ScrollViewer is not working in WPF WindowsFormHost

Question

I have WindowsFormHost with a RichTextBox in my WPF form, i have given ScrollViewer for

Answer 1

If your WindowsFormsHost is to be placed inside a UserControl, then the answer presented by Avinash may not work. So I had to tweak the ScrollViewerWindowsFormsHost class as follows.

    public class ScrollViewerWindowsFormsHost : WindowsFormsHost
    {
        protected override void OnWindowPositionChanged(Rect rcBoundingBox)
        {
            base.OnWindowPositionChanged(rcBoundingBox);

            if (ParentScrollViewer == null)
                //return; // Instead, you set the ParentScrollViewr by calling the following method.
                SetParentScrollViewer();

            GeneralTransform tr = ParentScrollViewer.TransformToAncestor(MainWindow);

            var scrollRect = new Rect(new Size(ParentScrollViewer.ViewportWidth, ParentScrollViewer.ViewportHeight));
            scrollRect = tr.TransformBounds(scrollRect);

            var intersect = Rect.Intersect(scrollRect, rcBoundingBox);
            if (!intersect.IsEmpty)
            {
                tr = MainWindow.TransformToDescendant(this);
                intersect = tr.TransformBounds(intersect);
            }

            SetRegion(intersect);
        }

        // This is new a new method. This is called from the above method.
        private void SetParentScrollViewer()
        {
            if (ParentScrollViewer is ScrollViewer)
                return; // that means its already set;

            var p = Parent as FrameworkElement;
            while (p != null)
            {
                if (p is ScrollViewer)
                {
                    ParentScrollViewer = (ScrollViewer)p;
                    break;
                }

                p = p.Parent as FrameworkElement;
            }
        }
        // Just comment out this method, you dont need this any more. You set the parent Scroll Viewer by calling SetParentScrollViewer Method.
        //protected override void OnVisualParentChanged(DependencyObject oldParent)
        //{
        //    base.OnVisualParentChanged(oldParent);
        //    ParentScrollViewer = null;

        //    var p = Parent as FrameworkElement;
        //    while (p != null)
        //    {
        //        if (p is ScrollViewer)
        //        {
        //            ParentScrollViewer = (ScrollViewer)p;
        //            break;
        //        }

        //        p = p.Parent as FrameworkElement;

        //    }
        //}

        private void SetRegion(Rect intersect)
        {
            using (var graphics = System.Drawing.Graphics.FromHwnd(Handle))
                SetWindowRgn(Handle, (new System.Drawing.Region(ConvertRect(intersect))).GetHrgn(graphics), true);
        }

        static System.Drawing.RectangleF ConvertRect(Rect r)
        {
            return new System.Drawing.RectangleF((float)r.X, (float)r.Y, (float)r.Width, (float)r.Height);
        }

        private Window _mainWindow;
        Window MainWindow
        {
            get
            {
                if (_mainWindow == null)
                    _mainWindow = Window.GetWindow(this);

                return _mainWindow;
            }
        }

        ScrollViewer ParentScrollViewer { get; set; }

        [DllImport("User32.dll", SetLastError = true)]
        public static extern int SetWindowRgn(IntPtr hWnd, IntPtr hRgn, bool bRedraw);
    }

Thats it. Every thing remains the same.