select的onchange事件

    <script type="text/javascript">
        function selectChange()
        {
            var ddlList = document.getElementById("ddlrole");
            var item = ddlList.options[ddlList.selectedIndex].value;
           
        }
    </script>

dropdownList解析到前台后台的Select控件选择事件

<asp:DropDownList ID="ddlrole" runat="server" onchange="selectChange()">

</asp:DropDownList>