save value on XML one time per process

Dear All,
i am creating XML file, and saving some values on it.
if user A trying to save some values on that xml, and user B came on the same time and trying to save another value,
i need to keep user B on hold untill user A finished saving, the user B process starts.
how can i do this?
XmlNode node = xmlDoc.SelectSingleNode("Orders/LastOrder/id"); if (node != null && node.InnerText != id) { node.InnerText = id; // handle coincident required here xmlDoc.Save(Directoy + "Orders.xml"); return true; }