我如何解决“参数无效”。我的代码出错？

Question

当我试图从PDF获取图像时，我的代码出错了。任何人都知道如何重新获得它。我的代码如下： -

        PdfReader pdf = new PdfReader(sourcePdf);
        RandomAccessFileOrArray raf = new iTextSharp.text.pdf.RandomAccessFileOrArray(sourcePdf);
        try
        {
            for (int pageNumber = 1; pageNumber <= pdf.NumberOfPages; pageNumber++)
            {
                PdfDictionary pg = pdf.GetPageN(pageNumber);
                // recursively search pages, forms and groups for images.
                PdfObject obj = FindImageInPDFDictionary(pg);
                if (obj != null)
                {
                    int XrefIndex = Convert.ToInt32(((PRIndirectReference)obj).Number.ToString(System.Globalization.CultureInfo.InvariantCulture));
                    PdfObject pdfObj = pdf.GetPdfObject(XrefIndex);
                    PdfStream pdfStrem = (PdfStream)pdfObj;
                    byte[] bytes = PdfReader.GetStreamBytesRaw((PRStream)pdfStrem);
                    if ((bytes != null))
                    {
                        using (System.IO.MemoryStream memStream = new System.IO.MemoryStream(bytes))
                        {
                            memStream.Position = 0;
                            **System.Drawing.Image img = System.Drawing.Image.FromStream(memStream);**
                            // must save the file while stream is open.
                            if (!Directory.Exists(outputPath))
                                Directory.CreateDirectory(outputPath);

                            string path = Path.Combine(outputPath, String.Format(@"{0}.jpg", pageNumber));
                            System.Drawing.Imaging.EncoderParameters parms = new System.Drawing.Imaging.EncoderParameters(1);
                            parms.Param[0] = new System.Drawing.Imaging.EncoderParameter(System.Drawing.Imaging.Encoder.Compression, 0);
                            System.Drawing.Imaging.ImageCodecInfo jpegEncoder =GetImageEncoder("JPEG");
                            img.Save(path, jpegEncoder, parms);
                        }
                    }
                }
            }
        }
        catch
        {
            throw;
        }

在 System.Drawing.Image img = System.Drawing.Image.FromStream（memStream）;  我遇到错误“参数无效。”