Creating a C# DLL to use in C++ executables

Question

I have been struggling with this topic for 1 week now and nothing I tried seems to work. I made a very simple C# class:

namespace SimpleMathLib
{
    public class SimpleMath
    {
        public float SumFloat(float a, float b)
        {
            return a + b;
        }

        public int SumInt(int a, int b)
        {
            return a + b;
        }
    }
}

in a Visual Studio 2022 solution configured to build a DLL:

I did follow a few online tutorial but the one that gave me less problems was the one about creating a C++ wrapper that uses CLR and compile it as a static library (.lib). Here are the scripts I added in this wrapper:

// SimpleMathLibWrapper.h
#pragma once

class SimpleMathLibWrapperPrivate;

class __declspec(dllexport) SimpleMathLibWrapper
{
    private:
        SimpleMathLibWrapperPrivate* _private;

    public:
        SimpleMathLibWrapper();
        ~SimpleMathLibWrapper();

        float SumFloat(const float a, const float b);
        int SumInt(const int a, const int b);
};

// SimpleMathLibWrapper.cpp
#include "..\public\SimpleMathLibWrapper.h"

#include <msclr\auto_gcroot.h>
#include <msclr\marshal_cppstd.h>

using namespace System::Runtime::InteropServices;
using namespace SimpleMathLib;

class SimpleMathLibWrapperPrivate
{
    public:
        msclr::auto_gcroot<SimpleMath^> simpleMathCSharp;
};

SimpleMathLibWrapper::SimpleMathLibWrapper()
{
    _private = new SimpleMathLibWrapperPrivate();
    _private->simpleMathCSharp = gcnew SimpleMath();
}

SimpleMathLibWrapper::~SimpleMathLibWrapper()
{
    delete _private;
}

float SimpleMathLibWrapper::SumFloat(const float a, const float b)
{
    return _private->simpleMathCSharp->SumFloat(a, b);
}

int SimpleMathLibWrapper::SumInt(const int a, const int b)
{
    return _private->simpleMathCSharp->SumInt(a, b);
}

and these are the settings of the Visual Studio CLR library project I had to change: moreover, I added the compiled C# DLL reference to the project

Now, for the C++ executable project, it is a Visual Studio C++ console project with the following script:

// SimpleMathLibUser.cpp : This file contains the 'main' function. Program execution begins and ends there.
//

//#define LOAD_DLL_MANUALLY

#include <iostream>
#include <Windows.h>

#ifndef LOAD_DLL_MANUALLY
    #include "SimpleMathLibWrapper.h"
#endif // !LOAD_DLL_MANUALLY


#ifdef LOAD_DLL_MANUALLY
    void PrintExecutablePath()
    {
        TCHAR exePath[MAX_PATH];
        GetModuleFileName(NULL, exePath, MAX_PATH);
        char narrowExePath[MAX_PATH];

        // convert the string to a narrow character string
        if (WideCharToMultiByte(CP_ACP, 0, exePath, -1, narrowExePath, MAX_PATH, 0, 0) == 0)
        {
            std::cerr << "Failed to convert the path to a narrow character string." << std::endl;
            return;
        }

        char* lastSlash = strrchr(narrowExePath, '\\');
        if (lastSlash != NULL)
        {
            *lastSlash = '\0';
        }

        std::cout << "Current directory: " << narrowExePath << std::endl << std::endl;
    }
#endif // LOAD_DLL_MANUALLY

int main()
{
#ifdef LOAD_DLL_MANUALLY
    PrintExecutablePath();

    // load the DLL.
    HMODULE mathLib = LoadLibrary(TEXT("..\\Plugins\\SimpleMathLibWrapper.dll"));

    if (mathLib == NULL)
    {
        std::cerr << "Failed to load the DLL." << std::endl;
        return 1;
    }

    // get a pointer to functions from the DLL.
    float (*SumFloat)(float, float) = (float (*)(const float, const float))GetProcAddress(mathLib, "SumFloat");
    int (*SumInt)(int, int) = (int (*)(const int, const int))GetProcAddress(mathLib, "SumInt");

    if (SumFloat == NULL)
    {
        std::cerr << "Failed to find the 'SumFloat' function in the DLL." << std::endl;
        return 1;
    }

    if (SumInt == NULL)
    {
        std::cerr << "Failed to find the 'SumInt' function in the DLL." << std::endl;
        return 1;
    }

