C#RichTextBoxコントロールの内容を印刷する方法

using System;
using System.Windows.Forms;
using System.Drawing;
using System.Runtime.InteropServices;
using System.Drawing.Printing;

namespace RichTextBoxPrintCtrl
{
	public class RichTextBoxPrintCtrl:RichTextBox
	{
		//Convert the unit used by the .NET framework (1/100 inch) 
		//and the unit used by Win32 API calls (twips 1/1440 inch)
		private const double anInch = 14.4;

		[StructLayout(LayoutKind.Sequential)] 
			private struct RECT
		{
			public int Left;
			public int Top;
			public int Right;
			public int Bottom;
		}

		[StructLayout(LayoutKind.Sequential)]
			private struct CHARRANGE
		{
			public int cpMin;         //First character of range (0 for start of doc)
			public int cpMax;           //Last character of range (-1 for end of doc)
		}

		[StructLayout(LayoutKind.Sequential)]
			private struct FORMATRANGE
		{
			public IntPtr hdc;             //Actual DC to draw on
			public IntPtr hdcTarget;       //Target DC for determining text formatting
			public RECT rc;                //Region of the DC to draw to (in twips)
			public RECT rcPage;            //Region of the whole DC (page size) (in twips)
			public CHARRANGE chrg;         //Range of text to draw (see earlier declaration)
		}

		private const int WM_USER  = 0x0400;
		private const int EM_FORMATRANGE  = WM_USER + 57;
		
		[DllImport("USER32.dll")]
		private static extern IntPtr SendMessage (IntPtr hWnd , int msg , IntPtr wp, IntPtr lp); 

		// Render the contents of the RichTextBox for printing
		//	Return the last character printed + 1 (printing start from this point for next page)
		public int Print( int charFrom, int charTo,PrintPageEventArgs e)
		{
			//Calculate the area to render and print
			RECT rectToPrint; 
			rectToPrint.Top = (int)(e.MarginBounds.Top * anInch);
			rectToPrint.Bottom = (int)(e.MarginBounds.Bottom * anInch);
			rectToPrint.Left = (int)(e.MarginBounds.Left * anInch);
			rectToPrint.Right = (int)(e.MarginBounds.Right * anInch);

			//Calculate the size of the page
			RECT rectPage; 
			rectPage.Top = (int)(e.PageBounds.Top * anInch);
			rectPage.Bottom = (int)(e.PageBounds.Bottom * anInch);
			rectPage.Left = (int)(e.PageBounds.Left * anInch);
			rectPage.Right = (int)(e.PageBounds.Right * anInch);

			IntPtr hdc = e.Graphics.GetHdc();

			FORMATRANGE fmtRange;
			fmtRange.chrg.cpMax = charTo;				//Indicate character from to character to 
			fmtRange.chrg.cpMin = charFrom;
			fmtRange.hdc = hdc;                    //Use the same DC for measuring and rendering
			fmtRange.hdcTarget = hdc;              //Point at printer hDC
			fmtRange.rc = rectToPrint;             //Indicate the area on page to print
			fmtRange.rcPage = rectPage;            //Indicate size of page

			IntPtr res = IntPtr.Zero;

			IntPtr wparam = IntPtr.Zero;
			wparam = new IntPtr(1);

			//Get the pointer to the FORMATRANGE structure in memory
			IntPtr lparam= IntPtr.Zero;
			lparam = Marshal.AllocCoTaskMem(Marshal.SizeOf(fmtRange));
			Marshal.StructureToPtr(fmtRange, lparam, false);

			//Send the rendered data for printing 
			res = SendMessage(Handle, EM_FORMATRANGE, wparam, lparam);

			//Free the block of memory allocated
			Marshal.FreeCoTaskMem(lparam);

			//Release the device context handle obtained by a previous call
			e.Graphics.ReleaseHdc(hdc);

			//Return last + 1 character printer
			return res.ToInt32();
		}

	}
}

		this.printDocument1.BeginPrint += new System.Drawing.Printing.PrintEventHandler(this.printDocument1_BeginPrint);
		this.printDocument1.PrintPage += new System.Drawing.Printing.PrintPageEventHandler(this.printDocument1_PrintPage);
		this.btnPrint.Click += new System.EventHandler(this.btnPrint_Click);
		this.btnPrintPreview.Click += new System.EventHandler(this.btnPrintPreview_Click);
		this.btnPageSetup.Click += new System.EventHandler(this.btnPageSetup_Click);

		private int checkPrint;
		private void btnPageSetup_Click(object sender, System.EventArgs e)
		{
			pageSetupDialog1.ShowDialog();
		}

		private void btnPrintPreview_Click(object sender, System.EventArgs e)
		{
			printPreviewDialog1.ShowDialog();
		}

		private void btnPrint_Click(object sender, System.EventArgs e)
		{
			if (printDialog1.ShowDialog() == DialogResult.OK)
				printDocument1.Print();
		}

		private void printDocument1_BeginPrint(object sender, System.Drawing.Printing.PrintEventArgs e)
		{
			checkPrint = 0;
		}

		private void printDocument1_PrintPage(object sender, System.Drawing.Printing.PrintPageEventArgs e)
		{
			// Print the content of RichTextBox. Store the last character printed.
			checkPrint = richTextBoxPrintCtrl1.Print(checkPrint, richTextBoxPrintCtrl1.TextLength, e);

			// Check for more pages
			if (checkPrint < richTextBoxPrintCtrl1.TextLength)
				e.HasMorePages = true;
			else
				e.HasMorePages = false;
		}