procedure SelectLine(cm: TCustomMemo; index: Integer);
begin
  with cm do
    if index < 0 then
      SelStart := 0 else
    if index < Lines.Count then
    begin
      SelStart := Perform(EM_LINEINDEX, index, 0);
      SelLength := Perform(EM_LINEINDEX, Succ(index), 0) - SelStart;
    end else
      SelStart := MaxInt;
end;

// SelectLine(RichEdit, Pred(RichEdit.Lines.Count));