SetWindowsHookEx in C#

Question

I'm trying to hook a 3rd party app so that I can draw to its screen. Drawing to the screen is easy, and I need no help with it, but I seem to be having issues with using SetWindowsHookEx to handle WH_GETMESSAGE. I can't figure out what to pass for the last two parameters.

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Windows.Forms;

namespace WindowDrawer
{
    public partial class Form1 : Form
    {
        private delegate int HookProc(int code, IntPtr wParam, IntPtr lParam);
        static IntPtr hHook;
        IntPtr windowHandle;
        uint processHandle;

        HookProc PaintHookProcedure;     

        [System.Runtime.InteropServices.DllImport("user32.dll", EntryPoint = "FindWindow", SetLastError = true)]
        static extern System.IntPtr FindWindowByCaption(int ZeroOnly, string lpWindowName);

        [System.Runtime.InteropServices.DllImport("user32.dll", EntryPoint = "SetWindowsHookEx", SetLastError = true)]
        static extern IntPtr SetWindowsHookEx(int idHook, HookProc lpfn, IntPtr hMod, uint dwThreadId);

        [System.Runtime.InteropServices.DllImport("user32.dll")]
        static extern int CallNextHookEx(IntPtr hhk, int nCode, IntPtr wParam, IntPtr lParam);

        // When you don't want the ProcessId, use this overload and pass IntPtr.Zero for the second parameter
        [System.Runtime.InteropServices.DllImport("user32.dll")]
        static extern uint GetWindowThreadProcessId(IntPtr hWnd, out uint lpdwProcessId);

        [System.Runtime.InteropServices.DllImport("kernel32.dll", CharSet =System.Runtime.InteropServices.CharSet.Auto)]
        public static extern IntPtr GetModuleHandle(string lpModuleName);

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        { 
            PaintHookProcedure = new HookProc(PaintHookProc);
            windowHandle = FindWindowByCaption(0, "Untitled - Notepad");
            uint threadID = GetWindowThreadProcessId(windowHandle, out processHandle);
            IntPtr hMod = System.Runtime.InteropServices.Marshal.GetHINSTANCE(typeof(Form1).Module);

            // HERE IS THE PROBLEM.  WHAT THE HECK DO I PASS INTO THE LAST 2 PARAMS?  I get a null pointer
            hHook = SetWindowsHookEx(WH_GETMESSAGE, PaintHookProcedure, hMod, threadID);
        }

        public int PaintHookProc(int nCode, IntPtr wParam, IntPtr lParam)
        {
           // Do some painting here.
            return CallNextHookEx(hHook, nCode, wParam, lParam); 
        }

        private const int WM_PAINT = 15;
        private const int WH_GETMESSAGE = 3;
    }
}

Answer 1

  internal enum HookType : uint {
    WH_JOURNALRECORD = 0,
    WH_JOURNALPLAYBACK = 1,
    WH_KEYBOARD = 2,
    WH_GETMESSAGE = 3,
    WH_CALLWNDPROC = 4,
    WH_CBT = 5,
    WH_SYSMSGFILTER = 6,
    WH_MOUSE = 7,
    WH_HARDWARE = 8,
    WH_DEBUG = 9,
    WH_SHELL = 10,
    WH_FOREGROUNDIDLE = 11,
    WH_CALLWNDPROCRET = 12,
    WH_KEYBOARD_LL = 13,
    WH_MOUSE_LL = 14
  }

Global hooks are not supported in the .NET Framework

Except for the WH_KEYBOARD_LL low-level hook and the WH_MOUSE_LL low-level hook, you cannot implement global hooks in the Microsoft .NET Framework.

To install a global hook, a hook must have a native DLL export to inject itself in another process that requires a valid, consistent function to call into. This behavior requires a DLL export.

The .NET Framework does not support DLL exports. Managed code has no concept of a consistent value for a function pointer because these function pointers are proxies that are built dynamically.

Low-level hook procedures are called on the thread that installed the hook. Low-level hooks do not require that the hook procedure be implemented in a DLL.

See also: Snoop - The WPF Spy Utility

Or my WPF => WF => Win32 LL_Keyboard Hook Proxy w/ Gestures

Answer 2

I know that this is an old question but I'm hoping that there is still someone who will find this useful. I think that you are mixing up int and IntPtr

[System.Runtime.InteropServices.DllImport("user32.dll")]
static extern int CallNextHookEx(IntPtr hhk, int nCode, IntPtr wParam, IntPtr lParam);

Answer 3

This work for me use 13...

 private static IntPtr SetHook(LowLevelKeyboardProc proc)
        {
            using (Process curProcess = Process.GetCurrentProcess())
            using (ProcessModule curModule = curProcess.MainModule)
            {
                return SetWindowsHookEx(13, proc,
                GetModuleHandle(curModule.ModuleName), 0);
            }
        }