how to make a function return an array? for making DLL to use in VB.NET

Question

i am trying to make a c++ DLL project in VS2017 to use in a VB.NET VS2017 project.

the c++ code;

MyCudaLib.h

#ifndef DLL3_H
#define DLL3_H

#ifdef DLL3_EXPORTS
#define DLL3_API __declspec(dllexport)
#else
#pragma message("automatic link to MyCudaLib.LIB")
//#pragma comment(lib, "MyCudaLib.lib")
#define DLL3_API __declspec(dllimport)
#endif

int* __stdcall test_array();

#endif //DLL3_H

MyCudaLib.cpp

#include <stdio.h>
#include "MyCudaLib.h"
#include <Windows.h>
#include <stdexcept>
#include <sstream>
using namespace std;

#define DLL3_EXPORTS


BOOL APIENTRY DllMain(HANDLE /*hModule*/,
    DWORD  ul_reason_for_call,
    LPVOID /*lpReserved*/
)
{
    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
    case DLL_THREAD_ATTACH:
    case DLL_THREAD_DETACH:
    case DLL_PROCESS_DETACH:
        break;
    }
    return TRUE;
}
int* __stdcall test_array()
{
int arr[]{ 30,50,60,70 };

return  arr;
}

in VB part of the code

Private Declare Function test_array Lib "MyCudaLib.dll" () As Integer()

Private Sub Form1_Load(sender As Object, e As EventArgs) Handles MyBase.Load

    Console.WriteLine(test_array(0))

End Sub

is my exporting part of code is wrong? c++ part compiled wtihout error but VB part of the code wont work gives error.

System.Runtime.InteropServices.MarshalDirectiveException: ''return value' cannot arranged.

Note: My point to make a proper working function in c++ is to write a CUDA code to program the GPU. I can write and compile code in CUDA in c++ without problem. But students in my class cannot program in c++ and i can't write more complex code with c++. So, i thought, if i can use the cuda function outside the c++ environment, we all can code it with vb or other languages.

Answer 1

Sure your function compiles fine - it is defined as a function returning a single int and does so as well (return *arr; is equivalent to return arr[0];).

From MarshalDirectiveException

The exception that is thrown by the marshaler when it encounters a MarshalAsAttribute it does not support.

As you did not provide an English error message, I can only guess what's happening now, but there are only few options available:

1. You have yet another error with your marshaling not visible here.
2. The function is not recognised due to C++ name mangling (see below).
3. Incompatibility between int returned in function and expected array is recognised.
4. The int is interpreted as pointer – but pointer size does not match and the type cannot be marshaled at all.

If you want to return an array, you need to define your function as such:

int* getArray()
// ^
{
    static int a[] = {1, 2, 3};
//  ^^^^^^ we cannot return an array with local storage duration, so make it
//         static, global, or allocate one on heap (new int[]; but assure the
//         array to be delete[]d appropriately again to avoid memory leak)

    return a; // NOT dereferenced!
}

Be aware that you have C++ name mangling applied. You might possibly prefer a C-kompatible interface:

extern "C" int* getArray();

However, you cannot return C++ data types this way like std::vector – which in most cases even is preferrable, though, as those types usually are incompatible across different implementations (DLL compiled with MSVS, EXE with GCC/MinGW) anyway.

Finally be aware that via a pointer, you lose information about array size! You need to find other means to provide this information to the user of your library.