WPF - Preventing Alt+F4 from closing the window

Sometimes I want to prevent from closing windows using Alt+F4. It's quite a rate case when I want the window to be closeable (so I can't bind to Window.Closing event and stop it), but I want to disable the Alt+F4. One of these scenarios is my clock showing window, a part of the WinRun application.

It's not a typical window. It basically acts like a live part of the desktop. It should almost be non-focusable. I'm using Esc to hide it, but sometimes, when it is the last opened window and I want to shutdown the computer using Alt+F4, invoked on the desktop, and instead of having focus on the dekstop I have focus on the clock window. It closes the window... Damn!

The code

Here is a code snippet to actualy prevent Alt+F4 on WPF window. The method is bound to a Window.PreviewKeyDown event.

private void OnPreviewKeyDown(object sender, KeyEventArgs e)
{
    if (e.Key == Key.System && e.SystemKey == Key.F4)
    {
        e.Handled = true;
    }
}

The "trick" is to test the pressed key for Key.System and then test KeyEventArgs.SystemKey.

That's it.

Hope it helps me some day in the future.