public new Polyline ToGeometry()

{
  var polylineBuilder = new PolylineBuilderEx(polyline);
  polylineBuilder.ReverseOrientation();
  Polyline reversedPolyline = polylineBuilder.ToGeometry();
}

{
  List<MapPoint> firstPoints = new List<MapPoint>();
  firstPoints.Add(MapPointBuilderEx.CreateMapPoint(1.0, 1.0));
  firstPoints.Add(MapPointBuilderEx.CreateMapPoint(1.0, 2.0));
  firstPoints.Add(MapPointBuilderEx.CreateMapPoint(2.0, 2.0));
  firstPoints.Add(MapPointBuilderEx.CreateMapPoint(2.0, 1.0));

  List<MapPoint> nextPoints = new List<MapPoint>();
  nextPoints.Add(MapPointBuilderEx.CreateMapPoint(11.0, 1.0));
  nextPoints.Add(MapPointBuilderEx.CreateMapPoint(11.0, 2.0));
  nextPoints.Add(MapPointBuilderEx.CreateMapPoint(12.0, 2.0));
  nextPoints.Add(MapPointBuilderEx.CreateMapPoint(12.0, 1.0));

  // use AttributeFlags.None since we have 2D points in the list
  PolylineBuilderEx pBuilder = new PolylineBuilderEx(firstPoints, AttributeFlags.None);
  pBuilder.AddPart(nextPoints);

  Polyline polylineFromBuilder = pBuilder.ToGeometry();
  // polyline has 2 parts

  pBuilder.RemovePart(0);
  polylineFromBuilder = pBuilder.ToGeometry();
  // polyline has 1 part
}