<?xml version="1.0" encoding="utf-8"?>
<Links>
  <Link>
    <List Id="56040516-e147-4929-957b-9ea771e09abf"> 65014720 -a4c0- 4994 -995f-1f29e5718a9e</List>
  </Link>
</Links>

XPathDocument xml = new XPathDocument(my.xml);
 XPathNavigator navigator = xml.CreateNavigator();
 string id = "56040516-e147-4929-957b-9ea771e09abf";
 XPathNodeIterator iterator = navigator.Select("//List[@Id=" + id + "]");
 while (iterator.MoveNext())
 {
      Console.WriteLine(iterator.Current.Value);
 }