Get IntPtr[] from double** type of C++ dll in C#?

Question

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I have a function in C++ dll with its return is 2-point, as follows:

C++

#include "stdafx.h"
#include <vector>
using namespace std;

double** _stdcall f(int *n)
    {
        vector<double> t;
        vector<double> X;
        int i=0, j=0;
        do
        {
            t.push_back(3*i-4);
            X.push_back(2*j);

            i++;
            j++;
        }
        while (i<15&&j<90);
        *n=i;

        double** ret = new double*[2];
        for (i=0;i<2;i++)
            ret[i]=new double[*n];
        for (i=0;i<*n;i++)
        {
            ret[0][i]=t[i];
            ret[1][i]=X[i];
        }
        return ret;
    }

Now, I declare this function in C# as follows:

C#

[DllImport("exDP.dll")]
public static extern IntPtr[] f(ref int n_);

Now, all of C# code as follows:

C#

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;
using System.IO;
using System.Runtime.InteropServices;

namespace pABC
{
    public partial class frmABC : Form
    {

        [DllImport("exDP.dll")]
        public static extern IntPtr[] f(ref int n_);


        public frmABC()
        {
            InitializeComponent();
        }

        private void cmdOK_Click(object sender, EventArgs e)
        {
                int n = 0, i;


                IntPtr[] ret = new IntPtr[2];
                ret = f(ref n);

                double[] t = new double[n];
                Marshal.Copy(ret[0], t, 0, n);
                double[] X = new double[n];
                Marshal.Copy(ret[1], X, 0, n);

                MessageBox.Show("X[0]= " + X[0].ToString());
        }

    }
}

I compile OK. When I run it, an error occurs at the line:

C#

ret = f(ref n);

That is: Cannot marshal 'return value': Invalid managed/unmanaged type combination.

How to fix it and get the results correctly. Thanks.

Posted 17-Aug-14 16:59pm

Andrewpeter

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2 solutions

Solution 1

My experience is: never allocate memory in C++ to use it in C#. Allocate the memory in C# and fill it in C++.

you should redesign the interface.
C+

_stdcall int f(int *n, double*arr);//return code signals sucess/error

in C#

double[] arrDoubles = new double[4];
int rc = f((n,arrDoubles);

Posted 17-Aug-14 21:40pm

KarstenK

Comments

Andrewpeter 18-Aug-14 4:47am

Thanks KarstenK, I will try your solution. Vote 5 for you.

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Fredrik Bornander · Accepted Answer · 2014-08-17T21:46:00

Try to type the return value from f as a IntPtr instead of a IntPtr[], then marshal that return value to a IntPtr[], iterate over that and marshal each entry to a double[].

Change

C#

[DllImport("exDP.dll")]
public static extern IntPtr[] f(ref int n_);

to

C#

[DllImport("exDP.dll")]
public static extern IntPtr f(ref int n_);

Then read the values like this;

C#

int n = 0, i;
IntPtr ret = f(ref n);

List<double[]> doubleArrays = new List<double[]>();

IntPtr[] intPtrs = new IntPtr[2];
Marshal.Copy(ret, intPtrs, 0, 2);
for(int c = 0; c < intPtrs.Length; ++c) {
    double[] array = new double[n];
    Marshal.Copy(intPtrs[c], array, 0, n);
    doubleArrays.Add(array);
}

The IntPtr can hold a reference to anything, including the double** result from the C++ function.

Hope this helps,
Fredrik