Windows Forms

private void vScrollBar1_Scroll(object sender, ScrollEventArgs e)
{
    MessageBox.Show("dummy");
}

private bool ok = true;
private void vScrollBar1_Scroll(object sender, ScrollEventArgs e)
{
    if (ok)
    {
        ok = false;
        MessageBox.Show("dummy");
        ok = true;
    }
}

class MyVScroll : VScrollBar
{
    private const int WM_VSCROLL = 0x2115;
    protected override void WndProc(ref Message m)
    {
        if ((m.Msg != WM_VSCROLL) || this.Capture)
        {
              base.WndProc(ref m);
        }
    }
}