One of the question asked often in the newsgroups is how to prevent the user from editing certain lines in a textbox. We've seen ridiculous solutions, like counting vbCrLf in the textbox, etc...

The right way to do it is to call a Windows API function to find out what line the cursor is located, then apply editing rules. Check out the example below.

• Start New Project (Standard EXE)
• Add 2 textboxes to the form
• Leave the name of the first textbox that will contain the text as is, but set the Multiline property to True and resize it to the height of the form
• Call the second textbox txtPrevent

'declarations
Private Const EM_LINEFROMCHAR = &HC9
Private Declare Function SendMessage Lib "user32" Alias _
    "SendMessageA" (ByVal hwnd As Long, ByVal wMsg As Long, _
    ByVal wParam As Long, lParam As Any) As Long

Private Sub Text1_KeyPress(KeyAscii As Integer)
    Dim iCurrentLineNumber As Long
    Dim sForbiddenLines() As String
    Dim x As Long
    
    'function call
    iCurrentLineNumber = SendMessage(Text1.hwnd, EM_LINEFROMCHAR, Text1.SelStart, 0&)

    'increment, because the API counts lines starting from zero
    iCurrentLineNumber = iCurrentLineNumber + 1

    'split the forbidden string into an array
    sForbiddenLines = Split(txtPrevent.Text, ",")
    
    'loop through the forbidden lines and check what lines are verbotten
    For x = 0 To UBound(sForbiddenLines)
        If iCurrentLineNumber = Val(sForbiddenLines(x)) Then
            'This line is forbidden to edit, cancel the entry
            KeyAscii = 0
            Exit For
        End If
    Next
End Sub

• Paste a bunch of text into Text1 and enter what lines are forbidden to edit (separate them by a comma)