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  [DllImport("user32", CharSet = CharSet.Ansi, SetLastError = true, ExactSpelling = true)]
        private static extern IntPtr GetSystemMenu(IntPtr hWnd, bool bRevert);

        [DllImport("user32.dll")]
        private static extern int AppendMenu(int hMenu, int Flagsw, int IDNewItem, string lpNewItem);

        [DllImport("user32.dll")]
        static extern uint TrackPopupMenuEx(IntPtr hmenu, uint uFlags, int x, int y, IntPtr hwnd, IntPtr lptpm);

        [DllImport("user32.dll")]
        public static extern bool SetForegroundWindow(IntPtr hWnd);

        [DllImport("user32.dll", CharSet = CharSet.Auto)]
        public static extern bool PostMessage(IntPtr hhwnd, uint msg, IntPtr wparam,

        IntPtr lparam);

        private const uint TPM_LEFTBUTTON = (uint)0x0000L;
        private const uint TPM_RIGHTBUTTON = (uint)0x0002L;

        private const uint TPM_NONOTIFY = (uint)0x0080L;
        private const uint TPM_RETURNCMD = (uint)0x0100L;

        private const uint WM_SYSCOMMAND = (uint)0x0112L;

        public static void ShowContextMenu(IntPtr hWnd)
        {
            SetForegroundWindow(hWnd);
            IntPtr wMenu = GetSystemMenu(hWnd, false);

            uint command = TrackPopupMenuEx(wMenu, TPM_LEFTBUTTON | TPM_RETURNCMD, 0, 0, hWnd, IntPtr.Zero);

            if (command == 0)
            {
                return;
            }

            PostMessage(hWnd, WM_SYSCOMMAND, new IntPtr(command), IntPtr.Zero);
        }

        private void button1_Click(object sender, EventArgs e)
        {
            ShowContextMenu(this.Handle);
        }                    

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        private void SetupSystemMenu()
        {
            IntPtr menu = GetSystemMenu(this.Handle, false);

            AppendMenu(menu, 0xA00, 0, null);
            AppendMenu(menu, 0, 111, "Test?");
        }

        protected override void WndProc(ref Message m)
        {
            base.WndProc(ref m);

            if (m.Msg == 0x112)
            {
                if (m.WParam.ToInt32() == 111)
                {
                    MessageBox.Show("Test!");
                }
            }
        }