Model popup with list of employees with check box grid

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="cc1" %>
//then add modal popup extender on page
<form id="form1" runat="server">
<asp:ScriptManager ID="ScriptManager1" runat="server">
</asp:ScriptManager>
<asp:Button ID="btnShow" runat="server" Text="Show Modal Popup" />
<!-- ModalPopupExtender -->
<cc1:ModalPopupExtender ID="mp1" runat="server" PopupControlID="Panel1" TargetControlID="btnShow"
    CancelControlID="btnClose" BackgroundCssClass="modalBackground">
</cc1:ModalPopupExtender>
<asp:Panel ID="Panel1" runat="server" CssClass="modalPopup" align="center" style = "display:none">
    This is an ASP.Net AJAX ModalPopupExtender Example<br />
    <asp:Button ID="btnClose" runat="server" Text="Close" />
</asp:Panel>
<!-- ModalPopupExtender -->
</form>

 <asp:ScriptManager ID="dd" runat="server"></asp:ScriptManager>

  <asp:Button ID="Btn1" runat="server" OnClick="Btn1_Click" Text="Show Popup" />

        <asp:Button ID="btnShowPopup" runat="server" style="display:none" />

        <ajax:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="btnShowPopup" PopupControlID="pnlpopup"
CancelControlID="btnCancel" BackgroundCssClass="modalBackground">
</ajax:ModalPopupExtender>
<asp:Panel ID="pnlpopup" runat="server" BackColor="White" Height="269px" Width="400px" style="display:none">


    <asp:GridView ID="gvDetails" runat="server" DataKeyNames="ID"
              AutoGenerateColumns="False" OnSelectedIndexChanged="gvDetails_SelectedIndexChanged"> 
              
<Columns>
<asp:TemplateField HeaderText="First Name" SortExpression="FirstName">
<ItemTemplate>
<asp:Label ID="lblFname" runat="server" Text='<%#Eval("FirstName")%>'/>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Last Name" SortExpression="LastName">
<ItemTemplate>
<asp:Label ID="lblLname" runat="server" Text='<%#Eval("LastName")%>'/>
</ItemTemplate>
</asp:TemplateField>
<asp:TemplateField HeaderText="Location" SortExpression="Location">
<ItemTemplate>
<asp:Label ID="lblLocation" runat="server" Text='<%#Eval("Location")%>'/>
</ItemTemplate>
</asp:TemplateField>
</Columns>
</asp:GridView>

    </asp:Panel>

DataTable dtGrid = new DataTable();
         int id = Convert.ToInt32(gvDetails.DataKeys[0].Values[0]);
         SqlConnection con = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString);
         string strSelect = "SELECT FirstName,LastName,Location FROM Details where Id="+id ;
         SqlCommand cmd = new SqlCommand(strSelect, con);
         SqlDataAdapter dAdapter = new SqlDataAdapter(cmd);
         dAdapter.Fill(dtGrid);
         TxtName.Text = dtGrid.DefaultView[0]["Name"];

<asp:Button ID="btnShowPopup" Text="Show Popup" runat="server" Style="display: none" />
<cc1:ModalPopupExtender ID="mpe" runat="server" TargetControlID="btnShowPopup"
    PopupControlID="pnlPopup" BackgroundCssClass="modalBackground">
</cc1:ModalPopupExtender>
<asp:Panel ID="pnlPopup" runat="server" BackColor="White" Height="100px"
    Width="400px" Style="display: none">
    <table>
        <tr>
            <td>
                <asp:GridView Id="gv" runat="server" autogeneratecolumns="False"
       autogenerateselectbutton="True"
       selectedindex="1"
       onselectedindexchanged="gv_SelectedIndexChanged">
		<Columns>
		</Columns>
		</asp:GridView>
            </td>
        </tr>
        <tr>
            <td>
		<asp:HiddenField Id="hdnId" runat="server"/>
		<asp:Button Id="btnGet" runat="server" Text="Get Record" OnClick="btnGet_OnClick"/>
            </td>
        </tr>
    </table>
</asp:Panel>

  Protected void gv_SelectedIndexChanged(Object sender, EventArgs e)
  {
    GridViewRow row = gv.SelectedRow;
    hdnId.Value= row.Cells[2].Text ;
  }
     Protected void btnGet_OnClick(Object sender, EventArgs e)
     {
            //pass hidden field value to main view
     }