Refer to active Window in WPF?

One possible way would be to scan the list of open windows in the application and check which one of them has IsActive = true:

Application.Current.Windows.OfType<Window>().SingleOrDefault(x => x.IsActive);

Not sure if there may be more than one active window if, for example, there's a modal dialog showing, in which case, the owner of the dialog and the dialog itself might be active.

Another way to do it is to use the native GetActiveWindow function from user32.dll.

[DllImport("user32.dll", CharSet = CharSet.Auto)]
public static extern IntPtr GetActiveWindow();

To convert it to an actual WPF Window:

IntPtr handle = GetActiveWindow();

HwndSource hwndSource = HwndSource.FromHwnd(handle);
var window = hwndSource?.RootVisual as Window;

If hosting a WPF Window in a WinForms app, WindowInteropHelper should be used. This makes for example the Window owner work correctly:

var wih = new WindowInteropHelper(window)
{
    Owner = GetActiveWindow()
};

I edited my old answer because the edge case I encountered disappeared after a Visual Studio update, but it can be checked from answer history. I encountered an issue there where I was getting null for active window in certain circumstances while debugging.