C# 无边框窗体随意拖动和改变尺寸

const int WM_NCHITTEST = 0x0084;
const int HTLEFT = 10;
const int HTRIGHT = 11;
const int HTTOP = 12;
const int HTTOPLEFT = 13;
const int HTTOPRIGHT = 14;
const int HTBOTTOM = 15;
const int HTBOTTOMLEFT = 0x10;
const int HTBOTTOMRIGHT = 17;

protected override void WndProc(ref Message m)
{
    base.WndProc(ref m);
    switch (m.Msg)
    {
        case WM_NCHITTEST:
            Point vPoint = new Point((int)m.LParam & 0xFFFF,
                (int)m.LParam >> 16 & 0xFFFF);
            vPoint = PointToClient(vPoint);
            if (vPoint.X <= 5)
                if (vPoint.Y <= 5)
                    m.Result = (IntPtr)HTTOPLEFT;
                else if (vPoint.Y >= ClientSize.Height - 5)
                    m.Result = (IntPtr)HTBOTTOMLEFT;
                else m.Result = (IntPtr)HTLEFT;
            else if (vPoint.X >= ClientSize.Width - 5)
                if (vPoint.Y <= 5)
                    m.Result = (IntPtr)HTTOPRIGHT;
                else if (vPoint.Y >= ClientSize.Height - 5)
                    m.Result = (IntPtr)HTBOTTOMRIGHT;
                else m.Result = (IntPtr)HTRIGHT;
            else if (vPoint.Y <= 5)
                m.Result = (IntPtr)HTTOP;
            else if (vPoint.Y >= ClientSize.Height - 5)
                m.Result = (IntPtr)HTBOTTOM;
            break;
    }
}