Check this sample, I have a Modal Popup with a DropDown with AutoPostback
<html xmlns="http://www.w3.org/1999/xhtml">
<head runat="server">
<title></title>
<style type="text/css">
.modalBackground
{
background-color: Black;
filter: alpha(opacity=90);
opacity: 0.8;
}
.modalPopup
{
background-color: #FFFFFF;
border-width: 3px;
border-style: solid;
border-color: black;
padding-top: 10px;
padding-left: 10px;
width: 300px;
height: 140px;
}
</style>
</head>
<body style = "height:3000px">
<form id="form1" runat="server">
<cc1:ToolkitScriptManager runat = "server"></cc1:ToolkitScriptManager>
<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">
Do you like this product?
<asp:DropDownList ID="DropDownList1" runat="server" AutoPostBack = "true" OnSelectedIndexChanged = "OnSelectedIndexChanged">
<asp:ListItem Text = "Please Select" Value = "0"></asp:ListItem>
<asp:ListItem Text = "Yes" Value = "1"></asp:ListItem>
<asp:ListItem Text = "No" Value = "2"></asp:ListItem>
</asp:DropDownList>
<br />
<asp:Button ID="btnClose" runat="server" Text="Close" />
</asp:Panel>
<!-- ModalPopupExtender -->
</form>
</body>
</html>
protected void OnSelectedIndexChanged(object sender, EventArgs e)
{
mp1.Show();
}
