CustomObjectGrips NewGeometry() not adding Extrusions?

Hi,

I am attempting to implement custom grips on Extrusions following the example:
SampleCsRectangleGrips

When I modified the code (see below) to return an Extrusion, I am expecting a solid Extrusion but a Surface is added to document instead. Is there a reason why this should happen?

T

    /// <summary>
    /// CustomObjectGrips override
    /// </summary>
    protected override GeometryBase NewGeometry()
    {
      UpdateGrips();
      if (GripsMoved && m_draw_rectangle)
        // Modified code to return Extrusion created from modified rectangle.
        return Extrusion.Create(new PolylineCurve(m_active_rectangle), 100.0, true);
      return null;
    }