public CombinedEnumerator(VirtualizingUICellCollection cellCollection)
            {
                if (cellCollection == null)
                {
                    throw new ArgumentNullException("cellCollection");
                }

                m_cellCollection = cellCollection;
            }
      public CombinedEnumerator( VirtualizingUICellCollection cellCollection )
      {
        if( cellCollection == null )
          throw new ArgumentNullException( "cellCollection" );

        m_cellCollection = cellCollection;
      }