protected void cmbNhomHang_SelectedIndexChanged1(object sender, EventArgs e)
        {
            if (Int32.Parse(cmbNhomHang.Value + "") == -1)
            {
                dataHangHoa dtHH = new dataHangHoa();
                DataTable   daHH = dtHH.LayDanhSachHangHoa_Ten();
                daHH.Rows.Add(-1, "Tất cả hàng hóa");

                cmbHangHoa.DataSource = daHH;
                cmbHangHoa.TextField  = "TenHangHoa";
                cmbHangHoa.ValueField = "ID";
                cmbHangHoa.DataBind();
                cmbHangHoa.SelectedIndex = daHH.Rows.Count;
            }
            else
            {
                dataHangHoa dtHH = new dataHangHoa();
                DataTable   daHH = dtHH.LayDanhSachHangHoa_IDNhom(cmbNhomHang.Value + "");
                daHH.Rows.Add(-1, "Tất cả hàng hóa");

                cmbHangHoa.DataSource = daHH;
                cmbHangHoa.TextField  = "TenHangHoa";
                cmbHangHoa.ValueField = "ID";
                cmbHangHoa.DataBind();
                cmbHangHoa.SelectedIndex = daHH.Rows.Count;
            }
        }
        protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                dataNhomHang dtNhomH = new dataNhomHang();
                DataTable    daNhomH = dtNhomH.getDanhSachNhomHang2();
                daNhomH.Rows.Add(-1, "Tất cả nhóm hàng");

                cmbNhomHang.DataSource = daNhomH;
                cmbNhomHang.TextField  = "TenNhomHang";
                cmbNhomHang.ValueField = "ID";
                cmbNhomHang.DataBind();
                cmbNhomHang.SelectedIndex = daNhomH.Rows.Count;

                dataHangHoa dtHH = new dataHangHoa();
                DataTable   daHH = dtHH.LayDanhSachHangHoa_Ten();
                daHH.Rows.Add(-1, "Tất cả hàng hóa");

                cmbHangHoa.DataSource = daHH;
                cmbHangHoa.TextField  = "TenHangHoa";
                cmbHangHoa.ValueField = "ID";
                cmbHangHoa.DataBind();
                cmbHangHoa.SelectedIndex = daHH.Rows.Count;
            }
        }