Global keyboard hook works, but global shell hook fails

Question

I have two pieces of code which are very similar in that they register global hooks.

Registering global keyboard hook:

public class KeyboardHook : IDisposable
{
    #region Events
    private delegate IntPtr LowLevelKeyboardProc(int nCode, IntPtr wParam, IntPtr lParam);
    public delegate void HookEventHandler(object sender, KeyboardHookEventArgs e);
    public event HookEventHandler KeyDown;
    public event HookEventHandler KeyUp;
    #endregion

    #region Constants
    private const int WH_KEYBOARD_LL = 13;
    private const int WM_KEYDOWN = 0x0100;
    private const int WM_SYSKEYDOWN = 0x0104;
    private LowLevelKeyboardProc _proc = null;
    private static IntPtr _hookID = IntPtr.Zero;
    #endregion

    #region Imports
    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr SetWindowsHookEx(int idHook,
        LowLevelKeyboardProc lpfn, IntPtr hMod, uint dwThreadId);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    [return: MarshalAs(UnmanagedType.Bool)]
    private static extern bool UnhookWindowsHookEx(IntPtr hhk);

    [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr CallNextHookEx(IntPtr hhk, int nCode,
        IntPtr wParam, IntPtr lParam);

    [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
    private static extern IntPtr GetModuleHandle(string lpModuleName);

    #endregion

    #region Constructor
    public KeyboardHook()
    {
        _proc = new LowLevelKeyboardProc(HookCallback); 
        _hookID = SetHook(_proc);
    }

    #endregion


    #region Methods
    private IntPtr SetHook(LowLevelKeyboardProc proc)
    {
        using (Process curProcess = Process.GetCurrentProcess())
        using (ProcessModule curModule = curProcess.MainModule)
        {
            return SetWindowsHookEx(WH_KEYBOARD_LL, proc,
                GetModuleHandle(curModule.ModuleName), 0);
        }
    }

    private IntPtr HookCallback(int nCode, IntPtr wParam, IntPtr lParam)
    {
        //if (nCode >= 0 && wParam == (IntPtr)WM_KEYDOWN)
        if (nCode >= 0 && (wParam == (IntPtr)WM_KEYDOWN || wParam == (IntPtr)WM_SYSKEYDOWN))
        {
            int vkCode = Marshal.ReadInt32(lParam);
            WinForms.Keys key = (WinForms.Keys)vkCode;
            if (this.KeyDown != null)
                this.KeyDown(this, new KeyboardHookEventArgs(vkCode));
        }
        return CallNextHookEx(_hookID, nCode, wParam, lParam);
    }

    #endregion

    #region Destructor
    public void Dispose()
    {
        UnhookWindowsHookEx(_hookID);
    }
    #endregion
}

Registering global shell hook:

    public enum ShellEvents
    {
        HSHELL_WINDOWCREATED = 1,
        HSHELL_WINDOWDESTROYED = 2,
        HSHELL_ACTIVATESHELLWINDOW = 3,
        HSHELL_WINDOWACTIVATED = 4,
        HSHELL_GETMINRECT = 5,
        HSHELL_REDRAW = 6,
        HSHELL_TASKMAN = 7,
        HSHELL_LANGUAGE = 8,
        HSHELL_ACCESSIBILITYSTATE = 11
    }

    public class ShellHook
    {
        #region Events
        private delegate IntPtr ShellProc(int nCode, IntPtr wParam, IntPtr lParam);
        public delegate void HookEventHandler(object sender, ShellHookEventArgs e);
        public event HookEventHandler WindowActivated;
        #endregion

        #region Constants
        private const int WH_SHELL = 10;
        private ShellProc _proc = null;
        private static IntPtr _hookID = IntPtr.Zero;
        #endregion

        #region Imports
        [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        private static extern IntPtr SetWindowsHookEx(int idHook,
            ShellProc lpfn, IntPtr hMod, uint dwThreadId);

        [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        [return: MarshalAs(UnmanagedType.Bool)]
        private static extern bool UnhookWindowsHookEx(IntPtr hhk);

        [DllImport("user32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        private static extern IntPtr CallNextHookEx(IntPtr hhk, int nCode,
            IntPtr wParam, IntPtr lParam);

        [DllImport("kernel32.dll", CharSet = CharSet.Auto, SetLastError = true)]
        private static extern IntPtr GetModuleHandle(string lpModuleName);

        [DllImport("user32.dll")]
        static extern int GetWindowText(IntPtr hWnd, StringBuilder text, int count);
        #endregion

        #region Fields
        #endregion

        #region Constructor
        public ShellHook()
        {
            _proc = new ShellProc(HookCallback); 
            _hookID = SetHook(_proc);
        }
        #endregion

        #region Methods
        private IntPtr SetHook(ShellProc proc)
        {
            using (Process curProcess = Process.GetCurrentProcess())
            using (ProcessModule curModule = curProcess.MainModule)
            {
                return SetWindowsHookEx(WH_SHELL, proc,
                    GetModuleHandle(curModule.ModuleName), 0);
            }
        }

        private IntPtr HookCallback(int nCode, IntPtr wParam, IntPtr lParam)
        {
            if (nCode >= 0 && wParam.Equals(ShellEvents.HSHELL_WINDOWACTIVATED))
            {
                string windowTitle = GetWindowTitle(wParam);
                if (this.WindowActivated != null)
                    this.WindowActivated(this, new ShellHookEventArgs(windowTitle));
            }
            return CallNextHookEx(_hookID, nCode, wParam, lParam);
        }

        private string GetWindowTitle(IntPtr hWnd)
        {
            const int nChars = 256;
            StringBuilder Buff = new StringBuilder(nChars);

            if (GetWindowText(hWnd, Buff, nChars) > 0)
            {
                return Buff.ToString();
            }
            return null;
        }
        #endregion

        #region Destructor
        public void Dispose()
        {
            UnhookWindowsHookEx(_hookID);
        }
        #endregion
    }

For some reason, the keyboard hook works, but the shell hook fails (SetWindowsHookEx returns 0 and the callback is never reached).

Any ideas why?

Answer 1

I think I'll answer my own question. This quote from pinvoke.net explains why...

However, you cannot implement global hooks in Microsoft .NET Framework except low level hooks. To install a global hook, a hook must have a native dynamic-link library (DLL) export to inject itself in another process that requires a valid, consistent function to call into. This requires a DLL export, which .NET Framework does not support. Managed code has no concept of a consistent value for a function pointer because these function pointers are proxies that are built dynamically.