Robin,
Yes one would think it would be that way, but alas it is not. VFP chaches some class stuff for optimization and now and then once everything is released the class library is still in memory. Typing CLEAR ALL in the command window will close the class library and allow you to build again.
It is not necessary to do a Thisform.Release in the unload of a form as the unload fires after the object has begun to go away.