public override Polygon 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));

// Builder constructors need to run on the MCT.
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  // use the PolygonBuilder as we wish to manipulate the parts
  using (PolygonBuilder pb = new PolygonBuilder(outerCoordinates))
  {
    Polygon donut = pb.ToGeometry();
    double area = donut.Area;       // area = 100

    pb.AddPart(innerCoordinates);
    donut = pb.ToGeometry();

    area = donut.Area;    // area = 84.0

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


// builderEx constructors don't need to run on the MCt
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