ASP.NET

<ajax:modalpopupextender id="MyExtender" behaviorid="MyExtenderBehaviorId" targetcontrolid="PostBack" popupcontrolid="test2" runat="server" />

<script type="text/javascript">

  var loadInitialization = function()
  {
    var extender = $find('MyExtenderBehaviorId');
    var handler = Function.createDelegate(extender, function(sender, args) { args.set_cancel(true); });
    extender.add_showing(handler);
  }
  
  Sys.Application.add_load(loadInitialization);

</script>

var h = $find("ModalPopupExtenderID");
h.dispose();

<asp:TemplateField ItemStyle-Width="16px" >
    <ItemTemplate>
        <asp:ImageButton runat="server" ID="imgAnnul" ImageUrl="~/Images/x16.png" AlternateText="Reservering annuleren" CommandName="AnnulGrid" OnCommand="Grid_OnCommand" />
    </ItemTemplate>
</asp:TemplateField>

protected void Grid_OnCommand(Object sender, CommandEventArgs e)
        {
            ImageButton btn = (ImageButton)sender;
            GridViewRow row = (GridViewRow)btn.NamingContainer;
            if (e.CommandName.Equals("AnnulGrid"))
            {
                grdReservation.SelectedIndex = row.RowIndex;
                 // youd code here

                grdReservation.SelectedIndex = -1;
            }
            LoadReservation(grdReservation.PageIndex);
        }

DataKeyNames="ID"

ImageButton btn = (ImageButton)sender;
GridViewRow row = (GridViewRow)btn.NamingContainer;
if (e.CommandName.Equals("AnnulGrid"))
{
   grdReservation.SelectedIndex = row.RowIndex;
   Int64 waardeID = Convert.ToInt64(grdReservation.SelectedValue);

   grdReservation.SelectedIndex = -1;
}