如何将const char*API导入C#？

// CPP_API.h
#pragma once

#include<string>

extern "C"
{
    __declspec(dllexport) const char* GetString();
}

// CPP_API.cpp
#include "stdafx.h"
#include "CPP_API.h"

const char* GetString()
{
    return "Hello from C++ DLL!";
}

using System;
using System.Runtime.InteropServices;

namespace CSharp_API
{
    class Program
    {
        [DllImport("CPP_API.dll", CallingConvention = CallingConvention.Cdecl, CharSet = CharSet.Ansi)]
        public static extern IntPtr GetString();

        static void Main(string[] args)
        {
            IntPtr ptr = GetString();
            string result = Marshal.PtrToStringAnsi(ptr);
            Console.WriteLine(result);
        }
    }
}