Exception System.ArgumentException When using DrawString to add text to an existing PDF

[netx123]

Issue

When trying to add text to a PDF (That already contains text) I'm getting an Exception Exception System.ArgumentException This issue is present in both 1.50 and 6.1.0 This issue only occurs if the input PDF already contains some text, if the input PDF only contains shapes, then the code below works and the text is added as expected.

Expected Behavior

I expected the additional text to be added to the the new PDF

Actual Behaviour

I get an exception System.ArgumentException "An item with the same key has already been added"

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)

Steps to Reproduce the Behavior

Using the following code, with a PDF that contains text, triggers the issue

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);
        }
    }
}

[netx123]

This issue may be PDF specific, so please find attached a PDF that triggers the problem. Empty.pdf

The PDF was generated with Xara Photo & Graphic Designer 19.0.1.65946

[StLange]

Bug is fixed in 6.1.0 Preview 3.