Call vector parameter from C++ dll in C#?

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Hello,
I have a function to create a dll file with C++, this is:

XML

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

void _stdcall fn(vector<double> &p, int* pLength)
{
    int i=0;
    double p_=0;
    do
    {
        p.push_back(p_);
        p_=2*p_+1;
        i++;
    }
    while (a condition of p_);    // The condition of loop....

    *pLength=i;

    return;
}

I compile it successfuly. (testFile.dll).
In C#, I call this function as follows:

C#

using System;
using System.Collections.Generic;
using System.Text;
using System.Runtime.InteropServices;

namespace ConsoleApplication1
{
    class Program
    {
        [DllImport("testFile.dll")]
        public static extern void fn(List<double> p, ref int pLength);
        static void Main(string[] args)
        {
            // I want to declare an array with unknown length (size),
            // because the length is calculated from fn() function!!!
            // And pass to fn() function to get p[i].....
            List<double> p = new List<double>();
            int pLength = 0;
            fn(p, ref pLength);

            // Display pLength and p[i]
            Console.WriteLine(pLength);
            for (int i = 0; i < pLength; i++)
                Console.WriteLine(p[i]);
            Console.ReadKey();
        }
    }
}

Build successfuly. When I run, I get an error:

C#

Cannot marshal 'parameter #1': Generic types cannot be marshaled.

at

C#

fn(p, ref pLength);

How to fix this error and to get correct result? Thanks.

Posted 31-Mar-14 4:43am

Andrewpeter

Updated 1-Apr-14 0:59am

v5

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Comments

[no name] 31-Mar-14 10:58am

Why don't you just use an array like you did before?

Andrewpeter 31-Mar-14 11:04am

Sorry, using array is not convenient! Because the array to pass for function is declared the unchanged size. I use List to improve that. What your idea about this? Thanks. It's difficult for me.

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bling · Answer 1 · 2014-03-31T05:30:00

C# List and C++ std::vector are not compatible. They are functionally equivalent but not binary compatible. To call native code from C#, you need data types that can be easily marshalled between managed and native code.

C++

#include "stdafx.h"
#include <vector>
using namespace std;
 
int _stdcall fn(double list[], int length)
{
    if (length < 20)
        return 0;

    double value = 0.f;
    for (int i = 0; i < 20; i++)
    {
        list[i] = value;
        value = value*2+1;
    }
    return 20;
}
</vector>

Import declared like so:

C#

[DllImport("testFile.dll")]
public static extern int fn(double[] list, int length);

Call the function from C# like so:

C#

double[] list = new double[20];
int length = 0;
length = fn(list, list.Length);

In this example, a return value of 0 means the list is too small.