最小化窗口

点击“X”或“Alt+F4”时，最小化窗口，如：

protected override void WndProc(ref Message m)
{
    const int WM_SYSCOMMAND = 0x0112;
    const int SC_CLOSE = 0xF060;
    if (m.Msg == WM_SYSCOMMAND && (int)m.WParam == SC_CLOSE)
    {
        // User clicked close button
        this.WindowState = FormWindowState.Minimized;
        return;
    }
    base.WndProc(ref m);
}