how to find when the form is hidden

Where are you hiding?

See if you are keeping the reference open, just call the reference to get the file.

Form frm = new Form();

now if you hide using
frm.Hide();

it will be hidden. To show the same file use
from.Show();

If you dont have frm with you, that means the object is out of scope and will be in process of Garbage collection.

On the other hand, if you want your form to be out of strong reference so that it will be left for garbage collection but still want to check whether the form is not yet garbage collected or not, you need to use WeakReference for that.

I have written one article on how you can use WeakReference to enhance performance of your application. You may read :
http://www.abhisheksur.com/2010/07/garbage-collection-algorithm-with-use.html[^]

:thumbsup: