Hooking into Windows message loop in WPF window adds white border on the inside

I am trying to create a WPF window with WindowStyle="None" (for custom buttons and no title) that cannot be resized. Setting ResizeMode to NoResize removes the aero border, which I want to keep.

I could set the min/max size properties and be done with it, except that:

1. The resize cursors are still visible, and
2. The window is displayed in response to a user action and fits to its contents. It displays an image, so the size changes.

So, I have a simple scheme that gets me 99% of the way there:

public class BorderedWindowNoResize : Window
{
    [DllImport( "DwmApi.dll" )]
    public static extern int DwmExtendFrameIntoClientArea(
        IntPtr hwnd,
        ref MARGINS pMarInset );

    [DllImport( "user32.dll", CharSet = CharSet.Auto )]
    public static extern IntPtr DefWindowProc(
        IntPtr hWnd,
        int msg,
        IntPtr wParam,
        IntPtr lParam );

    public BorderedWindowNoResize()
    {           
        Loaded += BorderedWindowNoResize_Loaded;
    }

    private void BorderedWindowNoResize_Loaded( object sender, RoutedEventArgs e )
    {           
        IntPtr mainWindowPtr = new WindowInteropHelper( this ).Handle;
        HwndSource mainWindowSrc = HwndSource.FromHwnd( mainWindowPtr );            
        mainWindowSrc.AddHook( WndProc );
    }

    private IntPtr WndProc( IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled )
    {           
        var htLocation = DefWindowProc( hwnd, msg, wParam, lParam ).ToInt32();

        if( ms开发者_JS百科g == (uint)WM.NCHITTEST )
        {
            handled = true;
            switch( htLocation )
            {
                case (int)HitTestResult.HTBOTTOM:
                case (int)HitTestResult.HTBOTTOMLEFT:
                case (int)HitTestResult.HTBOTTOMRIGHT:
                case (int)HitTestResult.HTLEFT:
                case (int)HitTestResult.HTRIGHT:
                case (int)HitTestResult.HTTOP:
                case (int)HitTestResult.HTTOPLEFT:
                case (int)HitTestResult.HTTOPRIGHT:
                    htLocation = (int)HitTestResult.HTBORDER;
                    break;
            }               
        }

        return new IntPtr( htLocation );
    }
}

Basically;

1. Override the window procedure.
2. Call the default window procedure.
3. If the message it is WM_NCHITTEST, check for the border results.
4. If it is a border, return the regular HTBORDER.

This works as far as allowing me to keep the aero window border and hiding the resize cursor(s), but it adds a ~5 pixel white border to the inside of my window.

In fact, even if I return the default windows procedure result at the top of WndPrc and do nothing else the border is still there. I need a different background color on my window, so this won't work for me.

Any ideas? Thanks in advance as always.

When you add your hook, you should only handle the messages you need to, and ignore the others. I believe you are handling certain messages twice, since you call DefWindowProc, but never set the handled parameter to true.

So in your case, you'd use:

private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled) {

    if (msg == (uint)WM.NCHITTEST) {
        handled = true;
        var htLocation = DefWindowProc(hwnd, msg, wParam, lParam).ToInt32();
        switch (htLocation) {
            case (int)HitTestResult.HTBOTTOM:
            case (int)HitTestResult.HTBOTTOMLEFT:
            case (int)HitTestResult.HTBOTTOMRIGHT:
            case (int)HitTestResult.HTLEFT:
            case (int)HitTestResult.HTRIGHT:
            case (int)HitTestResult.HTTOP:
            case (int)HitTestResult.HTTOPLEFT:
            case (int)HitTestResult.HTTOPRIGHT:
                htLocation = (int)HitTestResult.HTBORDER;
                break;
        }
        return new IntPtr(htLocation);
    }

    return IntPtr.Zero;
}

Also, I'd probably add the hook in an OnSourceInitialized override, like so:

protected override void OnSourceInitialized(EventArgs e) {
    base.OnSourceInitialized(e);

    IntPtr mainWindowPtr = new WindowInteropHelper(this).Handle;
    HwndSource mainWindowSrc = HwndSource.FromHwnd(mainWindowPtr);
    mainWindowSrc.AddHook(WndProc);
}

You can try from anywhere in a WPF App

ComponentDispatcher.ThreadFilterMessage += new ThreadMessageEventHandler(ComponentDispatcherThreadFilterMessage);

and:

// ******************************************************************
private static void ComponentDispatcherThreadFilterMessage(ref MSG msg, ref bool handled)
{
    if (!handled)
    {
        if (msg.message == WmHotKey)
        {
            HotKey hotKey;

            if (_dictHotKeyToCalBackProc.TryGetValue((int)msg.wParam, out hotKey))
            {
                if (hotKey.Action != null)
                {
                    hotKey.Action.Invoke(hotKey);
                }
                handled = true;
            }
        }
    }
}