C++ code:
#include <Windows.h>
#include "MSCorEE.h"
#include <metahost.h>
//#include "main.h"
#pragma unmanaged

void __cdecl StartDotNet(void);

BOOL APIENTRY DllMain(HANDLE hModule, DWORD ul_reason_for_call, LPVOID lpReeserved)
{
    switch (ul_reason_for_call)
    {
        case DLL_PROCESS_ATTACH:
            CreateThread(NULL, 0, (LPTHREAD_START_ROUTINE)&StartDotNet, 0, 0, NULL);
            break;
        case DLL_THREAD_ATTACH:
        case DLL_THREAD_DETACH:
        case DLL_PROCESS_DETACH:
            break;
    }
    return TRUE;
}

void StartDotNet()
{
    HRESULT hr;
    ICLRRuntimeHost *pClrHost = NULL;
    ICLRMetaHost *pMetaHost = NULL;
    hr = CLRCreateInstance(CLSID_CLRMetaHost, IID_ICLRMetaHost, (LPVOID*)&pMetaHost);
    //MessageBox(NULL, L"CLRCreateInstance Done.", NULL, NULL);

    ICLRRuntimeInfo * lpRuntimeInfo = NULL;

    hr = pMetaHost->GetRuntime(L"v4.0.30319", IID_ICLRRuntimeInfo, (LPVOID*)&lpRuntimeInfo);
    //MessageBox(NULL, L"pMetaHost->GetRuntime Done.", NULL, NULL);

    ICLRRuntimeHost * lpRuntimeHost = NULL;

    hr = lpRuntimeInfo->GetInterface(CLSID_CLRRuntimeHost, IID_ICLRRuntimeHost, (LPVOID *)&lpRuntimeHost);
    //MessageBox(NULL, L"lpRuntimeInfo->GetInterface Done.", NULL, NULL);

    hr = lpRuntimeHost->Start();
    //MessageBox(NULL, L"lpRuntimeHost->Start() Done.", NULL, NULL);

    DWORD dwRet = 0;

    hr = lpRuntimeHost->ExecuteInDefaultAppDomain(
        L"C:\\AOInject.dll",
        L"Namespace.Program", L"DllMain", L"Injection Worked", &dwRet);

    lpRuntimeHost->Release();

}

C# Code
using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.Linq;
using System.Runtime.InteropServices;
using System.Text;
using System.Windows.Forms;

namespace Namespace
{
    public class Program
    {
        [DllImport("User32", EntryPoint = "MessageBoxA")]
        static extern int MessageBox(IntPtr hWnd, string text, string title, int type);
        public static int DllMain(string args)
        {
            Process currentProcess = Process.GetCurrentProcess();
            string text = string.Format("Process: {0}\nMessage: {1}", currentProcess.ProcessName, args);
            MessageBox(IntPtr.Zero, text, "", 0);
            return 0;
        }
    }
}