title: How to get ALL available properties for a DirectoryEntry
body: <p>I have a reference to a user object, the <code>Properties</code> collection of this object will only contain properties that have values set but I need to check if a property (by name) exists for this object - I guess this would come from the schema.</p>
  
  <p>I have looked at <code>deUser.SchemaEntry</code>, but I can't find any useful property info from this object.</p>
  
  <p>Any ideas?</p>
  
  <pre><code>DirectoryEntry deUser = new DirectoryEntry(path);
  
  foreach (var prop in deUser.Properties)
  {
     //if user.Properties["company"] is not set on this user then 
     //it will not be available here although 'company' is 
     //a property defined for the user class
  }
  
  //How do I get to the list of all available properties using 
  //deUserSchema as below
  DirectoryEntry deUserSchema = deUser.SchemaEntry();
  </code></pre>