//đã xong
private void tsmiSell_Click(object sender, EventArgs e)
{
int _maHoaDon = _MaHoaDonHienTai++;
Staff _nhanVien = StaffDAO.Instance.LayNhanVienTheoMa(_Account.MaNV);
FSell _fSell = new FSell(_maHoaDon, _nhanVien, this);
_fSell.Show();
}