public ChiTietHDBanBLL()
 {
     db = new QuanLyBanHangDataContext();
 }
 public HoaDonBLL()
 {
     db = new QuanLyBanHangDataContext();
 }
 public KhachHangBLL()
 {
     db = new QuanLyBanHangDataContext();
 }
 public NhanVienBLL()
 {
     db = new QuanLyBanHangDataContext();
 }