In my application there are only 2 windows — win_a & win_b, on each of these windows there is button that call another window, e.g. click on btn1 of win_a will call win_b, click on btn2 of win_b will show win_a. Desired behaviour: 1. Only one instance of object is premitted at the same time, e.g. situation, where 2 instances of win_a running at the same time is not permitted. When you click on button that calls windows that already exist this action will only change a focus to needed window. If you call a window that previously had been created, but after this has been closed this action will create a new instance of this window. E.g. there are 2 running windows. you close one of them and after try to call this window back, so related button will create it. How to write it in WPF (XAML + C#). For the moment I wrote a version that can create a lot of instances of the same window (no number of instances control implemented), but I want to see only one instance of the same window, as we can see it in a lot of applications. Example of my code: Window win = new Window(); win.Show(); Thanks.