ListBox with scroll events

Simple extension of ListBox provides you vertical scroll event.

public class ListBoxScroll : ListBox
{
    private const int WM_VSCROLL = 0x0115;
    private const int WM_HSCROLL = 0x0114;
    private const int WM_MOUSEWHEEL = 0x020A;
    private const int WM_MOUSEHWHEEL = 0x020E;
    private const int SB_THUMBTRACK = 5;
    private const int SB_ENDSCROLL = 8;

    public delegate void ListBoxScrollDelegate(object a_sender, bool a_tracking);

    public event ListBoxScrollDelegate VerticalScroll;

    protected override void WndProc(ref Message m)
    {
        base.WndProc(ref m);

        if (m.Msg == WM_MOUSEWHEEL)
        {
            if (VerticalScroll != null)
                VerticalScroll(this, false);
        }

        if (m.Msg == WM_MOUSEHWHEEL)
        {
            if (VerticalScroll != null)
                VerticalScroll(this, false);
        }

        if (m.Msg == WM_VSCROLL)
        {
            int nfy = m.WParam.ToInt32() & 0xFFFF;
            if (VerticalScroll != null && (nfy == SB_THUMBTRACK || nfy ==
                   SB_ENDSCROLL))
            {
                VerticalScroll(this, nfy == SB_THUMBTRACK);
            }
        }
    }
}