using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace Sysmenu
{
    public partial class Form1 : Form
    {
        [StructLayout(LayoutKind.Sequential, CharSet = CharSet.Auto)]
        private struct MENUITEMINFO
        {
            public uint cbSize;
            public uint fMask;
            public uint fType;
            public uint fState;
            public uint wID;
            public IntPtr hSubMenu;
            public IntPtr hbmpChecked;
            public IntPtr hbmpUnchecked;
            public IntPtr dwItemData;
            public string dwTypeData;
            public uint cch;
            public IntPtr hbmpItem;
        }

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

        [DllImport("user32", CharSet = CharSet.Auto)]
        private static extern bool InsertMenuItem(IntPtr hMenu, uint uItem, bool fByPosition, ref MENUITEMINFO lpmii);

        private const uint MenuID = 0x1111;

        public Form1()
        {
            InitializeComponent();
        }

        protected override void OnCreateControl()
        {
            base.OnCreateControl();

            IntPtr hSysMenu = GetSystemMenu(this.Handle, false);

            MENUITEMINFO item = new MENUITEMINFO();
            item.cbSize = (uint)Marshal.SizeOf(item);
            item.fMask = 0x42;  // MIIM_STRING | MIIM_ID
            item.wID = MenuID;
            item.dwTypeData = "新しく追加した項目";

            InsertMenuItem(hSysMenu, 0, false, ref item);
        }

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

            if (m.Msg == 0x0112)    // WM_SYSCOMMAND
            {
                if ((m.WParam.ToInt32() & 0xfffF) == MenuID)
                {
                    MessageBox.Show("新しく追加した項目が選択されました。");
                }
            }
        }
    }
}