C# : Imprimer richTextBox avec visual Studio

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

namespace RichTextBoxPrintable
{
    public class RichTextBoxPrintable : RichTextBox
    {
        private const double Inch = 14.4;

        [StructLayout(LayoutKind.Sequential)]
        private struct MARG
        {
            public int iLeft, iRight, iTop, iBottom;
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct CARACTERE
        {
            public int iMin, iMax;
        }

        [StructLayout(LayoutKind.Sequential)]
        private struct FORMAT
        {
            public IntPtr hdc,Target;
            public MARG Mg, MgPage;
            public CARACTERE Carac;
        }

        private const int Utilisateur = 0x0400;
        private const int FormatUtilisateur = Utilisateur + 57;

        [DllImport("USER32.DLL")]
        private static extern IntPtr EnvoieMessage(IntPtr HWind, int msg,IntPtr Wrp, IntPtr Lrp);

        public int Impression(int CaracDebut, int CaracFin, PrintPageEventArgs arg)
        {
            MARG ZoneImprimable = new MARG { iTop = (int)(arg.MarginBounds.Top * Inch), iBottom = (int)(arg.MarginBounds.Bottom * Inch), iLeft = (int)(arg.MarginBounds.Left * Inch), iRight = (int)(arg.MarginBounds.Right * Inch) };
            MARG TaillePage = new MARG { iTop = (int)(arg.PageBounds.Top * Inch), iBottom = (int)(arg.PageBounds.Bottom * Inch), iLeft = (int)(arg.PageBounds.Left * Inch), iRight = (int)(arg.PageBounds.Right * Inch) };
            IntPtr hdc = arg.Graphics.GetHdc();

            FORMAT ZFormat;
            ZFormat.Carac.iMin = CaracDebut;
            ZFormat.Carac.iMax = CaracFin;
            ZFormat.hdc = hdc;
            ZFormat.Target = hdc;
            ZFormat.Mg = ZoneImprimable;
            ZFormat.MgPage = TaillePage;

            IntPtr res = IntPtr.Zero;
            IntPtr wParam = IntPtr.Zero;
            wParam = new IntPtr(1);
            IntPtr lParam = IntPtr.Zero;
            lParam = Marshal.AllocCoTaskMem(Marshal.SizeOf(ZFormat));
            Marshal.StructureToPtr(ZFormat, lParam, false);

            res = EnvoieMessage(Handle, FormatUtilisateur, wParam, lParam);

            Marshal.FreeCoTaskMem(lParam);
            arg.Graphics.ReleaseHdc(hdc);

            return res.ToInt32();
        }
        
    }
}