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

public static class RichTextBoxHelper
{
    private const int EM_SETPARAFORMAT = 0x447;
    private const int EM_SETTYPOGRAPHYOPTIONS = 0x4CA;
    private const int TO_ADVANCEDTYPOGRAPHY = 1;
    private const int PFM_ALIGNMENT = 8;
    private const int PFA_JUSTIFY = 4;

    [StructLayout(LayoutKind.Sequential)]
    private struct PARAFORMAT2
    {
        public int cbSize;
        public uint dwMask;
        public short wNumbering;
        public short wReserved;
        public int dxStartIndent;
        public int dxRightIndent;
        public int dxOffset;
        public short wAlignment;
        public short cTabCount;
        [MarshalAs(UnmanagedType.ByValArray, SizeConst = 32)]
        public int[] rgxTabs;
        public int dySpaceBefore;
        public int dySpaceAfter;
        public int dyLineSpacing;
        public short sStyle;
        public byte bLineSpacingRule;
        public byte bOutlineLevel;
        public short wShadingWeight;
        public short wShadingStyle;
        public short wNumberingStart;
        public short wNumberingStyle;
        public short wNumberingTab;
        public short wBorderSpace;
        public short wBorderWidth;
        public short wBorders;
    }

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

    [DllImport("user32.dll", CharSet = CharSet.Auto)]
    private static extern IntPtr SendMessage(IntPtr hWnd, int msg, IntPtr wParam, ref PARAFORMAT2 lParam);

    // Bekapcsolja a speciális szövegmegjelenítést (ez kell a sorkizárthoz).
    public static void EngedelyezSorkizartot(RichTextBox rtb)
    {
        SendMessage(rtb.Handle, EM_SETTYPOGRAPHYOPTIONS, (IntPtr)TO_ADVANCEDTYPOGRAPHY, (IntPtr)TO_ADVANCEDTYPOGRAPHY);
    }

    // Sorkizártra állítja a kijelölt bekezdéseket.
    public static void Sorkizart(RichTextBox rtb)
    {
        EngedelyezSorkizartot(rtb);

        PARAFORMAT2 pf = new PARAFORMAT2();
        pf.cbSize = Marshal.SizeOf(pf);
        pf.dwMask = PFM_ALIGNMENT;
        pf.wAlignment = PFA_JUSTIFY;
        pf.rgxTabs = new int[32];

        SendMessage(rtb.Handle, EM_SETPARAFORMAT, IntPtr.Zero, ref pf);
    }
}

private void btnSorkizart_Click(object sender, EventArgs e)
{
    RichTextBoxHelper.Sorkizart(rtbDokumentum);
    rtbDokumentum.Focus();
}

private void Form1_Load(object sender, EventArgs e)
{
    RichTextBoxHelper.EngedelyezSorkizartot(rtbDokumentum);
}