How to transfer mouse wheel events to WindowsFormsHost?

Question

I have a WinForms control in a WPF application. Luckily in WPF, mouse wheel events are normally handled by the control under the mouse cursor... but if it's a WinForms control, I have to click the control first to give it the keyboard focus. How do I bypass this behavior and send mouse wheel events to the WinForms control when it does not have the focus?

(Note: the MouseWheel and PreviewMouseWheel events do not seem to work for WindowsFormsHost. If you add a handler for PreviewMouseWheel in a parent of the WindowsFormsHost, it is not called when the mouse is on top of a non-WPF control. Obviously, the WinForms control does not get a MouseWheel event either; the event seems to vanish entirely.)

Answer 1

One possible solution is to handle all WM_MOUSEWHEEL messages and raise MouseWheel event for unfocused controls manually. My attached behavior implementing this idea:

public static class CaptureMouseWheelWhenUnfocusedBehavior
{
    private static readonly HashSet<WindowsFormsHost> TrackedHosts =
        new HashSet<WindowsFormsHost>();

    private static readonly System.Windows.Forms.IMessageFilter MessageFilter =
        new MouseWheelMessageFilter();

    private sealed class MouseWheelMessageFilter : System.Windows.Forms.IMessageFilter
    {
        [DllImport("User32.dll"), SuppressUnmanagedCodeSecurity]
        private static extern IntPtr WindowFromPoint(System.Drawing.Point point);

        private static System.Drawing.Point LocationFromLParam(IntPtr lParam)
        {
            int x = (int)((((long)lParam) >> 0) & 0xffff);
            int y = (int)((((long)lParam) >> 16) & 0xffff);
            return new System.Drawing.Point(x, y);
        }

        private static bool ConsiderRedirect(WindowsFormsHost host)
        {
            var control = host.Child;
            return control != null &&
                  !control.IsDisposed &&
                   control.IsHandleCreated &&
                   control.Visible &&
                  !control.Focused;
        }

        private static int DeltaFromWParam(IntPtr wParam)
        {
            return (short)((((long)wParam) >> 16) & 0xffff);
        }

        private static System.Windows.Forms.MouseButtons MouseButtonsFromWParam(IntPtr wParam)
        {
            const int MK_LBUTTON = 0x0001;
            const int MK_MBUTTON = 0x0010;
            const int MK_RBUTTON = 0x0002;
            const int MK_XBUTTON1 = 0x0020;
            const int MK_XBUTTON2 = 0x0040;
            int buttonFlags = (int)((((long)wParam) >> 0) & 0xffff);
            var buttons = System.Windows.Forms.MouseButtons.None;
            if(buttonFlags != 0)
            {
                if((buttonFlags & MK_LBUTTON) == MK_LBUTTON)
                {
                    buttons |= System.Windows.Forms.MouseButtons.Left;
                }
                if((buttonFlags & MK_MBUTTON) == MK_MBUTTON)
                {
                    buttons |= System.Windows.Forms.MouseButtons.Middle;
                }
                if((buttonFlags & MK_RBUTTON) == MK_RBUTTON)
                {
                    buttons |= System.Windows.Forms.MouseButtons.Right;
                }
                if((buttonFlags & MK_XBUTTON1) == MK_XBUTTON1)
                {
                    buttons |= System.Windows.Forms.MouseButtons.XButton1;
                }
                if((buttonFlags & MK_XBUTTON2) == MK_XBUTTON2)
                {
                    buttons |= System.Windows.Forms.MouseButtons.XButton2;
                }
            }
            return buttons;
        }

        public bool PreFilterMessage(ref System.Windows.Forms.Message m)
        {
            const int WM_MOUSEWHEEL = 0x020A;
            if(m.Msg == WM_MOUSEWHEEL)
            {
                var location = LocationFromLParam(m.LParam);
                var hwnd = WindowFromPoint(location);
                foreach(var host in TrackedHosts)
                {
                    if(!ConsiderRedirect(host)) continue;
                    if(hwnd == host.Child.Handle)
                    {
                        var delta = DeltaFromWParam(m.WParam);
                        {
                            // raise event for WPF control
                            var mouse = InputManager.Current.PrimaryMouseDevice;
                            var args = new MouseWheelEventArgs(mouse, Environment.TickCount, delta);
                            args.RoutedEvent = WindowsFormsHost.MouseWheelEvent;
                            host.RaiseEvent(args);
                        }
                        {
                            // raise event for winforms control
                            var buttons = MouseButtonsFromWParam(m.WParam);
                            var args = new System.Windows.Forms.MouseEventArgs(
                                buttons, 0, location.X, location.Y, delta);
                            var method = typeof(System.Windows.Forms.Control).GetMethod(
                                "OnMouseWheel",
                                System.Reflection.BindingFlags.Instance |
                                System.Reflection.BindingFlags.NonPublic);
                            method.Invoke(host.Child, new object[] { args });
                        }
                        return true;

                    }
                }
            }
            return false;
        }
    }

    public static bool GetIsEnabled(DependencyObject obj)
    {
        return (bool)obj.GetValue(IsEnabledProperty);
    }

    public static void SetIsEnabled(DependencyObject obj, bool value)
    {
        obj.SetValue(IsEnabledProperty, value);
    }

    public static readonly DependencyProperty IsEnabledProperty =
        DependencyProperty.RegisterAttached(
            "IsEnabled",
            typeof(bool),
            typeof(CaptureMouseWheelWhenUnfocusedBehavior),
            new PropertyMetadata(false, OnIsEnabledChanged));

    private static void OnIsEnabledChanged(DependencyObject o, DependencyPropertyChangedEventArgs e)
    {
        var wfh = o as WindowsFormsHost;
        if(wfh == null) return;

        if((bool)e.NewValue)
        {
            wfh.Loaded += OnHostLoaded;
            wfh.Unloaded += OnHostUnloaded;
            if(wfh.IsLoaded && TrackedHosts.Add(wfh))
            {
                if(TrackedHosts.Count == 1)
                {
                    System.Windows.Forms.Application.AddMessageFilter(MessageFilter);
                }
            }
        }
        else
        {
            wfh.Loaded -= OnHostLoaded;
            wfh.Unloaded -= OnHostUnloaded;
            if(TrackedHosts.Remove(wfh))
            {
                if(TrackedHosts.Count == 0)
                {
                    System.Windows.Forms.Application.RemoveMessageFilter(MessageFilter);
                }
            }
        }
    }

    private static void OnHostLoaded(object sender, EventArgs e)
    {
        var wfh = (WindowsFormsHost)sender;
        if(TrackedHosts.Add(wfh))
        {
            if(TrackedHosts.Count == 1)
            {
                System.Windows.Forms.Application.AddMessageFilter(MessageFilter);
            }
        }
    }

    private static void OnHostUnloaded(object sender, EventArgs e)
    {
        var wfh = (WindowsFormsHost)sender;
        if(TrackedHosts.Remove(wfh))
        {
            if(TrackedHosts.Count == 0)
            {
                System.Windows.Forms.Application.RemoveMessageFilter(MessageFilter);
            }
        }
    }
}

It can be used both in XAML:

<WindowsFormsHost namespace:CaptureMouseWheelWhenUnfocusedBehavior.IsEnabled="True" />

and code behind:

CaptureMouseWheelWhenUnfocusedBehavior.SetIsEnabled(host, true);

Update: added code to raise event for WinForms control also