Passing data between windows forms

When you are opening a 2^nd form what are you doing with 1^st form are you closing and re-instantiating the 1^st form when you are coming back? If yes you will lose all the data in 1^st form.

What are you doing on the second form?

Instead of closing the 1^st form hide the 1^st form and open the second form when you want to come back unhide the 1^st form.

Or

When you are calling the 2^nd form call ShowDialog

Hope this helps