Csharp 如何使用PDFSharp将文本添加到现有PDF

我正在使用PDFSharp(1.50或6.1.0 Preview 2)try 打开现有的pdf，添加一些文本，然后再次保存.

然而，在绘制文本的那一行，我得到了System. ArgumentResponse "An item with the same key has already been added."

我不明白这个错误想告诉我什么或如何解决它.

注释DrawString行即可编写新的pdf，但如何将文本添加到PDF中？

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using PdfSharp;
using PdfSharp.Pdf.IO;
using PdfSharp.Pdf;
using PdfSharp.Drawing;
using PdfSharp.Fonts;

namespace MakeAPDF
{
    internal class Program
    {
        static void Main(string[] args)
        {

            const string outfilename = @"C:\Test\Out.pdf";
            PdfDocument document = PdfReader.Open(@"C:\Test\In.pdf",PdfDocumentOpenMode.Modify);
            
            // Get an XGraphics object for drawing
            PdfPage page = document.Pages[0];

            XGraphics gfx = XGraphics.FromPdfPage(page);
            // Create a font

            XFont font = new XFont("Verdana", 20, XFontStyle.BoldItalic);
        
            // Draw the text
            // This line triggers System.ArgumentException
            // Message=An item with the same key has already been added.
            gfx.DrawString("Hello, World!", font, XBrushes.Black, new XRect(0, 0, page.Width, page.Height),  XStringFormats.Center);
        
            // Save the document...  
            document.Save(outfilename);
        }
    }
}

STACKTRACE:

 System.ArgumentException
 HResult=0x80070057
 Message=An item with the same key has already been added.
 Source=mscorlib
 StackTrace: 
 at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
 at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
 at PdfSharp.Pdf.Advanced.PdfResourceMap.CollectResourceNames(Dictionary`2 usedResourceNames)
 at PdfSharp.Pdf.Advanced.PdfResources.ExistsResourceNames(String name)
 at PdfSharp.Pdf.Advanced.PdfResources.get_NextExtGStateName()
 at PdfSharp.Pdf.Advanced.PdfResources.AddExtGState(PdfExtGState extGState)
 at PdfSharp.Drawing.Pdf.PdfGraphicsState.RealizeFillColor(XColor color, Boolean overPrint, PdfColorMode colorMode)
 at PdfSharp.Drawing.Pdf.PdfGraphicsState.RealizeBrush(XBrush brush, PdfColorMode colorMode, Int32 renderingMode, Double fontEmSize)
 at PdfSharp.Drawing.Pdf.PdfGraphicsState.RealizeFont(XFont font, XBrush brush, Int32 renderingMode)
 at PdfSharp.Drawing.Pdf.XGraphicsPdfRenderer.Realize(XFont font, XBrush brush, Int32 renderingMode)
 at PdfSharp.Drawing.Pdf.XGraphicsPdfRenderer.DrawString(String s, XFont font, XBrush brush, XRect rect, XStringFormat format)
 at PdfSharp.Drawing.XGraphics.DrawString(String text, XFont font, XBrush brush, XRect layoutRectangle, XStringFormat format)

UPDATE:

我发现，如果我开始使用的PDF完全空或仅包含形状，上述代码仍然有效.如果PDF仅包含一段文本，则会触发此异常.那么这可能是PDFSharp中的一个错误吗？

引发该问题的PDF可以从here下载.