If IsClosed, just return true. Otherwise, decide if curve can be closed as follows: Linear curves polylinear curves with 2 segments, Nurbs with 3 or less control points cannot be made closed. Also, if tolerance > 0 and the gap between start and end is larger than tolerance, curve cannot be made closed. Adjust the curve's endpoint to match its start point.

public bool MakeClosed(
	double tolerance
)

Public Function MakeClosed ( 
	tolerance As Double
) As Boolean