How to use ModalPopUpExtender On Edit Button in Griedview?

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C#

I want to use ModalPopupExtender Control in GridView
if i click on edit button popup should come but my code is not working

<form id="form1" runat="server">
    <div>
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>
        <asp:GridView ID="GridView1" runat="server" AllowPaging="True" PageSize="15"
            onselectedindexchanged="GridView1_SelectedIndexChanged" AutoGenerateColumns="false">
            <Columns>
                <asp:TemplateField HeaderText="Edit">
                    <ItemTemplate>
                        <asp:ImageButton ID="btnEdit" runat="server"  Height="20px" Width="20px" ImageUrl="~/images.jpg" OnClick="btnEdit_Click" />
                    </ItemTemplate>
                </asp:TemplateField>
                <asp:BoundField DataField="SalID" HeaderText="Id" />
                <asp:BoundField DataField="SalVal" HeaderText="Salary" />
            </Columns>
        </asp:GridView>
        <asp:Button ID="btnedit" runat="server" style="display:none" />
        <asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" TargetControlID="btnedit" PopupControlID="popup" CancelControlID="btnCancel" BackgroundCssClass="modalBackground">
        </asp:ModalPopupExtender>
        <asp:Panel ID="popup" runat="server" CssClass="popupPnl">
            <table width="100%" style="border:Solid 3px #D55500; width:100%; height:100%" cellpadding="0" cellspacing="0">
<tr style="background-color:#D55500">
<td colspan="2" style=" height:10%; color:White; font-weight:bold; font-size:larger" align="center">User Details</td>
</tr>
<tr>
<td align="right" style=" width:45%">
UserId:
</td>
<td>
<asp:Label ID="lblID" runat="server"></asp:Label>
</td>
</tr>
<tr>
<td align="right">
UserName:
</td>
<td>
<asp:Label ID="lblusername" runat="server"></asp:Label>
</td>
</tr>
<tr>
<td align="right">
SalaryId
</td>
<td>
<asp:TextBox ID="txtsalID" runat="server"/>
</td>
</tr>
<tr>
<td align="right">
SalName:
</td>
<td>
<asp:TextBox ID="txtSalName" runat="server"/>
</td>
</tr>

<tr>
<td>
</td>
<td>
<asp:Button ID="btnUpdate" CommandName="Update" runat="server" Text="Update" />
<asp:Button ID="btnCancel" runat="server" Text="Cancel" />
</td>
</tr>
</table>
        </asp:Panel>
    </div>
    </form>

protected void Page_Load(object sender, EventArgs e)
{
if (!IsPostBack)
{
SqlConnection con = new SqlConnection();
con.ConnectionString = ConfigurationManager.ConnectionStrings["GridexCon"].ConnectionString;
SqlCommand cmd = new SqlCommand("Select * from salary", con);
DataSet ds = new DataSet();
SqlDataAdapter da = new SqlDataAdapter(cmd);
da.Fill(ds);
GridView1.DataSource = ds;
GridView1.DataBind();
}

}
protected void GridView1_SelectedIndexChanged(object sender, EventArgs e)
{

}
protected void btnEdit_Click(object sender, ImageClickEventArgs e)
{
ImageButton btndetails = sender as ImageButton;
GridViewRow gvrows = (GridViewRow)btndetails.NamingContainer;
this.ModalPopupExtender1.Show();
}