public PolygonBuilder( 
   IEnumerable<MapPoint> points
)

// Use a builder convenience method or use a builder constructor.

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

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

// Builder convenience methods don't need to run on the MCT.
Polygon polygon = PolygonBuilder.CreatePolygon(list, SpatialReferences.WGS84);

// Builder constructors need to run on the MCT.
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  using (PolygonBuilder polygonBuilder = new PolygonBuilder(list))
  {
    polygonBuilder.SpatialReference = SpatialReferences.WGS84;
    polygon = polygonBuilder.ToGeometry();
  }
});


// builderEx constructor's don't need to run on the MCT
PolygonBuilderEx polygonBuilderEx = new PolygonBuilderEx(list);
polygonBuilderEx.SpatialReference = SpatialReferences.WGS84;
polygon = polygonBuilderEx.ToGeometry() as Polygon;

// builderEx convenience methods don't need to run on the MCT
//     use AttributeFlags.NoAttributes because we only have 2d points in the list
polygon = PolygonBuilderEx.CreatePolygon(list, AttributeFlags.NoAttributes);