How return vector<int> from C++ DLL to C#?

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C++

C#

Hello,
I have used return of function with std::vector<int> from C++ dll to C#.

1. C++ dll code file:

#include "pch.h"
#include <vector>

std::vector<int> _stdcall f(char* strTB)
{
    // Replace "-" to " "
    int len = strlen(strTB);
    for (int i = 0; i < len; i++)
        if (strTB[i] == '-')
            strTB[i] = ' ';

    // Split to int data !
    std::vector<int> ret;
    char separators[] = " ";
    char* token;
    char* next_token;
    int i = 0;
    token = strtok_s(strTB, separators, &next_token);
    while (token)
    {
        ret.push_back(atoi(token));
        token = strtok_s(NULL, separators, &next_token);
    }
    return ret;
}

2. .def file for compile DllTest.dll:

EXPORTS
	f

Compile to dll successful !
3. C# code file using function f() from DllTest.dll.

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

namespace ConsoleAppCSharp
{
    class Program
    {
        [DllImport("DllTest.dll", CharSet = CharSet.Ansi)]
        static extern List<int> f(string str);

        static void Main(string[] args)
        {
            string str = "12 -  34 56 78";
            List<int> ret = f(str);
            int n = ret.Count;
            for (int i = 0; i < n; i++)
                Console.WriteLine(ret[i]);
            Console.ReadKey();
        }
    }
}

Error was displayed when compile with C#:

System.Runtime.InteropServices.MarshalDirectiveException: 'Cannot marshal 'return value': Generic types cannot be marshaled.'

I can't know how to fix code C#. Help me. Thanks.

What I have tried:

I try to write code and to compile, but not get result !