HoloLens Callbacks with Native Library

Questions & Answers
1

My goal is to call methods, which are implemented in the Unity Code, from my UWP DLL. (So I can use them in my HoloLens Project)

I tried this with a bigger project but failed. Therefore I wrote a simple example to make it easier to find the mistake and exclude other influences.
But still, I get the same error.

My Working Environment:

  • 64-bit Computer with OS Windows 10
  • Micsrosoft Visual Studio Community
    2015 Version 14.0.25431.01 Update 3
  • HoloLens Emulator 10.0.14393.0
  • Unity 5.5.0f3 Personal (64 bit)

Creating the UWP DLL:

To approach this I created a C++ DLL(Windows Universal) in Visual Studio 2015 as followed:

New Project > Visual C++ > Windows > Universal > DLL(Universal Windows)

After the project was auto generated I added my code.
So the code looks like this:

Native Library Code:

SimpleProjectDLL.cpp:

#include "pch.h"
#define DLL_EXPORT __declspec(dllexport)

typedef void(*CB_V)();
typedef void(*CB_V_VI)(const char * a, int b);

CB_V_VI cb_native_log;
CB_V cb_call;

void log()
{
    // this method makes problems !
    cb_native_log("Call for callback", 1);
}

extern "C" {
    DLL_EXPORT void initInterfaceCallbacks(
        CB_V_VI native_log,
        CB_V call
    ) {
        cb_native_log = native_log;
        cb_call = call;
    }

    DLL_EXPORT void callSmth() 
    {
        cb_call();
    }

    DLL_EXPORT int getSomeInt()
    {
        return 42;
    }

    DLL_EXPORT void initCallback() 
    {
        log();
    }
}

SimpleProjectDLL.h is prepearing the delegates:

SimpleProjectDLL.h:

#pragma once
#include <cstdint>
#define DLL_EXPORT __declspec(dllexport)

extern "C" 
{
    typedef void(*CB_V)();
    typedef void(*CB_V_VI)(const char * a, int b);
}

I did not make any changes to the auto generated files dllmain.cpp, pch.cpp, pch.h or targetver.h.

Finally I build the project for “Release” mode and architecture “x86” to generate the DLL-file.
Location of the DLL-file is now: project-root-folder/Release/SimpleProject/SimpleProjectDLL.dll.

---------------------

Next step I created a new Unity Project added the HoloLens-Toolkit and made sure that the new project is running fine on the emulator.

Unity Project Code:

After that I added the SimpleProjectDLL.dll in the Asset-Folder and implemented the following code:

First of all we need to create the connection between the delegates.
Cpp.cs prepears the Delegates:

Cpp.cs

using UnityEngine;
using System;
using System.Runtime.InteropServices;

namespace Cpp
{
    delegate void DelegateV();
    delegate void DelegateVVi(IntPtr a, int b);
}

SimpleInterfaceCpp.cs initializes the connection:

SimpleInterfaceCpp.cs

using Cpp;
using System.Runtime.InteropServices;
using UnityEngine;

public static class SimpleInterfaceCpp
{
    public static void Init()
    {
         initInterfaceCallbacks(
            SimpleInterface.NativeLog,
            SimpleInterface.Call
        );
    }

    [DllImport(SimpleInterface.DLL)]
    private static extern void initInterfaceCallbacks(
        DelegateVVi native_log,
        DelegateV call
    );
}

Main:

MainController.cs

using UnityEngine;
using System.Collections;
using System.Runtime.InteropServices;

public class MainController : MonoBehaviour 
{
    void Start ()
    {
        SimpleInterfaceCpp.Init();
        SimpleInterface.TestCalls();
    }
}

SimpleInterface.cs is calling the methodes:

SimpleInterface.cs

using System;
using UnityEngine;
using System.Runtime.InteropServices;
using AOT;
using IntPtr = System.IntPtr;
using Cpp;

using StringReturn = System.IntPtr;

public class SimpleInterface
{
    public const string DLL = "SimpleProjectDLL";

    public static void TestCalls()
    {
        // This works fine
        int number = getSomeInt();
        Debug.Log("getSomeInt: " + number);

        // This also works fine and outputs "--- A callback ---"
        callSmth();

        // This call gives the output "call_log: native log" but crashes afterwards !
        initCallback();

    }

    [MonoPInvokeCallback(typeof(DelegateVVi))]
    public static void NativeLog(IntPtr logMessage,
         int logLevel)
    {
        string result = StringFromCReturn(logMessage);
        UnityEngine.Debug.Log(result); // outputs "call_log: native log"
    }

    [MonoPInvokeCallback(typeof(DelegateV))]
    public static void Call()
    {
        UnityEngine.Debug.Log("--- A callback---");
    }

    [DllImport(DLL)]
    private static extern void initCallback();
    [DllImport(DLL)]
    private static extern void callSmth();
    [DllImport(DLL)]
    private static extern int getSomeInt();

    public static string StringFromCReturn(StringReturn someReturnVal)
    {
        return Marshal.PtrToStringAnsi(someReturnVal);
    }
}

Now if I create a SLN, open the project in Visual Studio and start it with the “HoloLens Emulator” I get the following Output:

getSomeInt: 42
 
(Filename: C:/buildslave/unity/build/artifacts/generated/Metro/runtime/DebugBindings.gen.cpp Line: 51)

--- A callback---
 
(Filename: C:/buildslave/unity/build/artifacts/generated/Metro/runtime/DebugBindings.gen.cpp Line: 51)

call_log: native log
 
(Filename: C:/buildslave/unity/build/artifacts/generated/Metro/runtime/DebugBindings.gen.cpp Line: 51)

The program '[1932] SimpleProject.exe' has exited with code -1073740791 (0xc0000409).

After that the App just closes.

So my Question is, does anyone know what the problem could be?

Is this the right way to use callbacks in a HoloLens Project?

Or does someone know how to find an error description for the code “-1073740791 (0xc0000409)” ?

Additional Information:
I also tried it on a real HoloLens device, same issue, so the problem does not lays at the emulator.

2

did you ever get this working? Can you debug native plugins from within the Emulator?