ViewState有关问题 高手指教

<script type="text/javascript" language="javascript">
        function txt_onkeydown() {
            if (event.keyCode == 13) {
                <%bandRpt_ISDetail();%>

            }
        }
    </script>

protected void Page_Load(object sender, EventArgs e)
        {
            if (!IsPostBack)
            {
                                       DataTable dtInS = new DataTable();
                        dtInS.Columns.Add(new DataColumn("stockName"));
                        dtInS.Columns.Add(new DataColumn("scanNo"));
                        dtInS.Columns.Add(new DataColumn("Num"));
                        dtInS.Columns.Add(new DataColumn("Uint"));

                }
            }
        }
 public void bandRpt_ISDetail()
        {
            DataTable dt = (DataTable)ViewState["dtInS"];
            string scanNo = txt_scan_No.Text;
            if (!string.IsNullOrEmpty(scanNo))
            {
                DataRow dr = dt.NewRow();
                dr["stockName"] = hf_stockName.Value;
                dr["Uint"] = "台";
                dr["Num"] = "1";
                dr["scanNo"] = scanNo;
                dt.Rows.Add(dr);
                
                ViewState["dtInS"] = dt;

                rpt_ISDetail.DataSource = dt;
                rpt_ISDetail.DataBind();
            }
        }