C#：调用系统api移动无边框窗体，重载doubleclick无效？

Question

我新建一标签，想对其进行移动：
Label内：        [DllImport("user32.dll")]
        public static extern bool ReleaseCapture();
        [DllImport("user32.dll")]
        public static extern bool SendMessage(IntPtr hwnd, int wMsg, int wParam, int lParam);
        public const int WM_SYSCOMMAND = 0x0112;
        public const int SC_MOVE = 0xF010;
        public const int HTCAPTION = 0x0002;        protected override void OnMouseDown(MouseEventArgs e)
        {
            ReleaseCapture();
            SendMessage(this.Handle, WM_SYSCOMMAND, SC_MOVE + HTCAPTION, 0);
            base.OnMouseDown(e);
        }然后我重载其doubleclick方法，发现我虽然对其双击，但不执行！        protected override void OnDoubleClick(EventArgs e)
        {
            CustUserCtrl custCtrl = new CustUserCtrl();
            custCtrl.Left = this.Left;
            custCtrl.Top = this.Top + this.Height - 3;
            this.Controls.Add(custCtrl);
            base.OnDoubleClick(e);
        }
请问是什么原因啊？