IExportContext.OnElementBegin

/// <summary>
/// Often it is found beneficial to keep a stack of th element(s)
/// currently being processed, so it can be refer to it from other
/// methods of the export context.
/// </summary>
Stack<ElementId> m_elementStack = new Stack<ElementId>();

ElementId CurrentElementId()
{
   return (m_elementStack.Count > 0) ? m_elementStack.Peek() : ElementId.InvalidElementId;
}

/// <summary>
/// Method that indicates the start of processing an element.
/// </summary>
public RenderNodeAction OnElementBegin(ElementId elementId)
{
   // We may find it useful to remember this element's Id.
   // So we can refer to it in methods invoked during the 
   // export process of this element
   m_elementStack.Push(elementId);

   // We can use the element's Id to find out more about the element being processed.
   // For example, we can test if the element is a wall; if it is, we can get more
   // information about the wall and then we can proceed with the export, which will 
   // continue with processing geometry of the element. Elements that are not wall
   // will be skipped.

   Wall theWall = m_document.GetElement(elementId) as Wall;
   if (theWall != null)
   {
      double wallVolume = theWall.get_Parameter(BuiltInParameter.HOST_VOLUME_COMPUTED).AsDouble();
      return RenderNodeAction.Proceed;
   }
   else
   {
      return RenderNodeAction.Skip;
   }
}

/// <summary>
/// Method that indicates the end of processing an element
/// </summary>
public void OnElementEnd(ElementId elementId)
{
   // Note: this method is invoked even for elements that were skipped.

   m_elementStack.Pop();
}