DLLExport in .NET Core 3 for use in an unmanaged C/C++ Code

Question

I have found the DllExport Project on GitHub while searching for a way to use C# .NET Core 3 Code from Plain C/C++. My goal is being able to compile the C# to any dynamic library and use it on both Linux or Windows with dlopen (dlopen for Windows).

C# Library I'm trying to create:

using RGiesecke.DllExport;
using System;

namespace CSharpPart
{
    public class Test
    {
        [DllExport]
        public static int _add(int a, int b)
        {
            return a + b;
        }
    }
}

C/C++ Code where I'm trying to Reference the C# Library:

#include "CPartCMake.h"
    #include "dlfcn.h"
    #include "cstring"

    using namespace std;

    int main()
    {
        const char* pemodule = "path_to_dll\\CSharpPart.dll";
        void* handle = dlopen(pemodule, RTLD_NOW);

        typedef int(_cdecl* _add)(int a, int b);
        _add pAdd = (_add)dlsym(handle, "_add");
        cout << pAdd(5, 7);
        return 0;

    }

The Call to dlopen returns a non NULL handle which I guess means that the Library itself can be opened. dlsym return a NULL. When viewing the C# DLL through DLL Export Viewer it can be confirmed that the DLL doesn't contain the _add entry.

I tried Wrapping the C# project in an C++/CLI Project like proposed here but it doesn't seems to work with .NET Core.

I read there is also a possibility with using COM interop but I didn't really understood what it is about and I'm not sure if it would work cross-platform like.

I'm afraid that DLLExport just isn't yet compatible with .NET Core as well. Is there any way of achieving my goal like I described it?