private void cmbTimTheo_SelectedIndexChanged(object sender, EventArgs e)
        {
            AutoCompleteStringCollection auto = new AutoCompleteStringCollection();
            DataTable dt = new DataTable();

            dt = HoaDon_BLL.LayMaHoaDon();

            foreach (DataRow dr in dt.Rows)
            {
                if (cmbTimTheo.SelectedIndex == 0)
                {
                    auto.Add(dr[0].ToString());
                }
            }
            txtTuKhoa.AutoCompleteSource       = AutoCompleteSource.CustomSource;
            txtTuKhoa.AutoCompleteMode         = AutoCompleteMode.SuggestAppend;
            txtTuKhoa.AutoCompleteCustomSource = auto;
        }