Code
        const int WM_HOTKEY = 0x0312; // 热键消息
        const int ALT_TAB= 100;//自定义
        /// <summary>
        /// 注册系统所需热键
        /// </summary>
        private void RegisterHotKey()
        {
            this.SetHotKey(Keys.TAB, ALT_TAB);
        }

        protected override void WndProc(ref Message msg)
        {
            if (msg.Msg != WM_HOTKEY)
            {
                base.WndProc(ref msg);
            }
        }

        private void SetHotKey(Keys nowKey, int id,)
        {
            KeyModifiers modifier;
                modifier |= WinApi.KeyModifiers.Control;
            WinApi.RegisterHotKey(Handle, id, modifier, nowKey);
        }

                [DllImport("user32.dll", SetLastError = true)]
        public static extern bool RegisterHotKey(
            IntPtr hWnd,
            int id,
            KeyModifiers fsModifiers,
            Keys vk
            );

        [DllImport("user32.dll", SetLastError = true)]
        public static extern bool UnregisterHotKey(
            IntPtr hWnd,
            int id
            );
        [Flags()]
        public enum KeyModifiers
        {
            None = 0,
            Alt = 1,
            Control = 2,
            Shift = 4,
            Windows = 8
        }