La "Class level"

public const int WM_NCLBUTTONDOWN = 0xA1;
public const int HTCAPTION = 0x2;
[DllImport("User32.dll")]
public static extern bool ReleaseCapture();
[DllImport("User32.dll")]
public static extern int SendMessage(IntPtr hWnd, int Msg, int wParam, int lParam);

private void OnMouseDown(object sender, MouseEventArgs e)
{
    if (e.Button == MouseButtons.Left)
    {
        ReleaseCapture();
        SendMessage(Handle, WM_NCLBUTTONDOWN, HTCAPTION, 0);
    }
}

WM_NCHITTEST

WndProc

HTBOTTOMRIGHT

protected override void WndProc(ref Message m)
{
    if (m.Msg == 0x84)
    { 
        const int resizeArea = 10;
        Point cursorPosition = PointToClient(new Point(m.LParam.ToInt32() & 0xffff, m.LParam.ToInt32() >> 16));
        if (cursorPosition.X >= ClientSize.Width - resizeArea && cursorPosition.Y >= ClientSize.Height - resizeArea )
        {
            m.Result = (IntPtr)17;
            return;
        }
    }

    base.WndProc(ref m);
}

WM_NCHITTEST

WndProc

HTBOTTOMRIGHT