In C #, how to access DLL methods written in C ++

Question

The business partner created a DLL written in C ++.

Our partner describes that C ++ DLL methods can be accessed using this code in C ++ (we tested it and it works great with this code written in C ++)

create_func_ptr createInstance = (create_func_ptr)(GetProcAddress(hdl, "createReader"));
LudvigInterface* ludvigInterface = createInstance();
if (ludvigInterface)
{
    bool isRunning = ludvigInterface->isRunning();
    destroy_func_ptr closeInstance = (destroy_func_ptr)(GetProcAddress(hdl, "closeReader"));
    closeInstance(ludvigInterface);
}

But our task is that we want to do the same in C #.

We can open the DLL through DLLImport instructions, and everything works well until we try to access the isRunning method.

class Program
{
    [DllImport(@"\Libraries\LudvigImplementation.dll")]
    private static extern IntPtr createReader();

    [DllImport(@"\Libraries\LudvigImplementation.dll")]
    private static extern bool isRunning();

    static void Main(string[] args)
    {

        IntPtr LudvigInterfacePtr = createReader(); //this line runs well

        var a = isRunning(); //This line gives a System.EntryPointNotFoundException: 'Unable to find an entry point named 'isRunning' in DLL LudvigImplementation.dll'
    }
}

How can we solve this? How can we access the isRunning DLL method in C #?

Thank you so much

UPDATED 2. CAN AT 13:31 CET:

I have questions about what LudvigInterface looks like. Here it is (LudvigInterface.h)

class LudvigInterface
{
public:
    virtual bool isRunning() = 0;
    virtual char* getVersion() = 0;
};

Thanks again