public static MapPoint CreateMapPoint( 
   double x,
   double y,
   SpatialReference spatialReference
)

// Use a builderEx convenience method or a builderEx constructor.
// neither need to run on the MCT

MapPoint startPt = MapPointBuilderEx.CreateMapPoint(1.0, 1.0);
MapPoint endPt = MapPointBuilderEx.CreateMapPoint(2.0, 1.0);

List<MapPoint> list = new List<MapPoint>();
list.Add(startPt);
list.Add(endPt);

Polyline polyline = PolylineBuilderEx.CreatePolyline(list, SpatialReferences.WGS84);

// use AttributeFlags.None since we only have 2D points in the list
PolylineBuilderEx pb = new PolylineBuilderEx(list, AttributeFlags.None);
pb.SpatialReference = SpatialReferences.WGS84;
Polyline polyline2 = pb.ToGeometry();

//     use AttributeFlags.NoAttributes because we only have 2d points in the list
Polyline polyline4 = PolylineBuilderEx.CreatePolyline(list, AttributeFlags.None);