Restricting the Drag of a WPF Window in Predefined Bounds

Environment

Product	UI for WPF
Version	2025.2.521

Description

How to restict the dragging of a WPF Window to a specific bounding rectangle.

Solution

To restrict the dragging of a window, you can add a WndProct hook to the HwndSource of the Window object.

    private RECT restrictedBounds = new RECT() { Left = 150, Top = 150, Right = 795, Bottom = 577 };

    private void HookWindowHandler(Window window)
    {   
        // note that in order for the PresentationSource.FromVisual() method call to return a HwndSource the Show() method of the window should be already called
        mSource = (HwndSource)PresentationSource.FromVisual(window);
        mSource.AddHook(WndProc);
    }    

    private IntPtr WndProc(IntPtr hwnd, int msg, IntPtr wParam, IntPtr lParam, ref bool handled)
    {
        if (msg == (int)WindowsMessage.WM_MOVING)
        {
            RECT window = (RECT)Marshal.PtrToStructure(lParam, typeof(RECT));
            if (window.Left < restrictedBounds.Left)
            {
                window.Right = window.Right + restrictedBounds.Left - window.Left;
                window.Left = restrictedBounds.Left;
            }
            if (window.Top < restrictedBounds.Top)
            {
                window.Bottom = window.Bottom + restrictedBounds.Top - window.Top;
                window.Top = restrictedBounds.Top;
            }
            if (window.Right >= restrictedBounds.Right)
            {
                window.Left = restrictedBounds.Right - window.Right + window.Left - 1;
                window.Right = restrictedBounds.Right - 1;
            }
            if (window.Bottom >= restrictedBounds.Bottom)
            {
                window.Top = restrictedBounds.Bottom - window.Bottom + window.Top - 1;
                window.Bottom = restrictedBounds.Bottom - 1;
            }
            Marshal.StructureToPtr(window, lParam, true);

            handled = true;
            return new IntPtr(1);
        }

        handled = false;
        return IntPtr.Zero;
    }   

    [StructLayout(LayoutKind.Sequential)]
    struct RECT
    {
        public int Left, Top, Right, Bottom;
    }

    enum WindowsMessage
    {
        WM_MOVING = 0x0216
    }