hi,when i set datasource to webdropdown,the dropdownitem's text display looks good ,but when i  raise client SelectionChanged events,the sender.get_selectedItem().get_value() always return empty, how can i get the selecteditem's value.

//json  

var faulData = [{ "SMSFaultCodeID": "1", "FaultCodeName": "error_A"}];

        function onDataBind(sender, evntArgs) {
            evntArgs.set_cancel(true);
            sender._pi.show(sender);
            $.ajax({
                url: "products",
                contentType: "text/json; charset=utf-8",
                dataType: "json",
                success: function (datatsource, st) {
                    //dataSource = datatsource.data.results;
                     dataSource=faulData ;

                    sender.set_dataSource(dataSource);
                    sender.dataBind();
                    sender._pi.hide(sender);
                },
                error: function () {
                    alert("Error with AJAX callback");
                }
            });
        }

        function onSelectionChanged(sender, evntArgs) {
            var selectvalue= sender.get_selectedItem().get_value().toString();
        }

 <ig:WebDropDown ID="myDropDown" runat="server" ClientIDMode="Static" Width="200px"
        EnableClientRendering="true" DropDownContainerMaxHeight="200" EnableDropDownAsChild="false"
        AutoPostBackFlags-SelectionChanged="Off" TextField="FaultCodeName" ValueField="SMSFaultCodeID"
        AutoPostBack="false" ClientEvents-DataBinding="onDataBind" DisplayMode="DropDown">
        <ClientEvents SelectionChanged="onSelectionChanged" />
    </ig:WebDropDown>