    // call the functions.
    float resultFloat = SumFloat(10.f, 5.f);
    std::cout << "Float Sum: " << resultFloat << std::endl;

    int resultInt = SumInt(2, 3);
    std::cout << "Int Sum: " << resultInt << std::endl;

    // unload the DLL.
    FreeLibrary(mathLib);
#else
    SimpleMathLibWrapper wrapper;

    std::cout << "Float Sum: " << wrapper.SumFloat(10.f, 5.f) << std::endl;
    std::cout << "Int Sum: " << wrapper.SumInt(2, 3) << std::endl;
#endif // LOAD_DLL_MANUALLY

    return 0;
}

As you can see, for this I tried 2 approaches:

1. loading the wrapper manually
2. using the linker

These are the settings I added for the linker solution (which is currently the one that at least compile and seems to work until a certain point):

It doesn't matter what .NET version the C# dll is compiled with, when I run the executable, I always get the following error:

Unhandled Exception: System.IO.FileNotFoundException: Could not load file or assembly 'netstandard, Version=2.1.0.0, Culture=neutral, PublicKeyToken=cc7b13ffcd2ddd51' or one of its dependencies. Impossible to find the scpecified file.
   in SimpleMathLibWrapper.{ctor}(SimpleMathLibWrapper* )
   in mainCRTStartup()

What I know is that both Runtime and SDK for the .NET version are installed and properly referenced in the system. Whatever version of the target framework I select in the C# VS project, the result is the same; only the assembly name and version changes.

I also looked for this specific issue on the internet but most of the solutions where to try to open the project directly from the .sln file and not from VS which didn't work for me.

Loading the DLL manually causes a set of different issues that I wasn't able to figure out, so, I kept the code for reference but bailed on trying to fix it.

I know this is a very long post and I encourage you to ask me for more details in case I missed something. Hopefully, finding a solution here will be able to help a lot more people in the future.

Thanks !!

Answer 1

After reading some of the comments I received, I was able to finally have everything working !!!

First thing, the C# DLL needs to be compiled with a .NET Framework version that is less or equal than 4.7.2 because of CLR.

What should you do to achieve this? Edit the .csproj file like so:

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <TargetFramework>net472</TargetFramework>
    <ImplicitUsings>disable</ImplicitUsings>
    <Nullable>disable</Nullable>
    <SignAssembly>False</SignAssembly>
    <AssemblyOriginatorKeyFile>SimpleMathManaged</AssemblyOriginatorKeyFile>
  </PropertyGroup>

</Project>

making sure that <TargetFramework> points to a correct version, the <ImplicitUsing> is disabled so the project will not use the GlobalUsing that are not available for that framework and you also need to disable the <Nullable>, also not supported.

After that, recompile the DLL, the wrapper (I didn't change anything there) and also the C++ executable to update to the latest wrapper LIB. Make sure the C# DLL is copied where the .exe file is (I forgot to mention this on my original post), run the project and everything should properly work !!! :) Finally...

Basically my initial mistake was to compile the DLL using the .NET 6.0 (the default when you create a new C# project with Visual Studio) which is higher then 4.7.2 and not supported. The Wrapper compilation process was not launching any warning or error but when used from the executable it wasn't working.

The second mistake was to try with .Net Standard 2.1 which, again, is not supported and no warnings thrown.

When I tried the .NET 4.8 as suggested by JonasH and Hans Passant (thank you, guys) in the comments, I finally got a warning from the wrapper that was telling me that the targeted .NET Framework was not supported because higher than 4.7.2 and then, finally targeting the correct version, everything worked.

I really hope that this solution can help everyone with the same problem because it was not fun to find a solution.

UPDATE !!!

For the sake of completeness, I would like to add another thing. I compiled the C++ wrapper as a static library (.lib) because when I tried to link the DLL in the C++ executable Visual Studio project: I was getting this error:

Error LNK1302 only support linking safe .netmodules; unable to link ijw/native .netmodule
    SimpleMathLibUser D:\dev\C++\SimpleMathLibWrapper\x64\Debug\SimpleMathLibWrapper.dll

If you want to have your C++ wrapper as a dynamic library (.dll), the linker of the C++ executable Visual Studio project should link the .obj file instead of directly specifying the .dll one: