Note:
Form1
stays hidden, but wasn't closed.
WinForm:
You can check if Form1 is already opened by using
Application.OpenForms Property (System.Windows.Forms) | Microsoft Docs[
^]
Form frm = Application.OpenForms["Form1"];
frm.Show();
Note: please, read carefully
Security
section.
WPF:
There's a collection:
Application.Current.Windows
Application.Current.Windows.OfType<Window>().Where(x => x.Name == "Form1").FirstOrDefault()