Showing a hidden WPF window

foreach (Window window in App.Current.Windows)

        {
            if (!window.IsActive)
            {
                window.Show();
            }
        }

Works fine for me

 Window2 window2  = new Window2();
        this.Visibility = Visibility.Collapsed;
        window2.ShowDialog();
        this.Visibility = Visibility.Visible;

Do this instead:

this.Visibility = Visibility.Collapsed;    
...    
this.Visibility = Visibility.Visible;

Also, I saw your comment above that this doesn't work. However, I started a new WPF project, did this, built and ran it. It works.

Note that there are no errors.