You could use the cool C++ Marshaling library provided by Microsoft, something like this:

#include "cSharpRiJHarn"
#include "stdafx.h"
#include <string>
#include <stdio.h>
#include "msclr\marshal_cppstd.h" // marshaling library

using namespace cSharpRiJHarn;
using namespace System;
using namespace msclr::interop; // marshaling library

std::wstring Encrypt(std::wstring s)
{
    return marshal_as<std::wstring>(RijndaelLink::encrypt(marshal_as<String^>(s)));
}

std::wstring Decrypt(std::wstring s)
{
    return marshal_as<std::wstring>(RijndaelLink::decrypt(marshal_as<String^>(s)));
}