WPF Borderless window resize

Question

I am designing my own custom window in WPF and I have been trying to implement the resizing functionality I have used previously in WinForms. For some reason the return value of my WndProc isn't giving me the proper result.

I have a NativeMethods class for all my WndProc messages and results:

public class NativeMethods
{
    public const int WM_NCHITTEST  = 0x84;
    public const int HTCAPTION     = 2;
    public const int HTLEFT        = 10;
    public const int HTRIGHT       = 11;
    public const int HTTOP         = 12;
    public const int HTTOPLEFT     = 13;
    public const int HTTOPRIGHT    = 14;
    public const int HTBOTTOM      = 15;
    public const int HTBOTTOMLEFT  = 16;
    public const int HTBOTTOMRIGHT = 17;
}

And here is the code behind for my window:

public partial class MainWindow : Window
{
    const int GripSize   = 16;
    const int BorderSize = 7;

    public MainWindow()
    {
        InitializeComponent();
    }

    protected override void OnSourceInitialized(EventArgs e)
    {
        base.OnSourceInitialized(e);
        IntPtr windowHandle = new WindowInteropHelper(this).Handle;
        HwndSource windowSource = HwndSource.FromHwnd(windowHandle);
        windowSource.AddHook(WndProc);
    }

    private IntPtr WndProc(IntPtr hwnd, int msg,
        IntPtr wParam, IntPtr lParam, ref bool handled)
    {
        if (msg == NativeMethods.WM_NCHITTEST)
        {
            int x = lParam.ToInt32() << 16 >> 16, y = lParam.ToInt32() >> 16;
            Point pos = PointFromScreen(new Point(x, y));

            if (pos.X > GripSize && 
                pos.X < ActualWidth - GripSize &&
                pos.Y >= ActualHeight - BorderSize)
            {
                return (IntPtr)NativeMethods.HTBOTTOM; // This doesn't work?
            }

            // Top, Left, Right, Corners, Etc.
        }

        return IntPtr.Zero;
    }
}

I expected the cursor to change to the "resize down arrow" and the resizing functionality to work as it did in my WinForms project. I have set breakpoints and the HTBOTTOM return is firing when the cursor is in the expected location. In XAML I have ResizeMode set to CanResize and the WindowStyle set to None. What am I doing wrong?

Answer 1

Well this was a stupid mistake. I forgot to add handled = true; before I returned the result. Now the window is functioning as normal. As a note if you set the ResizeMode to NoResize this code won't work at all.