Option Explicit

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 Const EM_GETLINECOUNT = &HBA
Private Const EM_LINEINDEX = &HBB
Private Const EM_LINELENGTH = &HC1
Private Const EM_SETSEL = &HB1

Private Sub optSelection_Click(Index As Integer)
  Dim lngLineIndex As Long
  Dim lngLineCount As Long

  lngLineIndex = CLng(txtLineIndex.Text)
  SelectTextBoxLine Text1, lngLineIndex, Index
End Sub

Private Sub SelectTextBoxLine(ByVal TxtBox As TextBox, _
    ByVal LineIndex As Long, ByVal Index As Integer)

  Dim lngFirstCharPos As Long
  Dim lngLineLength As Long

  If LineIndex <= 0 Then Exit Sub

    With TxtBox
      'Position des ersten Zeichens in der Zeile ermitteln:
      lngFirstCharPos = SendMessage(.hwnd, EM_LINEINDEX, _
            LineIndex - 1, ByVal 0&)

      'Anzahl Zeichen in der Zeile ermitteln:
      lngLineLength = SendMessage(.hwnd, EM_LINELENGTH, _
            lngFirstCharPos, ByVal 0&)

      Select Case Index
        Case 0
          'Die gesamte Zeile markieren:
          .SelStart = lngFirstCharPos
          .SelLength = lngLineLength

        Case 1
          'Cursor ans Ende der Zeile setzen:
          .SelStart = lngFirstCharPos + lngLineLength

        Case 2
          'Cursor an den Anfang der Zeile setzen:
          .SelStart = lngFirstCharPos
      End Select

      .SetFocus
    End With
End Sub