' <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>
Private m_elementStack As New Stack(Of ElementId)()

Private Function CurrentElementId() As ElementId
    Return If((m_elementStack.Count > 0), m_elementStack.Peek(), ElementId.InvalidElementId)
End Function

' <summary>
' Method that indicates the start of processing an element.
' </summary>
Public Function OnElementBegin(elementId As ElementId) As RenderNodeAction Implements IExportContext.OnElementBegin
    ' 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.


    Dim theWall As Wall = TryCast(m_document.GetElement(elementId), Wall)
    If theWall IsNot Nothing Then
        Dim wallVolume As Double = theWall.Parameter(BuiltInParameter.HOST_VOLUME_COMPUTED).AsDouble()
        Return RenderNodeAction.Proceed
    Else
        Return RenderNodeAction.Skip
    End If
End Function

' <summary>
' Method that indicates the end of processing an element
' </summary>
Public Sub OnElementEnd(elementId As ElementId) Implements IExportContext.OnElementEnd
    ' Note: this method is invoked even for elements that were skipped.


    m_elementStack.Pop()
End Sub