Numbered lined richtextbox, synchronized scrolling - 3 Jan 2018 update

Learning from NickThissen's post on synchronized scrolling of two richtextboxes, I've simplified his code, slapping together this numbered version.

Attachment 154971

Besides typing, you can paste text into it by pressing Ctrl-P.

RichTextBox1's backcolor is set to "Control" in properties, borderstyle "None", scrollbars "None", readonly "True", docked "Left", with a width of 70.

RichTextbox2's backcolor is "Window", borderstyle "None", wordwrap "False", docked "Fill".

Enjoy! :)

Code:

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Public Class Form1

    Const WM_USER As Integer = &H400
    Const EM_GETSCROLLPOS As Integer = WM_USER + 221
    Const EM_SETSCROLLPOS As Integer = WM_USER + 222
    Declare Function SendMessage Lib "user32.dll" Alias "SendMessageW" (ByVal hWnd As IntPtr, ByVal msg As Integer, ByVal wParam As Integer, ByRef lParam As Point) As Integer


    Private Sub Form1_Load_1(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

        RichTextBox1.SelectionAlignment = HorizontalAlignment.Right
        RichTextBox1.Text = "1"
        RichTextBox1.Enabled = False
    End Sub


    Private Sub RichTextBox2_KeyUp(ByVal sender As Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles RichTextBox2.KeyUp
        If (e.KeyCode = Keys.P AndAlso e.Modifiers = Keys.Control) Then
            RichTextBox2.Text = Clipboard.GetText()
            RichTextBox2.SelectAll()
            RichTextBox2.SelectionFont = New Font("Microsoft Sans Serif", 10, FontStyle.Regular)
            RichTextBox1.SelectAll()
            RichTextBox1.SelectionFont = New Font("Microsoft Sans Serif", 10, FontStyle.Regular)
        End If
    End Sub


    Private Sub RichTextBox2_TextChanged(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles RichTextBox2.TextChanged
        Dim count As Integer
        Dim count2 As String
        Dim myList As List(Of String) = RichTextBox1.Lines.ToList()
        Dim minus As Integer
        Dim holder As String

        count = RichTextBox2.Lines.Length
        holder = CStr(RichTextBox1.Lines.Length)
        count2 = " "

        If RichTextBox1.Lines.Length > count Then
            For minus = 1 To CInt(CDbl(holder) - count)
                If myList.Count > count Then
                    myList.RemoveAt(myList.Count - 1)
                    RichTextBox1.Lines = myList.ToArray()
                    RichTextBox1.Refresh()
                End If
            Next
        Else
            If count = RichTextBox1.Lines.Length Then
                count = RichTextBox1.Lines.Length
            End If
        End If

        If count > RichTextBox1.Lines.Length + 1 Then
            count2 = CStr(count - RichTextBox1.Lines.Length)
            For minus = 1 To CInt(count2)
                RichTextBox1.AppendText(Environment.NewLine & minus + count - CDbl(count2))
            Next
        End If

        If RichTextBox1.Text.Length = 0 Then
            RichTextBox1.Text = "1"
        Else
        End If

        If RichTextBox1.Lines.Length < count Then
            RichTextBox1.AppendText(Environment.NewLine & count)
        Else
        End If

        If RichTextBox1.Text.Length = 2 Then
            RichTextBox1.AppendText("2")
        Else
        End If

        Dim pt2 As Point
        SendMessage(RichTextBox2.Handle, EM_GETSCROLLPOS, 0, pt2)
        SendMessage(RichTextBox1.Handle, EM_SETSCROLLPOS, 0, New Point(0, pt2.Y))
    End Sub


    Private Sub RichTextBox2_VScroll(ByVal sender As Object, ByVal e As System.EventArgs) Handles RichTextBox2.VScroll
        Dim pt2 As Point
        SendMessage(RichTextBox2.Handle, EM_GETSCROLLPOS, 0, pt2)
        SendMessage(RichTextBox1.Handle, EM_SETSCROLLPOS, 0, New Point(0, pt2.Y))
    End Sub


    Private Sub Form1_Resize(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Resize
        Dim pt2 As Point
        SendMessage(RichTextBox2.Handle, EM_GETSCROLLPOS, 0, pt2)
        SendMessage(RichTextBox1.Handle, EM_SETSCROLLPOS, 0, New Point(0, pt2.Y))
    End Sub


End Class

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The zipped project below uses an image divider separating the richtextboxes.