public static LineSegment CreateLineSegment( 
   Coordinate3D startPoint,
   Coordinate3D endPoint,
   SpatialReference spatialReference
)

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

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

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

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

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

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

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

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


// builderEx constructors don't need to run on the MCT
LineBuilderEx lbEx = new LineBuilderEx(startPt, endPt);
lineFromMapPoint = lbEx.ToSegment() as LineSegment;

lbEx = new LineBuilderEx(start2d, end2d);
lineFromCoordinate2D = lbEx.ToSegment() as LineSegment;

lbEx = new LineBuilderEx(start3d, end3d);
lineFromCoordinate3D = lbEx.ToSegment() as LineSegment;

lbEx = new LineBuilderEx(startPt, endPt);
lineFromMapPoint = lbEx.ToSegment() as LineSegment;

lbEx = new LineBuilderEx(lineFromCoordinate3D);
anotherLineFromLineSegment = lbEx.ToSegment() as LineSegment;