public override double Area {get;}

{
  List<Coordinate2D> outerCoordinates = new List<Coordinate2D>();
  outerCoordinates.Add(new Coordinate2D(10.0, 10.0));
  outerCoordinates.Add(new Coordinate2D(10.0, 20.0));
  outerCoordinates.Add(new Coordinate2D(20.0, 20.0));
  outerCoordinates.Add(new Coordinate2D(20.0, 10.0));

  // define the inner polygon as anti-clockwise
  List<Coordinate2D> innerCoordinates = new List<Coordinate2D>();
  innerCoordinates.Add(new Coordinate2D(13.0, 13.0));
  innerCoordinates.Add(new Coordinate2D(17.0, 13.0));
  innerCoordinates.Add(new Coordinate2D(17.0, 17.0));
  innerCoordinates.Add(new Coordinate2D(13.0, 17.0));

  PolygonBuilderEx pbEx = new PolygonBuilderEx(outerCoordinates);
  Polygon donutEx = pbEx.ToGeometry() as Polygon;
  double areaEx = donutEx.Area;       // area = 100

  pbEx.AddPart(innerCoordinates);
  donutEx = pbEx.ToGeometry() as Polygon;

  areaEx = donutEx.Area;    // area = 84.0

  areaEx = GeometryEngine.Instance.Area(donutEx);    // area = 84.0
}