function CountMemoLineHeights(Memo: TMemo): Integer;
var
DC: HDC;
SaveFont: HFont;
Size: TSize;
I: Integer;
begin
DC:= GetDC(Memo.Handle);
SaveFont:= SelectObject(DC, Memo.Font.Handle);
Size.cX := 0;
Size.cY := 0;
// I have not noticed difference in actual line heights for TMemo,
// so the next line should work OK
Windows.GetTextExtentPoint32(DC, 'W', 1, Size);
// BTW next (commented) line returns Size.cY = 0 for empty line (Memo.Lines[I] = '')
// Windows.GetTextExtentPoint32(DC, Memo.Lines[I], Length(Memo.Lines[I]), Size);
Result:= Memo.Lines.Count * Size.cY;
SelectObject(DC, SaveFont);
ReleaseDC(Memo.Handle, DC);
end;