// create the polygon
List<MapPoint> pts = new List<MapPoint>();
pts.Add(MapPointBuilder.CreateMapPoint(1.0, 1.0, SpatialReferences.WGS84));
pts.Add(MapPointBuilder.CreateMapPoint(1.0, 2.0, SpatialReferences.WGS84));
pts.Add(MapPointBuilder.CreateMapPoint(2.0, 2.0, SpatialReferences.WGS84));
pts.Add(MapPointBuilder.CreateMapPoint(2.0, 1.0, SpatialReferences.WGS84));
Polygon polygon = PolygonBuilder.CreatePolygon(pts);
// ensure it is simple
bool isSimple = GeometryEngine.Instance.IsSimpleAsFeature(polygon);
// create the spatial reference to project to
SpatialReference northPole = SpatialReferenceBuilder.CreateSpatialReference(102018); // north pole stereographic
// project
Geometry geometry = GeometryEngine.Instance.Project(polygon, northPole);