C#/C++ Interop with string passing

backup.cpp

#include "backup.h"

namespace Backup
{
	int Backup(WCHAR* test) {
		return wcslen(test);
	}
}

backup.h

#include <stdexcept>
using namespace std;

namespace Backup
{
	extern "C" { __declspec(dllexport) int Backup(WCHAR* test); }
}

Program.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.InteropServices;

namespace TestInterop
{
    class Program
    {
        [DllImport(@"C:\Users\Administrator\workspace\3rdmantools\SQLSnapManager\backup\x64\Debug\backup.dll", 
            CallingConvention = CallingConvention.Cdecl)]
        public static extern int Backup([MarshalAs(UnmanagedType.LPWStr)] StringBuilder test);

        static void Main(string[] args)
        {
            StringBuilder testStr = new StringBuilder(1024);
            testStr.Append("asdf");
            Console.WriteLine(Backup(testStr)); // 4
            Console.ReadLine();
        }
    }
}