Catching resize action with WndProc

Question

I'm trying to catpure the moment when the user resize a custom control dragging one of its border using WndProc.

I want to call a particular method only when, for example, the left border is dragged (mouse left button down). For now I only catch the mouse position and not the proper dragging action

This is the code I use:

const int WM_NCHITTEST = 0x84;
const int WM_SETCURSOR = 0x20;
const int WM_NCLBUTTONDBLCLK = 0xA3;

protected override void WndProc(ref Message m)
    {
        int borderWidth = (int)_drawingControlPanel.ImageScale;
        if (m.Msg == WM_SETCURSOR)  /*Setting cursor to SizeAll*/
        {
            if ((m.LParam.ToInt32() & 0xffff) == 0x2 /*Move*/)
            {
                Cursor.Current = Cursors.SizeAll;
                m.Result = (IntPtr)1;
                return;
            }
        }

        if ((m.Msg == WM_NCLBUTTONDBLCLK)) /*Disable Mazimiz on Double click*/
        {
            m.Result = (IntPtr)1;
            return;
        }

        base.WndProc(ref m);

        if (m.Msg == WM_NCHITTEST)
        {
            var pos = PointToClient(new Point(m.LParam.ToInt32() & 0xffff, m.LParam.ToInt32() >> 16));

            if (pos.X <= ClientRectangle.Left + borderWidth && pos.Y <= ClientRectangle.Top + borderWidth)
                m.Result = new IntPtr(13); //TOPLEFT
            else if (pos.X >= ClientRectangle.Right - borderWidth && pos.Y <= ClientRectangle.Top + borderWidth)
                m.Result = new IntPtr(14); //TOPRIGHT
            else if (pos.X <= ClientRectangle.Left + borderWidth && pos.Y >= ClientRectangle.Bottom - borderWidth)
                m.Result = new IntPtr(16); //BOTTOMLEFT
            else if (pos.X >= ClientRectangle.Right - borderWidth && pos.Y >= ClientRectangle.Bottom - borderWidth)
                m.Result = new IntPtr(17); //BOTTOMRIGHT
            else if (pos.X <= ClientRectangle.Left + borderWidth)
                m.Result = new IntPtr(10); //LEFT
            else if (pos.Y <= ClientRectangle.Top + borderWidth)
                m.Result = new IntPtr(12); //TOP
            else if (pos.X >= ClientRectangle.Right - borderWidth)
                m.Result = new IntPtr(11); //RIGHT
            else if (pos.Y >= ClientRectangle.Bottom - borderWidth)
                m.Result = new IntPtr(15); //Bottom
            else
                m.Result = new IntPtr(2); //Move
        }
    }

Answer 1

How about using the WM_SIZING message instead? From MSDN:

Sent to a window that the user is resizing. By processing this message, an application can monitor the size and position of the drag rectangle and, if needed, change its size or position.

The lParam represents the edge (border) being resized.