How to implement callback functions from C++ DLL

Unity Engine
1

Hi,
I created a DLL from a C++ project and wanted to use its functions in Unity, but I still have difficulties with callbacks. I keep on getting the error EntryPointNotFoundException: WacomMTRegisterAttachCallback.
Here the C++ file:

extern "C" {
#include <stdlib.h>

typedef enum _WacomMTError
{
    WMTErrorSuccess                = 0,
    WMTErrorDriverNotFound        = 1,
    WMTErrorBadVersion            = 2,
    WMTErrorAPIOutdated            = 3,
    WMTErrorInvalidParam            = 4,
    WMTErrorQuit                    = 5,
    WMTErrorBufferTooSmall        = 6
} WacomMTError;

typedef void (__stdcall * WMT_ATTACH_CALLBACK)(WacomMTCapability deviceInfo, void *userData);

__declspec(dllexport) WacomMTError __stdcall WacomMTInitialize(int libraryAPIVersion);

__declspec(dllexport) WacomMTError __stdcall WacomMTRegisterAttachCallback(WMT_ATTACH_CALLBACK attachCallback, void *userData);

}

The C# file in Unity:

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


enum WacomMTError
{
    WMTErrorSuccess = 0,
    WMTErrorDriverNotFound = 1,
    WMTErrorBadVersion = 2,
    WMTErrorAPIOutdated = 3,
    WMTErrorInvalidParam = 4,
    WMTErrorQuit = 5,
    WMTErrorBufferTooSmall = 6
}


public class PluginTest : MonoBehaviour {

    [DllImport("WacomFeel")]
    private static extern WacomMTError WacomMTInitialize(int libraryAPIVersion);

    private delegate void AttachCallbackDel(WacomMTCapability deviceInfo, IntPtr userData);
    [DllImport("WacomFeel")]
    private static extern WacomMTError WacomMTRegisterAttachCallback(AttachCallbackDel attachCallback, IntPtr userData);

    private AttachCallbackDel attachCallbackDel;

    private static void AttachCallback(WacomMTCapability deviceInfo, IntPtr userData)
    {
        //do something
    }

    void Start () {
        WacomMTError test = WacomMTInitialize(4);
        Debug.Log(test); //returns success

        attachCallbackDel = new AttachCallbackDel(AttachCallback);

        test = WacomMTRegisterAttachCallback(attachCallbackDel, IntPtr.Zero);
        Debug.Log(test); //no return due to error
    }

}

The function WacomMTInitialize works fine, but calling WacomMTRegisterAttachCallback brings an error. Any suggestions?

2

My understanding is you cannot call back like that. You have to use the UnitySendMessage() method on the native side, and Unity will get the call to your C# function, but on the next frame.

See here:

This is specifically the iOS plugins page, but I believe it applies to ALL native-C# interop under Unity.

Quote:

Calling C# back from native code

Unity iOS supports limited native-to-managed callback functionality via UnitySendMessage:

UnitySendMessage(“GameObjectName1”, “MethodName1”, “Message to send”);
This function has three parameters: the name of the target GameObject
, the script method to call on that object and the message string to pass to the called method.

Known limitations:

Only script methods that correspond to the following signature can be called from native code: function MethodName(message:string)
Calls to UnitySendMessage are asynchronous and have a delay of one frame.

3

Are you sure?

In this thread someone seemed to make it work, and he used quite a similar approach: