Thank Jim and Michael!
I apologize for my absence from the forum!
I have seen your last posts and I undertand how to implement your solution, but, as Michael sais "[...] Sometimes the easiest solution is to just remove the requirement. [...]". I found an anlternative (better?) way to implement the interaction with the user! I can dispose the controls in different order to make them all visible, so the user can interact with them!
Anyway, thank you very must: in your forum I find a lot of willingness!
Federico