How to get selected datavalueField in c#

[Guru]

I need to get all the selected items from RadDropDownTree and change it to XML format

<telerik:RadDropDownTree ID="rddt" runat="server" CheckBoxes="TriState" DefaultMessage="Select documents" DataFieldID="ID" DataFieldParentID="parentID" DataTextField="name" DataValueField="ID" DataSourceID="ds2">

                    <ButtonSettings ShowCheckAll="true" ShowClear="true" />

                    <Localization CheckAll="Select All Resources" />                  

</telerik:RadDropDownTree>

How can I get the DataFieldID and DataFieldParentID value from dropdowntree selected value?

var xdoc = new XDocument(
                           new XElement("Root",
                                  new XElement("Resource",
                                          rddt.SelectedValue.Select(x => new XElement("ID", x)),
                                          rddt.SelectedValue.Select(x=>new XElement("ParentID",x)))));

Thanks,