        byte[] buffer = new byte[1024];
        for (num_read = image_stream.Read(buffer, 0, 1024); num_read > 0; num_read = image_stream.Read(buffer, 0, 1024))
        {
            ms.Write(buffer, 0, num_read);
        }

        // Base 64 Encode the image data
        byte[] image_bytes = ms.ToArray();
        string encodedImage = Convert.ToBase64String(image_bytes);
        ms.Position = 0;
        System.Drawing.Image image_original = System.Drawing.Image.FromStream(ms); // <---error here: parameter is not valid
        image_stream.Close();

        image_content = string.Format("<w:p>{4}<w:r><w:pict><w:binData w:name=\"wordml://{0}\">{1}</w:binData>" +
            "<v:shape style=\"width:{2}px;height:{3}px\">" +
            "<v:imagedata src=\"wordml://{0}\"/>" +
            "</v:shape>" +
            "</w:pict></w:r></w:p>", _word_image_id, encodedImage, 800, 400, alignment.center);

        image_content = "<w:br w:type=\"text-wrapping\"/>" + image_content + "<w:br w:type=\"text-wrapping\"/>";
    }
    catch (Exception ex) 
    {
        return ex.ToString();
    }
    return image_content;

Using a static URI it works fine. If I replace "staticURL" with 开发者_开发问答"fullURL" in the WebRequest.Create method I get the error. Any ideas as to why the page request doesn't fully resolve?

And yes, the full URL resolves fine and shows an image if I post it in the address bar.

UPDATE:

Just read your updated question. Since you're running into login issues, try doing this before you execute the request:

request.Credentials = CredentialCache.DefaultCredentials

If this doesn't work, then perhaps the problem is that authentication is not being enforced on static files, but is being enforced on dynamic files. In this case, you'll need to log in first (using your client code) and retain the login cookie (using HttpWebRequest.CookieContainer on the login request as well as on the second request) or turn off authentication on the page you're trying to access.

ORIGINAL:

Since it works with one HTTP URL and doesn't work with another, the place to start diagnosing this is figuring out what's different between the two requests, at the HTTP level, which accounts for the difference in behavior in your code.

To figure out the difference, I'd use Fiddler (http://fiddlertool.com) to compare the two requests. Compare the HTTP headers. Are they the same? In particular, are they the same HTTP content type? If not, that's likely the source of your problem.

If headers are the same, make sure both the static and dynamic image are exactly the same content and file type on the server. (e.g. use File...Save As to save the image in a browser to your disk). Then use Fiddler's Hex View to compare the image content. Can you see any obvious differences?

Finally, I'm sure you've already checked this, but just making sure: /Publication/render_citation_chart.aspx refers to an actual image file, not an HTML wrapper around an IMG element, right? This would account for the behavior you're seeing, where a browser renders the image OK but your code doesn't.