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

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