How can I prevent focus stealing, but still get focus when returning to my app using Alt+Tab?

Question

Following MS guidelines, my WPF application's App constructor includes the following code for proper focus behavior:

HwndSource.DefaultAcquireHwndFocusInMenuMode = false;
Keyboard.DefaultRestoreFocusMode = RestoreFocusMode.None;

As explained in this article, these settings prevent focus stealing.

However, setting DefaultRestoreFocusMode to None has a bad side effect. When using Alt+Tab to leave a WPF application and then return to it, the WPF application doesn't get focus. However, if I don't set DefaultRestoreFocusMode to none, it does get focus as expected. Is there a way to prevent focus stealing but have focus still set when returning to a WPF application via Alt+Tab?

-Craig

Answer 1

I prevent my wpf window from getting focus by doing the below and i can still activate it by using ALT-TAB or clicking on it's taskbar item.

Here you change the window styles on your window so that it has no activate.

var yourWindow = new YourWindowType();
//set the windowstyle to noactivate so the window doesn't get focus
yourWindow.SourceInitialized += (s, e) =>
{
  var interopHelper = new WindowInteropHelper(yourWindow);
  int exStyle = User32.GetWindowLong(interopHelper.Handle, (int)WindowLongFlags.GWL_EXSTYLE);
  User32.SetWindowLong(interopHelper.Handle, (int)WindowLongFlags.GWL_EXSTYLE, exStyle | (int)WindowStylesEx.WS_EX_NOACTIVATE);

  //If you have trouble typing into your form's textboxes then do this
  ElementHost.EnableModelessKeyboardInterop(yourWindow);
};

This is something i added as an extra precaution, plus it lets you drag your window around if it is borderless:

private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
{
  switch (msg)
  {
    //don't activate the window when you click on it.
    case WindowMessage.WM_MOUSEACTIVATE:  
      handled = true;
      return (IntPtr)MouseActivate.MA_NOACTIVATE;
    //For Borderless Windows: occurs while dragging. it reports new position before it has been finalized.
    //otherwise you wont see the window moving while you're dragging it 
    case WindowMessage.WM_MOVING: 
      RECT rect = (RECT)Marshal.PtrToStructure(lParam, typeof(RECT));
      User32.SetWindowPos(new WindowInteropHelper(this).Handle, Hwnd.HWND_TOPMOST, 
                          rect.Left, rect.Top, rect.Width, rect.Height, 
                          SetWindowPosFlags.SWP_NOACTIVATE | SetWindowPosFlags.SWP_NOSIZE);
      break;
  }
  return IntPtr.Zero;
}

These add a hook so that WndProc is actually called in WPF:

private void Window_Loaded(object sender, RoutedEventArgs e)
{
  HwndSource source = HwndSource.FromHwnd(new WindowInteropHelper(this).Handle);
  if (source == null) return;
  source.AddHook(WndProc);
}

private void Window_Closing(object sender, System.ComponentModel.CancelEventArgs e)
{
  HwndSource source = HwndSource.FromHwnd(new WindowInteropHelper(this).Handle);
  if (source == null) return;
  source.RemoveHook(WndProc);
}

Just an FYI.. this still works even though you don't get focus:

   private void WpfPillForm_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
    {
      this.DragMove();
    }

Here's the Win32 API declarations so you don't have to look them up:

    [StructLayout(LayoutKind.Sequential)]
    public struct WINDOWPOS
    {
        public IntPtr hwnd;
        public IntPtr hwndInsertAfter;
        public int x;
        public int y;
        public int cx;
        public int cy;
        public int flags;
    }

[StructLayout(LayoutKind.Sequential)]
struct RECT
{
    public int left, top, right, bottom;
}

  public static class MouseActivate
  {
    public const int MA_ACTIVATE = 1;
    public const int MA_ACTIVATEANDEAT = 2;
    public const int MA_NOACTIVATE = 3;
    public const int MA_NOACTIVATEANDEAT = 4;
  }

  public enum WindowLongFlags : int
  {
    GWL_EXSTYLE = -20,
    GWLP_HINSTANCE = -6,
    GWLP_HWNDPARENT = -8,
    GWL_ID = -12,
    GWL_STYLE = -16,
    GWL_USERDATA = -21,
    GWL_WNDPROC = -4,
    DWLP_USER = 0x8,
    DWLP_MSGRESULT = 0x0,
    DWLP_DLGPROC = 0x4
  }

    public const int WM_MOVING = 0x0216;
    public const uint WS_EX_NOACTIVATE = 0x08000000,

    [DllImport("user32.dll")]
    [return: MarshalAs(UnmanagedType.Bool)]
    public static extern bool SetWindowPos(IntPtr hWnd, IntPtr hWndInsertAfter, int X, int Y, int cx, int cy, SetWindowPosFlags uFlags);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    public static extern int SetWindowLong(IntPtr hWnd, int nIndex, int dwNewLong);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    public static extern int GetWindowLong(IntPtr hwnd, int index);