How to get return array from function with global variable from C++ dll in C#?

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C++

C#

Hi all,
I have a dll file which is written by C++ (the name of file is "DllForTest.dll"), this is its code:

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

double *ret;
double* _stdcall f(int* n)
{
	vector<double> vret;
	int i=0;
	do
	{
		vret.push_back(i);
		i++;
	} while (the condition to stop this loop);
	*n=i;
	ret = new double[*n];
	for (i=0;i<*n;i++)
		ret[i]=vret[i];
	return ret;
}

This is C# code to call f function from the dll file above to get return value:

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

namespace WindowForm
{
    public partial class Form1 : Form
    {
        [DllImport("DllForTest.dll")]
        public static extern double[] f(ref int n);
        
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            int n=0;
            double[] x;
            x = f(ref n);
            MessageBox.Show("x[0]= " + x[0]);
        }
    }
}

When I run, it generate an error:

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

How to fix it to gain wanted result? Thanks.