PDFStamper不适用于较新版本的iTextSharp

Question

iTextSharp的最新版本是5.5.11。

当我对5.5.0运行完全相同的代码时，它可以工作。

这是一个非常简单的请求 - 使用iTextSharp在内存中的现有PDF上设置水印。

以下是有问题的代码：

private static Stream Watermark(Stream inStream, WatermarkRequest request)
    {
        using (var outStream = new MemoryStream((int)(inStream.Length + 4096)))
        {
            using (var reader = new PdfReader(inStream))
            using (var stamper = new PdfStamper(reader, outStream))
            {
                var opacity = request.Opacity.HasValue ? request.Opacity.Value : 0.75f;
                var angle = request.Angle.HasValue ? request.Angle.Value : 45;
                var size = request.Size.HasValue ? request.Size.Value : 72f;
                var color = new BaseColor(1f, 0f, 0f, 0f);

                if (!String.IsNullOrEmpty(request.Color))
                {
                    var rgba = PDF.Lib.RGBA.convert(request.Color);

                    color = new BaseColor((float)rgba.R, (float)rgba.G, (float)rgba.B, 0f);
                }

                for (var n = 1; n <= reader.NumberOfPages; n++)
                {
                    var pcb = stamper.GetOverContent(n);
                    var gstate = new PdfGState() { FillOpacity = opacity, StrokeOpacity = opacity };
                    pcb.SaveState();
                    pcb.SetGState(gstate);
                    pcb.SetColorFill(color);
                    pcb.BeginText();
                    pcb.SetFontAndSize(BaseFont.CreateFont(BaseFont.HELVETICA, Encoding.Default.WebName, true), size);
                    var ps = reader.GetPageSize(n);
                    var x = (ps.Right + ps.Left) / 2;
                    var y = (ps.Bottom + ps.Top) / 2;
                    pcb.ShowTextAligned(Element.ALIGN_CENTER, request.Text, x, y, angle);
                    pcb.EndText();
                    pcb.RestoreState();
                }
            }

            return new MemoryStream(outStream.ToArray());
        }
    }