Repeater后台取前台textbox值

<asp:HiddenField ID="hfValues" runat="server" /> 

 <table class="li_Tb">
            <tr>
                <td width="5%">
                    名称
                </td>
                <td width="8%">
                    原有顺序
                </td>
                <td width="5%">
                   更改顺序
                </td>
                <td width="5%">
                    更改
                </td>
            </tr>

<asp:Repeater ID="rptProductList" runat="server" OnItemCommand="rptProductList_ItemCommand">

                <ItemTemplate>
                    <tr>
                        <td>
                            <%#Eval("Name_CH")%>
                        </td>
                        <td>
                            <%# Eval("SortOrder")%>
                        </td>
                        <td>
                            <%# "<input type='text'  id='txtRanking" + Eval("SeqNo") + "' />"%>
                        </td>
                        <td>
                            <asp:Button ID="btnRanking" OnClientClick='<%#Eval("SeqNo","javascript:return assignment({0});")%>'
                                runat="server" Text="修改" CommandArgument='<%#Eval("SeqNo")%>' CommandName="Edit" />
                        </td>
                    </tr>
                </ItemTemplate>

            </asp:Repeater>

  </table>

<script type="text/javascript">

         var hfValues = document.getElementById("ContentPlace_hfValues");
         
         function assignment(id) {
             var txtRanking = document.getElementById("txtRanking" + id);
             if (txtRanking.value != "") {
                 hfValues.value = txtRanking.value;
                 return true;
             } else {
                 alert("更改顺序不能为空！");
                 return false;
             }
             return false;
         }

    </script>