完整代碼，原創無藏私，絕對實用。Windows10 X64 下調試通過，對 w3wp.exe, sqlserver.exe,notepad.exe,iexporer.exe 註入後，長時間運行穩定，未見異常。

要註入的全局dll（需強命名）：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Runtime.InteropServices;
using EasyHook;
using System.Threading;
using System.Diagnostics;
 
using System.Windows.Forms;

namespace ClassLibrary1
{
    [Serializable]
    public class HookParameter
    {
        public string Msg { get; set; }
        public int HostProcessId { get; set; }
    }

    public class Main : EasyHook.IEntryPoint
    {
        public LocalHook MessageBoxWHook = null
 
;
        public LocalHook MessageBoxAHook = null;

        public Main(
            RemoteHooking.IContext context,
            String channelName
            , HookParameter parameter
            )
        {
            MessageBox.Show(parameter.Msg, "Hooked");
        }

        public void Run(
            RemoteHooking.IContext context,
            String channelName
            , HookParameter parameter
            )
        {
            
 
try
            {
                MessageBoxWHook = LocalHook.Create(
                    LocalHook.GetProcAddress("user32.dll", "MessageBoxW"),
                    new DMessageBoxW(MessageBoxW_Hooked),
                    this);
                MessageBoxWHook.ThreadACL.SetExclusiveACL(new Int32[1]);

                MessageBoxAHook = LocalHook.Create(
                    LocalHook.GetProcAddress("user32.dll", "MessageBoxA"),
                    new DMessageBoxW(MessageBoxA_Hooked),
                    this);
                MessageBoxAHook.ThreadACL.SetExclusiveACL(new Int32[1]);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message);
                return;
            }

            try
            {
                while (true)
                {
                    Thread.Sleep(10);
                }
            }
            catch
            {

            }
        }

        #region MessageBoxW

        [DllImport("user32.dll", EntryPoint = "MessageBoxW", CharSet = CharSet.Unicode)]
        public static extern IntPtr MessageBoxW(int hWnd, string text, string caption, uint type);

        [UnmanagedFunctionPointer(CallingConvention.StdCall, CharSet = CharSet.Unicode)]
        delegate IntPtr DMessageBoxW(int hWnd, string text, string caption, uint type);

        static IntPtr MessageBoxW_Hooked(int hWnd, string text, string caption, uint type)
        {
            return MessageBoxW(hWnd, "Hooked - " + text, "Hooked - " + caption, type);
        }

        #endregion

        #region MessageBoxA

        [DllImport("user32.dll", EntryPoint = "MessageBoxA", CharSet = CharSet.Ansi)]
        public static extern IntPtr MessageBoxA(int hWnd, string text, string caption, uint type);

        [UnmanagedFunctionPointer(CallingConvention.StdCall, CharSet = CharSet.Ansi)]
        delegate IntPtr DMessageBoxA(int hWnd, string text, string caption, uint type);

        static IntPtr MessageBoxA_Hooked(int hWnd, string text, string caption, uint type)
        {
            return MessageBoxA(hWnd, "Hooked - " + text, "Hooked - " + caption, type);
        }

        #endregion
    }
}

註入主程序：

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;
using System.IO;
using System.Threading;
using System.Reflection;
using ClassLibrary1;
using EasyHook;
using System.Diagnostics;
using System.Runtime.InteropServices;

namespace WindowsFormsApplication8
{
    public partial class Form1 : Form
    {
        [DllImport("kernel32.dll", SetLastError = true, CallingConvention = CallingConvention.Winapi)]
        [return: MarshalAs(UnmanagedType.Bool)]
        internal static extern bool IsWow64Process([In] IntPtr process, [Out] out bool wow64Process);

        public Form1()
        {
            InitializeComponent();
        }

        private bool RegGACAssembly()
        {
            var dllName = "EasyHook.dll";
            var dllPath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, dllName);
            if (!System.Runtime.InteropServices.RuntimeEnvironment.FromGlobalAccessCache(Assembly.LoadFrom(dllPath)))
            {
                new System.EnterpriseServices.Internal.Publish().GacInstall(dllPath);
                Thread.Sleep(100);
            }

            dllName = "ClassLibrary1.dll";
            dllPath = Path.Combine(AppDomain.CurrentDomain.BaseDirectory, dllName);
            new System.EnterpriseServices.Internal.Publish().GacRemove(dllPath);
            if (!System.Runtime.InteropServices.RuntimeEnvironment.FromGlobalAccessCache(Assembly.LoadFrom(dllPath)))
            {
                new System.EnterpriseServices.Internal.Publish().GacInstall(dllPath);
                Thread.Sleep(100);
            }

            return true;
        }

        private static bool InstallHookInternal(int processId)
        {
            try
            {
                var parameter = new HookParameter
                {
                    Msg = "已經成功註入目標進程",
                    HostProcessId = RemoteHooking.GetCurrentProcessId()
                };

                RemoteHooking.Inject(
                    processId,
                    InjectionOptions.Default,
                    typeof(HookParameter).Assembly.Location,
                    typeof(HookParameter).Assembly.Location,
                    string.Empty,
                    parameter
                );
            }
            catch (Exception ex)
            {
                Debug.Print(ex.ToString());
                return false;
            }

            return true;
        }

        private static bool IsWin64Emulator(int processId)
        {
            var process = Process.GetProcessById(processId);
            if (process == null)
                return false;

            if ((Environment.OSVersion.Version.Major > 5)
                || ((Environment.OSVersion.Version.Major == 5) && (Environment.OSVersion.Version.Minor >= 1)))
            {
                bool retVal;

                return !(IsWow64Process(process.Handle, out retVal) && retVal);
            }

            return false; // not on 64-bit Windows Emulator
        }

        private void button1_Click(object sender, EventArgs e)
        {
            var p = Process.GetProcessById(int.Parse(textBox1.Text));
            if (p == null)
            {
                MessageBox.Show("指定的進程不存在!");
                return;
            }

            if(IsWin64Emulator(p.Id) != IsWin64Emulator(Process.GetCurrentProcess().Id))
            {
                var currentPlat = IsWin64Emulator(Process.GetCurrentProcess().Id) ? 64 : 32;
                var targetPlat = IsWin64Emulator(p.Id) ? 64 : 32;
                MessageBox.Show(string.Format("當前程序是{0}位程序，目標進程是{1}位程序，請調整編譯選項重新編譯後重試！", currentPlat, targetPlat));
                return;
            }

            RegGACAssembly();
            InstallHookInternal(p.Id);
        }

        private void Form1_Load(object sender, EventArgs e)
        {

        }
    }
}

完整代碼下載地址：http://download.csdn.net/download/nanfei01055/9999598

C# EasyHook MessageBox 示例（極簡而全）