Return C++ array to C#

Question

I can't seem to figure out how to return an array from an exported C++ DLL to my C# program. The only thing I've found from googling was using Marshal.Copy() to copy the array into a buffer but that doesn't give me the values I'm trying to return, I don't know what it's giving me.

Here's what I've been trying:

Exported function:

extern "C" __declspec(dllexport) int* Test() 
{
    int arr[] = {1,2,3,4,5};
    return arr;
}

C# portion:

    [DllImport("Dump.dll")]
    public extern static int[] test();

    static void Main(string[] args)
    {

        Console.WriteLine(test()[0]); 
        Console.ReadKey();


    }

I know the return type int[] is probably wrong because of the managed/unmanaged differences, I just have no idea where to go from here. I can't seem to find an answer for anything but returning character arrays to strings, not integer arrays.

I figured the reason the values I'm getting with Marshal.Copy are not the ones I'm returning is because the 'arr' array in the exported function gets deleted but I'm not 100% sure, if anyone can clear this up that would be great.

Answer 1

I have implemented the solution Sriram has proposed. In case someone wants it here it is.

In C++ you create a DLL with this code:

extern "C" __declspec(dllexport) int* test() 
{
    int len = 5;
    int * arr=new int[len+1];
    arr[0]=len;
    arr[1]=1;
    arr[2]=2;
    arr[3]=3;
    arr[4]=4;
    arr[5]=5;
        return arr;
}

extern "C" __declspec(dllexport) int ReleaseMemory(int* pArray)
{
    delete[] pArray;
    return 0;
}

The DLL will be called InteropTestApp.

Then you create a console application in C#.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.InteropServices;

namespace DLLCall
{
    class Program
    {
        [DllImport("C:\\Devs\\C++\\Projects\\Interop\\InteropTestApp\\Debug\\InteropTestApp.dll")]
        public static extern IntPtr test();

        [DllImport("C:\\Devs\\C++\\Projects\\Interop\\InteropTestApp\\Debug\\InteropTestApp.dll", CallingConvention = CallingConvention.Cdecl)]
        public static extern int ReleaseMemory(IntPtr ptr);

        static void Main(string[] args)
        {
            IntPtr ptr = test();
            int arrayLength = Marshal.ReadInt32(ptr);
            // points to arr[1], which is first value
            IntPtr start = IntPtr.Add(ptr, 4);
            int[] result = new int[arrayLength];
            Marshal.Copy(start, result, 0, arrayLength);

            ReleaseMemory(ptr);

            Console.ReadKey();
        }
    }
}

result now contains the values 1,2,3,4,5.

Hope that helps.

Answer 2

More or less the same as the answer above, but this worked for me. For an array returned from a c method like this:

EXPORT char** methodname(void);

I did not succeed in getting this to work with the MarshalAs attribute, but this did the trick: (condensed code)

[DllImport(libName, EntryPoint = "methodname", CallingConvention = CallingConvention.Cdecl)]
public static extern IntPtr methodname();

var listPtr = methodname();
var list = new List<string>();
IntPtr itemPtr = IntPtr.Zero;
var offset = 0;
while ((itemPtr = Marshal.ReadIntPtr(listPtr, offset)) != IntPtr.Zero) 
{
    Console.WriteLine(Marshal.PtrToStringAnsi(itemPtr));
    offset += 4;
}