c# - 保存缩略图，获取 GDI+ 中发生一般性错误

Question

我正在使用以下方法为在线图像创建缩略图:

public static string SaveThumbnail(string imageUrl, int newWidth, int newHeight, string id)
{
    string uploadImagePath = "/UploadedImages/";
    Stream str = null;
    HttpWebRequest wReq = (HttpWebRequest)WebRequest.Create(imageUrl);
    HttpWebResponse wRes = (HttpWebResponse)(wReq).GetResponse();
    str = wRes.GetResponseStream();

    using (var image = Image.FromStream(str, false, true))
    {
        Image thumb = image.GetThumbnailImage(newWidth, newHeight, () => false, IntPtr.Zero);
        string pathAndName = Path.ChangeExtension(uploadImagePath + id, "thumb");
        thumb.Save(pathAndName);
        return pathAndName;
    }
}

但是我得到了这个错误:

Description: An unhandled exception occurred during the execution of the current web
request. Please review the stack trace for more information about the error and where it
originated in the code.

异常详情:

System.Runtime.InteropServices.ExternalException: A generic error occurred in GDI+.

有什么想法吗？

Answer 1

Iirc 这是因为它希望流是可搜索的，而网络流不是。尝试先将流复制到 MemoryStream(如果很大，则复制到临时 FileStream)，然后从那里开始工作(记住在读取之前将位置设置回 0)。

using(var ms = new MemoryStream()) {
    str.CopyTo(ms); // a 4.0 extension method
    ms.Position = 0;
    // TODO: read from ms etc
}

对于 FileStream，您可能会发现 WebClient.DownloadFile 更方便。对于内存中方法，同上 WebClient.DownloadData。