What is wrong with my DLLImport(s) in my C# project for my custom C++ DLL?

Question

I am currently learning C++ (for eventually creating DLL's for Hooks). I have strengths in C# and would like to access the DLL's via pInvoke/DllImports.

I am using VS 2019 (and .NET Framework 4.7.2). As a sample for learning purposes, I have replicated the DLL as well as the C++ (console app) as described here: https://learn.microsoft.com/en-us/cpp/build/walkthrough-creating-and-using-a-dynamic-link-library-cpp?view=vs-2019. This is working perfectly fine (as expected).

Now when I start trying to import the DLL into a C# Project I am running into problems (which I believe are DllImport related)

In the sample (c++) console app (which is working):

int main()
{
    // Initialize a Fibonacci relation sequence.
    fibonacci_init(1, 1);
    // Write out the sequence values until overflow.
    do {
        std::cout << fibonacci_index() << ": " << fibonacci_current() << std::endl;
    } while (fibonacci_next());
    // Report count of values written before overflow.
    std::cout << fibonacci_index() + 1 << " Fibonacci sequence values fit in an " << "unsigned 64-bit integer." << std::endl;
}

In the C# test console app (not working properly):

    class Program
    {

        [DllImport("MathLibrary.dll", CallingConvention = CallingConvention.Cdecl)]
        internal static extern void fibonacci_init(ulong a, ulong b);

        [DllImport("MathLibrary.dll", CallingConvention = CallingConvention.Cdecl))]
        [return: MarshalAs(UnmanagedType.Bool)]
        public static extern bool fibonacci_next();

        [DllImport("MathLibrary.dll", CallingConvention = CallingConvention.Cdecl)]
        internal static extern ulong fibonacci_current();

        [DllImport("MathLibrary.dll", CallingConvention = CallingConvention.Cdecl)]
        internal static extern uint fibonacci_index();

        static void Main(string[] args)
        {

            // Initialize a Fibonacci relation sequence.
            fibonacci_init(1, 1);
            // Write out the sequence values until overflow.
            do {
                Console.WriteLine($"{fibonacci_index()}: {fibonacci_current()}");
            } while (fibonacci_next());
            // Report count of values written before overflow.
            Console.WriteLine($"{fibonacci_index() + 1} Fibonacci sequence values fit in an unsigned 64-bit integer.");
        }
    }

In the working (C++) app, the loop reaches 92 and prints 92: 12200160415121876738 and then exits the loop and prints 93 Fibonacci sequence values fit in an unsigned 64-bit integer.

However, in the failing (C#) app, it works up until the 92: 12200160415121876738, but rather than exiting the loop, fabonacci_next() continues to return true and it keeps looping the line 92: 12200160415121876738 (no more increments, and never exits).

For the DllImport in C# I have tried both:

[DllImport("MathLibrary.dll", CallingConvention = CallingConvention.Cdecl))]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool fibonacci_next();

and:

[DllImport("MathLibrary.dll", CallingConvention = CallingConvention.Cdecl))]
public static extern bool fibonacci_next();

Same results either way. If someone could explain to me what I am doing wrong with the DllImport (causing it not to ever return true, when the same DLL works properly when linked to a C++ console app).

--- UPDATE ---

So the solution to the example above changing [return: MarshalAs(UnmanagedType.Bool)] to [return: MarshalAs(UnmanagedType.I1)] does work (with further important related information provided in the links given by GSerg in the comments below).

--- UPDATE 2 ---

(@David) I did intentionally (and incorrectly) leave out the exports as they were in the link I provided to the original DLL 'walkthrough' (my apologies). For completeness of the question the exports are:

#ifdef MATHLIBRARY_EXPORTS
#define MATHLIBRARY_API __declspec(dllexport)
#else
#define MATHLIBRARY_API __declspec(dllimport)
#endif

extern "C" MATHLIBRARY_API void fibonacci_init(unsigned long long a, unsigned long long b);
extern "C" MATHLIBRARY_API bool fibonacci_next();
extern "C" MATHLIBRARY_API unsigned long long fibonacci_current();
extern "C" MATHLIBRARY_API unsigned fibonacci_index();

What is wrong with my DLLImport(s) in my C# project for my custom C++ DLL?

Answers (1)

Related Questions