Line Numbers in a RichTextBox.

[dynamic_sysop]

using the PARAFORMAT2 Structure which you can find info on msdn about ...

'/// very top of your Form / Class
Imports System.Runtime.InteropServices
 
'/// then to use ...
 
    <StructLayout(LayoutKind.Sequential)> _
    Public Structure PARAFORMAT2
        Public cbSize As Int32
        Public dwMask As Int32
        Public wNumbering As Int16
        Public wReserved As Int16
        Public dxStartIndent As IntPtr
        Public dxRightIndent As IntPtr
        Public dxOffset As IntPtr
        Public wAlignment As Int16
        Public cTabCount As Short
        <MarshalAs(UnmanagedType.ByValArray, SizeConst:=32)> _
        Public rgxTabs() As IntPtr
        Public dySpaceBefore As IntPtr
        Public dySpaceAfter As IntPtr
        Public dyLineSpacing As IntPtr
        Public sStyle As Short
        Public bLineSpacingRule As Byte
        Public bOutlineLevel As Byte
        Public wShadingWeight As Int16
        Public wShadingStyle As Int16
        Public wNumberingStart As Int16
        Public wNumberingStyle As Int16
        Public wNumberingTab As Int16
        Public wBorderSpace As Int16
        Public wBorderWidth As Int16
        Public wBorders As Int16
    End Structure
 
    <DllImport("user32", CharSet:=CharSet.Auto)> _
    Private Shared Function SendMessage( _
            ByVal hWnd As HandleRef, _
            ByVal msg As Int32, _
            ByVal wParam As Int32, _
            ByRef lParam As PARAFORMAT2) As Int32
    End Function
 
    Private Const EM_GETPARAFORMAT = 1085
    Private Const EM_SETPARAFORMAT = 1095
    Private Const PFM_NUMBERING As Int32 = &H20
    Private Const BULLET_NUMBER = 2
 
    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        Dim param As New PARAFORMAT2
        '/// you must specify the size of the structure here, using Marshal.SizeOf , in VB6 it was Len( )
        param.cbSize = Marshal.SizeOf(param)
        '/// build up the param structure with the current layout...
        SendMessage(New HandleRef(rtb, rtb.Handle), EM_GETPARAFORMAT, 0, param)
        '/// wNumbering
        '/// Options used for bulleted or numbered paragraphs. To use this member, set the PFM_NUMBERING flag in the dwMask member.
        '/// PFN_BULLET ( 1 ) = Insert a bullet at the beginning of each selected paragraph.
        '/// 2 = Uses Arabic numbers (1, 2, 3, ...).  , hence BULLET_NUMBER.
        param.dwMask = PFM_NUMBERING
        param.wNumbering = BULLET_NUMBER
        '/// update the richtextbox...
        SendMessage(New HandleRef(rtb, rtb.Handle), EM_SETPARAFORMAT, 0, param)
    End Sub