typedef void(* CallbackResult)(int progress, char *result);
extern"C" _declspec(dllexport)
void subscribe0(char* eventName, CallbackResult callback)
{
    callback(8, "AAABBB");
}

C#
public delegate void CallBack(int progress, string lParam);
[DllImport("deepstream_for_csharp")]
public static extern void subscribe0(string eventName, CallBack func);
CallBack myCallBack = new CallBack(ReportView);
subscribe0("adsm", myCallBack);
public static void ReportView(int progress, string lParam)
    {
        Debug.Log("AAAAAA");
    }

C++:
EXPORTS_U3DDLL
void subscribe(char* eventName,void(*func)(const char*))
{
    if (client == 0)
    {
        std::cout << "client has not been created!!";
        return;
    }
    const deepstream::SubscriptionId event_sub_id =
        client->event.subscribeForUnity(eventName, func);
    subEventID[eventName] = event_sub_id;
}

SubscriptionId subscribeForUnity(const std::string &name, std::function<void(const char* data)> callback)
{
Buffer name_buff(name);
Event::SubscribeFn core_callback([callback, this](const Buffer &prefixed_buff) {
const json &data = type_serializer_.prefixed_to_json(prefixed_buff);
const std::string &data_string(data);
callback(data_string.c_str());
});
SubscriptionId subscription_id = client_.event.subscribe(name_buff, core_callback);
return subscription_id;
}

C#:
[DllImport("deepstream_for_csharp")]
public static extern void subscribe(string eventName, SubscribeCallBack callback);
public delegate void SubscribeCallBack(string data);
public static void eventPosCallBack(string data)
{
    Debug.Log("pos:" + data+data.Length);
}
subscribe("pos", new SubscribeCallBack(eventPosCallBack));