title: How to implement: Express (ISO 10303-21) TYPE typename = SELECT in C#
body: <p>At the moment I am writing a really basic Early Binding for IFC, that is mapped following the Express-standard described in ISO 10303-21. 
  As I've only just started coding two months ago, everything I do is fairly basic still. However, I can already create all the entities and types with all their properties within the given schema. I'm also able to map that to a file, following the ISO. 
  There are still a problem I'm coming across:</p>
  
  <ul>
  <li><p>in the schema, there are types that are SELECT types, looking like the follwoing:</p>
  
  <pre><code>TYPE IfcGridPlacementDirectionSelect = SELECT
  (IfcDirection
  ,IfcVirtualGridIntersection);
  END_TYPE; 
  </code></pre>
  
  <p>IfcDirection and IfcVirtualGridIntersection are both entities or types themselves ( I create a class for a type and an entity likewise). 
  My TYPE IfcGridPlacementDirectionSelect - class looks something like this:</p>
  
  <pre><code>public class IfcGridPlacementDirectionSelect :IfcBase
  {
     public IfcBase _value { get; set; }
  }
  </code></pre></li>
  </ul>
  
  <p>Is there any way to implement the select structure into csharp so the property-type of _value is either "IfcDirection" or "IfcVirtualGridIntersection" instead of what I'm making it now "IfcBase". So that the SELECT-structure would be represented?</p>