private void addRoomButton_Click(object sender, EventArgs e)
{
string maPhong = cbMaPhong.SelectedValue.ToString();
string tenPhong = phongBLL.loadTenPhong(cbMaPhong.SelectedValue.ToString());
DateTime ngayThue = phongBLL.loadNgayThuePhong(cbMaPhong.SelectedValue.ToString());
DateTime ngayHienTai = DateTime.Now;
double songayThue = (ngayHienTai.Date - ngayThue.Date).TotalDays;
double donGia = phongBLL.loadDonGia(cbMaPhong.SelectedValue.ToString());
double phuThu = phongBLL.loadPhuThuNuocNgoai(cbMaPhong.SelectedValue.ToString());
double tiLePhuThu = phongBLL.loadTiLePhuThuNuocNgoai(cbMaPhong.SelectedValue.ToString());
double thanhTien = donGia * songayThue * phuThu * tiLePhuThu;
string[] row = new string[] { (index + 1).ToString(), maPhong, tenPhong, ngayThue.ToString("yyyy-MM-dd"), songayThue.ToString(), donGia.ToString(), thanhTien.ToString() };
dgvDanhSachPhongThanhToan.Rows.Add(row);
txtTongGiaTri.Text = (double.Parse(txtTongGiaTri.Text) + thanhTien).ToString();
index++;
}