private void btnSua_Click(object sender, EventArgs e)
        {
            try
            {
                HoaDonNhapNL hd = db.HoaDonNhapNLs.Find(int.Parse(txtMaHDN.Text));
                hd.MaPDNL  = int.Parse(txtMaPhieuDatNL.Text);
                hd.NgayLap = dateTimePicker1.Value;
                hd.MaNV    = int.Parse(comboBoxMaNV.Text);

                SqlParameter[] idparam =
                {
                    new SqlParameter {
                        ParameterName = "MaHDN", Value = int.Parse(txtMaHDN.Text)
                    },
                    new SqlParameter {
                        ParameterName = "MaPDNL", Value = int.Parse(txtMaPhieuDatNL.Text)
                    },
                    new SqlParameter {
                        ParameterName = "NgayLap", Value = dateTimePicker1.Value
                    },
                    new SqlParameter {
                        ParameterName = "MaNV", Value = int.Parse(comboBoxMaNV.Text)
                    },
                };
                db.Database.ExecuteSqlCommand("suahdn @mahdn ,@mapdnl , @ngaylap , @manv ", idparam);
                db.SaveChanges();
                MessageBox.Show("Sua thanh cong");
                listHoaDon.Items.Clear();
                show();
            }
            catch (Exception ex)
            {
                MessageBox.Show("Sua that bai. Chi tiet loi: " + ex.Message);
            }
        }
        private void btnThem_Click(object sender, EventArgs e)
        {
            try
            {
                HoaDonNhapNL ThemHD = new HoaDonNhapNL()
                {
                    MaHDN   = int.Parse(txtMaHDN.Text),
                    MaPDNL  = int.Parse(txtMaPhieuDatNL.Text),
                    NgayLap = dateTimePicker1.Value,
                    MaNV    = int.Parse(comboBox1.SelectedValue.ToString())
                };
                db.HoaDonNhapNLs.Add(ThemHD);
                db.SaveChanges();

                ListViewItem list = new ListViewItem(ThemHD.MaHDN.ToString());
                list.SubItems.Add(ThemHD.MaPDNL.ToString());
                list.SubItems.Add(ThemHD.NgayLap.ToString());
                list.SubItems.Add(ThemHD.MaHDN.ToString());
                list.SubItems.Add(ThemHD.MaNV.ToString());

                listHoaDon.Items.Add(list);
            }
            catch (Exception a)
            {
                MessageBox.Show("Có lỗi rồi!!!" + a.Message);
            }
        }