Monitoring context menu opened (SetWinEventHook)

ContextMenuObserverWin32.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

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

class ContextMenuObserverWin32 : Form
{
    [STAThread]
    public static void Main()
    {
        Application.Run(new ContextMenuObserverWin32());
    }

    [DllImport("user32.dll")]
    static extern IntPtr SetWinEventHook(uint eventMin, uint eventMax, IntPtr hmodWinEventProc,
                                         WinEventDelegate lpfnWinEventProc, uint idProcess,
                                         uint idThread, uint dwFlags);

    [DllImport("user32.dll")]
    static extern bool UnhookWinEvent(IntPtr hWinEventHook);

    delegate void WinEventDelegate(IntPtr hWinEventHook, uint eventType,
                                   IntPtr hwnd, int idObject, int idChild, uint dwEventThread, uint dwmsEventTime);

    public const uint WINEVENT_OUTOFCONTEXT = 0x0000;
    public const uint WINEVENT_SKIPOWNTHREAD = 0x0001;
    public const uint WINEVENT_SKIPOWNPROCESS = 0x0002;
    public const uint WINEVENT_INCONTEXT = 0x0004;
    public const uint EVENT_SYSTEM_MENUPOPUPSTART = 0x0006;

    private Label _result;
    private IntPtr _hook = IntPtr.Zero;

    ContextMenuObserverWin32()
    {
        _result = new Label();
        _result.AutoSize = true;
        _result.Location = new Point(10, 10);

        Controls.AddRange(new Control[] { _result });
        this.TopMost = true;
        this.AutoSize = true;
        this.MinimizeBox = false;
        this.MaximizeBox = false;
        this.Text = "ContextMenu Observer Win32";

        _hook = SetWinEventHook(EVENT_SYSTEM_MENUPOPUPSTART, EVENT_SYSTEM_MENUPOPUPSTART, IntPtr.Zero,
                                WindowEventCallback, 0, 0, WINEVENT_OUTOFCONTEXT | WINEVENT_SKIPOWNPROCESS);
    }

    protected override void OnClosed(EventArgs e)
    {
        if (_hook != IntPtr.Zero)
        {
            UnhookWinEvent(_hook);
            _hook = IntPtr.Zero;
        }
        base.OnClosed(e);
    }

    private void WindowEventCallback(IntPtr hWinEventHook, uint eventType,
                                     IntPtr hwnd, int idObject, int idChild, uint dwEventThread, uint dwmsEventTime)
    {
        _result.Text = Win32Menu.GetMenuTexts(hwnd);
        if (string.IsNullOrEmpty(_result.Text))
        {
            _result.Text = "コールバックされたけど、メニュー取得できなかったよ";
        }
    }
}

class Win32Menu
{
    #region Win32Interop
    const int MFS_DISABLED = 0x00000003;
    const int MIIM_STATE = 0x00000001;
    const int MIIM_ID = 0x00000002;
    const int MIIM_SUBMENU = 0x00000004;
    const int MIIM_STRING = 0x00000040;
    const int MN_GETHMENU = 0x01E1;

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    static extern IntPtr SendMessage(IntPtr hWnd, int Msg, IntPtr wParam, IntPtr lParam);

    [DllImport("user32.dll")]
    static extern int GetMenuItemCount(IntPtr hMenu);

    [DllImport("user32.dll", CharSet = CharSet.Unicode)]
    static extern bool GetMenuItemInfo(IntPtr hMenu, int uItem, bool fByPosition, [In, Out] MENUITEMINFO lpmii);

    [StructLayout(LayoutKind.Sequential, CharSet=CharSet.Unicode)]
    public class MENUITEMINFO
    {
        public int cbSize = Marshal.SizeOf(typeof(MENUITEMINFO));
        public int fMask;
        public int fType;
        public int fState;
        public int wID;
        public IntPtr hSubMenu;
        public IntPtr hbmpChecked;
        public IntPtr hbmpUnchecked;
        public IntPtr dwItemData;
        public IntPtr dwTypeData;
        public int cch;
    }
    #endregion

    public static string GetMenuTexts(IntPtr hwnd)
    {
        var results = "";
        var hMenu = SendMessage(hwnd, MN_GETHMENU, IntPtr.Zero, IntPtr.Zero);

        if (hMenu == IntPtr.Zero)
        {
            return results;
        }

        var count = GetMenuItemCount(hMenu);

        var itemInfo = new MENUITEMINFO();
        itemInfo.fMask = MIIM_STRING | MIIM_STATE | MIIM_SUBMENU | MIIM_ID;

        for (var i = 0; i < count; ++i)
        {
            var itemName = GetMenuItemName(hMenu, i, ref itemInfo);

            if (string.IsNullOrEmpty(itemName))
            {
                continue;
            }

            results += itemName + "\n\n";
        }

        return results;
    }
            
    private static string GetMenuItemName(IntPtr hMenu, int index, ref MENUITEMINFO itemInfo)
    {
        itemInfo.dwTypeData = IntPtr.Zero;
        GetMenuItemInfo(hMenu, index, true, itemInfo);

        ++itemInfo.cch;
        itemInfo.dwTypeData = Marshal.AllocHGlobal(itemInfo.cch * sizeof(char));
        GetMenuItemInfo(hMenu, index, true, itemInfo);

        var itemName = Marshal.PtrToStringUni(itemInfo.dwTypeData);
        Marshal.FreeHGlobal(itemInfo.dwTypeData);
        return itemName;
    }
}