public new Polygon ToGeometry()

{
  // Use a builderEx convenience method or use a builderEx constructor.

  MapPoint pt1 = MapPointBuilderEx.CreateMapPoint(1.0, 1.0);
  MapPoint pt2 = MapPointBuilderEx.CreateMapPoint(1.0, 2.0);
  MapPoint pt3 = MapPointBuilderEx.CreateMapPoint(2.0, 2.0);
  MapPoint pt4 = MapPointBuilderEx.CreateMapPoint(2.0, 1.0);

  List<MapPoint> list = new List<MapPoint>() { pt1, pt2, pt3, pt4 };

  Polygon polygonFromPoints = PolygonBuilderEx.CreatePolygon(list, SpatialReferences.WGS84);
  // polygon.HasZ will be false - it is determined by the HasZ flag of the points in the list

  // or specifically use AttributeFlags.NoAttributes
  polygonFromPoints = PolygonBuilderEx.CreatePolygon(list, AttributeFlags.None);

  // use AttributeFlags.None as we have 2D points
  PolygonBuilderEx polygonBuilder = new PolygonBuilderEx(list, AttributeFlags.None);
  polygonBuilder.SpatialReference = SpatialReferences.WGS84;
  polygonFromPoints = polygonBuilder.ToGeometry();
}

{
  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
}