Get CheeckBox changed value in jQuery Modal Dialog inside UpdatePanel after PostBack in ASP.Net

 i have gridview inside jquery dialog. inside gridview i have checkbox. but while i submit gridview data on button click ckeckbox changed value not shown in cs file. it shows previous value. all code is inside update panel

<asp:Content ID="Content1" ContentPlaceHolderID="head" runat="server">
    <script type="text/javascript">
        function Check_Click(objRef) {
            var row = objRef.parentNode.parentNode;
            var GridView = row.parentNode;
            var inputList = GridView.getElementsByTagName("input");
            for (var i = 0; i < inputList.length; i++) {
                var headerCheckBox = inputList[0];
                var checked = true;
                if (inputList[i].type == "checkbox" && inputList[i] != headerCheckBox) {
                    if (!inputList[i].checked) {
                        checked = false;
                        break;
                    }
                }
            }
            headerCheckBox.checked = checked;
        }
        function checkAll(objRef) {
            var GridView = objRef.parentNode.parentNode.parentNode;
            var inputList = GridView.getElementsByTagName("input");
            for (var i = 0; i < inputList.length; i++) {
                var row = inputList[i].parentNode.parentNode;
                if (inputList[i].type == "checkbox" && objRef != inputList[i]) {
                    if (objRef.checked) {
                        inputList[i].checked = true;
                    }
                    else {
                        inputList[i].checked = false;
                    }
                }
            }
            document.getElementById('<%=Button1.ClientID%>').click();
        }
        function gridmodalpopup() {
            $("#dialog2").dialog({
                title: "Details",
                width: 700,
                height: 410,
                modal: true,
                closeOnEscape: false,
 
                buttons: [{
                    text: "Confirm",
 
                    click: function () {
                        document.getElementById('<%=btnsave.ClientID%>').click();
                        $(this).dialog('destroy').remove();
                    }
                }, {
                    text: "Close",
                    click: function () {
                        $(this).dialog('destroy').remove();
                    }
                }]
 
            });
        };
 
    </script>
</asp:Content>
<asp:Content ID="Content2" ContentPlaceHolderID="ContentPlaceHolder1" runat="server">
     <asp:ScriptManager runat="server" ID="ScriptManager1" />
     <asp:UpdatePanel ID="updatePanel1" runat="server">
        <ContentTemplate>
      <div id="dialog2" style="display: none" class="modals modtable special">
                <div class="conte" style="display: inline-block;">
                    <asp:GridView ID="Gridview2" runat="server" AutoGenerateColumns="false">
                        <Columns>
                            <asp:TemplateField>
                                <ItemTemplate>
                                  <asp:Label runat="server" ID="lblpackno" Text='<%#Eval("packno") %>' Visible="false"></asp:Label>                              
                                </ItemTemplate>
                            </asp:TemplateField>
                            
                            <asp:TemplateField>
                                <HeaderTemplate>
                                    <asp:CheckBox ID="CheckBox1" runat="server" onclick="checkAll(this);"/></th>
                                                         
                                </HeaderTemplate>
                                <ItemTemplate>
                                   <asp:CheckBox ID="CheckBox2" runat="server" onclick="Check_Click(this)" /></td>
                                </ItemTemplate>
                            </asp:TemplateField>
                        </Columns> 
                    </asp:GridView>
                    <asp:Button ID="btnsave" runat="server" Text="btnsave" OnClick="btn_Click" UseSubmitBehavior="false" />
                </div>
            </div>
          </ContentTemplate>
        <Triggers>
          <asp:AsyncPostBackTrigger ControlID="btnsave" />
            <asp:PostBackTrigger ControlID="GridView2" />
        </Triggers>
    </asp:UpdatePanel>
</asp:Content>

This is my code. on btnsave click i can not get checked value of checkbox.