ModalPopup div not displaying - asp.net

Question

I have a ModalPopup window in my asp.net application, that I want to display when a Listview control item is clicked.
<div id="ModalPopup" style="visibility:hidden" runat="server">
<div style="position: absolute; width: 100%; height: 100%; z-index: 10002; background-color: Gray; filter: alpha(opacity=70); opacity: 0.7;">
</div>
<table style="position: absolute; width: 100%; height: 100%; z-index: 10003;">
<tr>
<td align="center" valign="middle">
<div style="color: Black; font-weight: bolder; background-color: White; padding: 15px; width: 200px;">
<asp:Image ID="Image4" runat="server" ImageUrl="~/Images/ajax-loader.gif" />...Processing....
</div>
</td>
</tr>
</table>
</div>
However, in my RadListView1_SelectedIndexChanged event, my code is: ModalPopup.Attributes.Add("style", "visibility:visible"); but the modal popup does not display.
How can I display it when a ListView item is selected?

Answer 1

Since you've already defined your ModalPopup div as a server control (e.g. runat=server) and you're trying to decide if to show it or not in codebehind - just use the Visible property...
<div id="ModalPopup" Visible="false" runat="server">
....
</div>
And in your RadListView1_SelectedIndexChanged event in code behind just change Visible to true:
protected void RadListView1_SelectedIndexChanged()
{
ModalPopup.Visible = true;
}
and if you insist on changing the visibility attribute itself, you can use RegisterStartupScript like this:
protected void RadListView1_SelectedIndexChanged()
{
ClientScript.RegisterStartupScript(this.GetType(), "ShowPopup", "document.getElementById('" + ModalPopup.ClientID + "').style.visibility = 'visible';", true);
}