C#でRTF形式データの印刷処理を作成したい

「.NET RichTextBox から WYSIWYG 印刷結果を取得する」を改造して、

namespace WinFormsApp1
{
    using System;
    using System.Windows.Forms;
    using System.Drawing;

    public partial class Form1 : System.Windows.Forms.Form
    {
        private RichTextBox richText1;
        private Button button1;

        public Form1()
        {
            InitializeComponent();

            button1 = new Button();
            button1.Text = "Test";
            button1.Click += button1_Click;
            this.Controls.Add(button1);


            this.richText1 = new RichTextBox();
            this.richText1.Location = new Point(0, button1.Height);
            this.richText1.Size = new Size(this.ClientSize.Width, this.ClientSize.Height - this.button1.Height);
            this.richText1.Anchor = AnchorStyles.Left | AnchorStyles.Right | AnchorStyles.Top | AnchorStyles.Bottom;
            this.Controls.Add(richText1);

            var data = Clipboard.GetDataObject();
            if (data.GetDataPresent(DataFormats.Rtf))
            {
                var oRtf = data.GetData(DataFormats.Rtf);
                if (oRtf is string rtf)
                {
                    this.richText1.Rtf = rtf;
                }
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            PrintDialog dlg = new PrintDialog();
            if (dlg.ShowDialog() == DialogResult.OK)
            {
                System.Drawing.Printing.PrintDocument printDoc
                = new System.Drawing.Printing.PrintDocument();

                printDoc.PrinterSettings = dlg.PrinterSettings;

                var richPrint = new RichTextBoxPrint(this.richText1);
                richPrint.PrintRichTextContents(printDoc);
            }
        }
    }
}

namespace WinFormsApp1
{
    using System;
    using System.Windows.Forms;
    using System.Runtime.InteropServices;
    using System.Drawing.Printing;

    public class RichTextBoxPrint
    {
        public RichTextBoxPrint(RichTextBox rich)
        {
            this.richTextBox = rich;
        }

        private RichTextBox richTextBox;
        private int m_nFirstCharOnPage;

        public void PrintRichTextContents(PrintDocument printDoc)
        {
            printDoc.BeginPrint += new PrintEventHandler(printDoc_BeginPrint);
            printDoc.PrintPage += new PrintPageEventHandler(printDoc_PrintPage);
            try
            {
                printDoc.Print();
            }
            finally
            {
                printDoc.BeginPrint -= new PrintEventHandler(printDoc_BeginPrint);
                printDoc.PrintPage -= new PrintPageEventHandler(printDoc_PrintPage);
            }
        }


        private void printDoc_BeginPrint(object sender, System.Drawing.Printing.PrintEventArgs e)
        {
            m_nFirstCharOnPage = 0;
        }

        private void printDoc_PrintPage(object sender, System.Drawing.Printing.PrintPageEventArgs e)
        {
            m_nFirstCharOnPage = this.FormatRange(false, e, m_nFirstCharOnPage, this.richTextBox.TextLength);
            e.HasMorePages = (m_nFirstCharOnPage < this.richTextBox.TextLength);
        }


        private int FormatRange(bool measureOnly, PrintPageEventArgs e, int charFrom, int charTo)
        {
            STRUCT_CHARRANGE cr;
            cr.cpMin = charFrom;
            cr.cpMax = charTo;

            STRUCT_RECT rc;
            rc.top = HundredthInchToTwips(e.MarginBounds.Top);
            rc.bottom = HundredthInchToTwips(e.MarginBounds.Bottom);
            rc.left = HundredthInchToTwips(e.MarginBounds.Left);
            rc.right = HundredthInchToTwips(e.MarginBounds.Right);

            STRUCT_RECT rcPage;
            rcPage.top = HundredthInchToTwips(e.PageBounds.Top);
            rcPage.bottom = HundredthInchToTwips(e.PageBounds.Bottom);
            rcPage.left = HundredthInchToTwips(e.PageBounds.Left);
            rcPage.right = HundredthInchToTwips(e.PageBounds.Right);
            IntPtr hdc = e.Graphics.GetHdc();

            STRUCT_FORMATRANGE fr;
            fr.chrg = cr;
            fr.hdc = hdc;
            fr.hdcTarget = hdc;
            fr.rc = rc;
            fr.rcPage = rcPage;

            Int32 wParam = (measureOnly ? 0 : 1);
            IntPtr lParam = Marshal.AllocCoTaskMem(Marshal.SizeOf(fr));
            Marshal.StructureToPtr(fr, lParam, false);
            int res = SendMessage(this.richTextBox.Handle, EM_FORMATRANGE, wParam, lParam);
            Marshal.FreeCoTaskMem(lParam);
            e.Graphics.ReleaseHdc(hdc);

            return res;
        }

        private Int32 HundredthInchToTwips(int n)
        {
            return (Int32)(n * 14.4);
        }

        #region Win32
        [StructLayout(LayoutKind.Sequential)]
        private struct STRUCT_RECT
        {
            public Int32 left;
            public Int32 top;
            public Int32 right;
            public Int32 bottom;
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct STRUCT_CHARRANGE
        {
            public Int32 cpMin;
            public Int32 cpMax;
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct STRUCT_FORMATRANGE
        {
            public IntPtr hdc;
            public IntPtr hdcTarget;
            public STRUCT_RECT rc;
            public STRUCT_RECT rcPage;
            public STRUCT_CHARRANGE chrg;
        }

        [DllImport("user32.dll")]
        private static extern Int32 SendMessage(IntPtr hWnd, Int32 msg, Int32 wParam, IntPtr lParam);

        private const Int32 WM_USER = 0x400;
        private const Int32 EM_FORMATRANGE = WM_USER + 57;

        #endregion
    }
}