title: UserPrincipal GetUnderlyingObject: properties missing
body: <p>I'm trying to load the attribute <code>physicalDeliveryOfficeName</code> from a <code>DirectoryEntry</code> which is returned by the GetUnderlyingObject method of a UserPrincipal instance:</p>
  
  <pre><code>DirectoryEntry directoryEntry = principal.GetUnderlyingObject() as DirectoryEntry;
  </code></pre>
  
  <p>which means that the following statement returns false:</p>
  
  <pre><code>directoryEntry.Properties.Contains("physicalDeliveryOfficeName");
  </code></pre>
  
  <p>I know that this property can be loaded by adding the name to the <code>StringCollection</code> <code>DirectorySearcher.PropertiesToLoad</code> when using said <code>DirectorySearcher</code>.</p>
  
  <p>My questions are, why doesn't the <code>DirectoryEntry</code> returned by the method <code>GetUnderlyingObject</code> contain all properties? And how can I load this property without using a <code>DirectorySearcher</code>?</p>