Loading a WPF Window without showing it

I create a global hot key to show a window by PInvoking RegisterHotKey(). But to do this I need that window's HWND, which doesn't exist until the window is loaded, that means shown for the first time. But I don't want to show the window before I can set the hot key. Is there a way to create a HWND for that window that is invisible to the user?

If you are targeting .NET 4.0 you can make use of the new EnsureHandle method available on the WindowInteropHelper:

public void InitHwnd()
{
    var helper = new WindowInteropHelper(this);
    helper.EnsureHandle();
}

(thanks to Thomas Levesque for pointing this out.)

If you are targeting an older version of the .NET Framework, the easiest way is to show the window to get to the HWND while setting a few properties to make sure that the window is invisible and doesn't steal focus:

var window = new Window() //make sure the window is invisible
{
    Width = 0,
    Height = 0,
    WindowStyle = WindowStyle.None,
    ShowInTaskbar = false,
    ShowActivated = false
};
window.Show();

Once you want to show the actual window you can then set the Content, the size and change the style back to a normal window.

You can also change the window into a so called message-only window. As this window type does not support graphical elements it will never be shown. Basically it comes down to calling:

    SetParent(hwnd, (IntPtr)HWND_MESSAGE);

Either create a dedicated message window which will always be hidden, or use the real GUI window and change it back to a normal window when you want to display it. See the code below for a more complete example.

    [DllImport("user32.dll")]
    static extern IntPtr SetParent(IntPtr hwnd, IntPtr hwndNewParent);

    private const int HWND_MESSAGE = -3;

    private IntPtr hwnd;
    private IntPtr oldParent;

    protected override void OnSourceInitialized(EventArgs e)
    {
        base.OnSourceInitialized(e);
        HwndSource hwndSource = PresentationSource.FromVisual(this) as HwndSource;

        if (hwndSource != null)
        {
            hwnd = hwndSource.Handle;
            oldParent = SetParent(hwnd, (IntPtr)HWND_MESSAGE);
            Visibility = Visibility.Hidden;
        }
    }

    private void OpenWindowMenuItem_Click(object sender, RoutedEventArgs e)
    {
        SetParent(hwnd, oldParent);
        Show();
        Activate();
    }

For me the solution of setting the width, height to zero and style to none didn't work out, as it still showed a tiny window, with an annoying shadow of what seems to be the border around a 0x0 window (tested on Windows 7). Therefore I'm providing this alternative option.

This is a dirty hack, but it should work, and doesn't have the downsides of changing the opacity :

• set the WindowStartupLocation to Manual
• set the Top and Left properties to somewhere outside the screen
• set ShowInTaskbar to false so that the user doesn't realize there is a new window
• Show and Hide the window

You should now be able to retrieve the HWND

EDIT: another option, probably better : set ShowInTaskBar to false and WindowState to Minimized, then show it : it won't be visible at all