private static XElement ToXml(DrawingElementBase drawingElement)
        {
            var lineElement = drawingElement as LineElement;
            if (lineElement != null) return ToXml(lineElement);

            var textElement = drawingElement as TextElement;
            if (textElement != null) return ToXml(textElement);

            var polygoneElement = drawingElement as PolygonElement;
            if (polygoneElement != null) return ToXml(polygoneElement);

            throw new NotSupportedException(drawingElement.GetType().Name + " is not supported");
        }