title: Converting XDocument to XmlDocument and vice versa
body: <p>It's a very simple problem that I have. I use XDocument to generate an XML file. I then want to return it as a XmlDocument class.
  And I have an XmlDocument variable which I need to convert back to XDocument to append more nodes.</p>
  
  <p>So, what is the <em>most efficient</em> method to convert XML between XDocument and XmlDocument? (Without using any temporary storage in a file.)</p>