Cannot get the ModalPopupExtender to run via code

I cannnot get the ModalPopupExtender to work via code. If I set the ModalPopupExtender's property TargetControlID="btn", it works. When I set TargetControlID="HiddenField1" to use the mpe.show() method in the code, it does not work (the dgvResults_RowCommand event fires fine, I have tested it). I have gone through many sites looking at code, and cannot figure out whats wrong. Thanks in advance for any help

<%@ Page Title="Home Page" Language="C#" MasterPageFile="~/Site.master" AutoEventWireup="true"
    CodeBehind="Default.aspx.cs" Inherits="Michlala._Default" %>

<%@ Register Assembly="AjaxControlToolkit" Namespace="AjaxControlToolkit" TagPrefix="asp" %>

<asp:Content ID="HeaderContent" runat="server" ContentPlaceHolderID="HeadContent">
    <style type="text/css">
        .modalPopup 
        {               
            background-color: Black;
            filter: alpha(opacity=80);
            opacity: 0.8;
            z-index: 10000;             
        }
        .style1
        {
            width: 100%;
        }
        .style5
        {
            text-align: center;
        }
    </style>
</asp:Content>
<asp:Content ID="BodyContent" runat="server" ContentPlaceHolderID="MainContent">
    <h1> Student Feedback</h1>
        <asp:ScriptManager ID="ScriptManager1" runat="server">
        </asp:ScriptManager>       
    <br />
    <asp:TextBox ID="txtSID" runat="server" Width="61px" Height="25px"></asp:TextBox>  
    &nbsp;&nbsp;&nbsp;&nbsp;  
    <asp:Button ID="btnSID" runat="server" onclick="Button1_Click" 
        Text="Submit Student ID" />

    <asp:GridView ID="dgvResults" runat="server" Visible="False" 
        onrowcommand="dgvResults_RowCommand">
        <Columns>
            <asp:TemplateField>
                <EditItemTemplate>
                    <asp:TextBox ID="TextBox1" runat="server"></asp:TextBox>                        
                </EditItemTemplate>
                <ItemTemplate>
                    <asp:Button ID="btnFeedback" runat="server" onclick="Button1_Click1" 
                        Text="Add FeedBack" CommandName="InsertFeedback" CommandArgument="<%# ((GridViewRow) Container).RowIndex %>"  />
                       <asp:HiddenField ID="HiddenField1" runat="server" />
                    <asp:ModalPopupExtender ID="ModalPopupExtender1" runat="server" 
                         TargetControlID="HiddenField1"
                         PopupControlID="pnlModalPanel"
                         CancelControlI开发者_StackOverflow社区D="btnCancel"
                         BackgroundCssClass="modalPopup" > 
                    </asp:ModalPopupExtender>
                </ItemTemplate>
            </asp:TemplateField>
        </Columns>
    </asp:GridView>
    <asp:Panel ID="pnlModalPanel" runat="server"  style="display:none" Width="481px" >
        <table class="style1">
            <tr>
                <td>
                    Course
                </td>
                <td>
                    Semester
                </td>
                <td class="style5">
                    Teacher Assitant
                </td>
                <td class="style5">
                    Lecturer
                </td>
            </tr>
            <tr>
                <td>
                    &nbsp
                    <asp:TextBox ID="txtCourse" runat="server"></asp:TextBox>
                </td>
                <td>
                    &nbsp
                </td>
                <td>
                    &nbsp
                </td>
                <td>
                    &nbsp
                </td>
            </tr>
            <tr>
                <td colspan="2" style="text-align: center">
                    <asp:Button ID="btnSubmit" runat="server" Text="Submit" />
                    &nbsp;
                </td>
                <td colspan="2">
                    &nbsp
                    <asp:Button ID="btnCancel" runat="server" Text="Cancel" />
                </td>
            </tr>
        </table>    
    </asp:Panel>
    <asp:DropDownList ID="ddlSemester" runat="server">
    </asp:DropDownList>
    <asp:Label ID="lblRes" runat="server" Text="Label"></asp:Label>        
    <br />        
</asp:Content>

in the server side code

protected void dgvResults_RowCommand(object sender, GridViewCommandEventArgs e)
{
    ModalPopupExtender mpe = new ModalPopupExtender();
    mpe.Show();
}

You need to fin the ModalPopupExtender in the selected row and then call it with the name:

if (e.CommandName.Equals("InsertFeedback"))
{
    int index = Convert.ToInt32(e.CommandArgument);

    // Retrieve the row that contains the button clicked 
    // by the user from the Rows collection.
    GridViewRow row = dgvResults.Rows[index];

    ModalPopupExtender mpe = (ModalPopupExtender)row.FindControl("ModalPopupExtender1");
    mpe.Show();
}