How to distinguish 'Window close button clicked (X)' vs. window.Close() in closing handler

Question

Is there a clever way to detect whether a window was closed by

• A user pressing the (X) button in the upper right corner of the window or
• window.Close() has been called programatically.

I would like to detect this in the window.Closing handler. I could set a flag whenever I call window.Close(), but this is not a very pretty solution.

Answer 1

I'm not sure I like this at all but it's a question that you obviously have a reason for asking. if you were to take a stack trace in the OnClosing event you could look up for the Window.Close event.

protected override void OnClosing(System.ComponentModel.CancelEventArgs e)
{
   bool wasCodeClosed = new StackTrace().GetFrames().FirstOrDefault(x => x.GetMethod() == typeof(Window).GetMethod("Close")) != null;
   if (wasCodeClosed)
   {
       // Closed with this.Close()
   }
   else
   {
       // Closed some other way.
   }

   base.OnClosing(e);
}

Answer 2

The difference is the following:

Window.Close() causes WM_CLOSE to be send to window.

Alt+F4 and X button causes WM_SYSCOMMAND message with SC_CLOSE type. You can decide if you wish to route this message further ( and cause WM_CLOSE in the end ).

Here's a piece of code to catch this message. Return "True" from delegate if you wish to cancel default behaviour:

class SystemMenu : IDisposable
{
    const int WM_SYSCOMMAND = 0x0112;
    const int SC_CLOSE = 0xF060;

    public delegate bool HandleSystemCommand();
    HwndSource _source;
    HandleSystemCommand _handler;

    public SystemMenu(Window window, HandleSystemCommand handler )
    {
        _handler = handler;
        _source  = HwndSource.FromHwnd(new WindowInteropHelper( window ).Handle);
        _source.AddHook(WndProc);
    }

    public void Dispose() {
        _source.RemoveHook(WndProc);
    }

    private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
    {
        switch (msg)
        {
            case WM_SYSCOMMAND:
                int command = wParam.ToInt32() & 0xfff0;
                if (command == SC_CLOSE)
                    handled = _handler();
                break;
            default:
                break;
        }
        return IntPtr.Zero;
    }
}