public static LineSegment CreateLineSegment( 
   MapPoint startPt,
   MapPoint endPt,
   SpatialReference spatialReference
)

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

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

// Builder convenience methods don't need to run on the MCT.
LineSegment lineFromMapPoint = LineBuilder.CreateLineSegment(startPt, endPt);

// coordinate2D
Coordinate2D start2d = (Coordinate2D)startPt;
Coordinate2D end2d = (Coordinate2D)endPt;

LineSegment lineFromCoordinate2D = LineBuilder.CreateLineSegment(start2d, end2d);

// coordinate3D
Coordinate3D start3d = (Coordinate3D)startPt;
Coordinate3D end3d = (Coordinate3D)endPt;

LineSegment lineFromCoordinate3D = LineBuilder.CreateLineSegment(start3d, end3d);

// lineSegment
LineSegment anotherLineFromLineSegment = LineBuilder.CreateLineSegment(lineFromCoordinate3D);

// Builder constructors need to run on the MCT.
ArcGIS.Desktop.Framework.Threading.Tasks.QueuedTask.Run(() =>
{
  using (LineBuilder lb = new LineBuilder(startPt, endPt))
  {
    // do something with the builder

    lineFromMapPoint = lb.ToSegment();
  }

  using (LineBuilder lb = new LineBuilder(start2d, end2d))
  {
    // do something with the builder

    lineFromCoordinate2D = lb.ToSegment();
  }

  using (LineBuilder lb = new LineBuilder(start3d, end3d))
  {
    // do something with the builder

    lineFromCoordinate3D = lb.ToSegment();
  }

  using (LineBuilder lb = new LineBuilder(lineFromCoordinate3D))
  {
    // do something with the builder

    LineSegment lineFromLineSegment = lb.ToSegment();
  }
});