Return List/Array from unmanaged C# DLL

Question

I have Unmanaged C# DLL:

[DllExport(ExportName = "GetStudentsList", CallingConvention = CallingConvention.StdCall)]
static public List<StudentsStruct>GetStudentsList() {  return List<StudentsStruct>;   }


[DllExport(ExportName = "maxElement", CallingConvention = CallingConvention.StdCall)]
static public int maxElement(int a, int b) { return c; }

I want to return List<StudentsStruct> from the function.

And I want to run above function in a C++ Application:

using GetStudentsListFn = List<StudentsStruct> (__stdcall *) (void);
GetStudentsListFn  GetStudentsList = reinterpret_cast<GetStudentsListFn> (GetProcAddress(mod, "GetStudentsList"));
List<StudentsStruct> myList = GetStudentsList();

using MaxElementFn = int(__stdcall *) (int a, int b);
MaxElementFn maxElement = reinterpret_cast<MaxElementFn> (GetProcAddress(mod, "maxElement"));
std::printf("max: %d\n", maxElement(1, 2));

MaxElement( ) function is working perfectly because it return an int. But i want to return List/Array of "StudentsStruct", from C# to C++.

Answer 1

I would do this with out array parameter and return the size, like this:

using ExportDllAttribute.DllExport;
using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public struct StudentsStruct
{
    public string Name;
    public int SomeInt;
    public double SomeDouble;

    [DllExport]
    public static int GetStudentsList([Out] out StudentsStruct[] students)
    {
        students = new StudentsStruct[] { new StudentsStruct { Name = "Satan", SomeInt = 666, SomeDouble = 666 },
                new StudentsStruct { Name = "Techno", SomeInt = 777, SomeDouble = 777 } };
        return students.Length;
    }
}

and the C++ code:

#include<Windows.h>

struct StudentsStruct
{
public:
    LPWSTR Name;
    int SomeInt;
    double SomeDouble;
};

using GetStudentsListFn = int (__stdcall*) (StudentsStruct **);

int main()
{
    HMODULE hModule = LoadLibraryA("DllExportArrayTest.dll");
    if (hModule)
    {
        GetStudentsListFn GetStudentsList = reinterpret_cast<GetStudentsListFn>(GetProcAddress(hModule, "GetStudentsList"));
        StudentsStruct* students = NULL;
        auto size = GetStudentsList(&students);
        for (int i = 0; i < size; i++)
            auto student = students[i];
        FreeLibrary(hModule);
    }
}