C#调用返回C++指针的缓冲区，并释放

在C++的函数中new了一段内存

char *test(char* filename)
{
     new一段内存
}

void release(char *mm);
{
   delete一段内存
}


        [DllImport("test.dll", EntryPoint = "test", CallingConvention = CallingConvention.Cdecl)]
        public static extern IntPtr test( string filename);
        [DllImport("test.dll", EntryPoint = "release", CallingConvention = CallingConvention.Cdecl)]
        public static extern void release(IntPtr sb);

        public static IntPtr GetPtr(string filename)
        {
            return test(filename);
        }



        static unsafe void Main(string[] args)
        {
            string sb = @"路径名";
            IntPtr re = XX.GetPtr(sb);
            string result = Marshal.PtrToStringAnsi(re);
            //释放非托管代码申请的内存
            XX.release(re);
            //内存已释放，所以打印结果为空
            Console.WriteLine(Marshal.PtrToStringAnsi(re));
            //result引用的对象是已释放的对象的副本，所以打印结果为所需结果
            Console.WriteLine(result);
            Console.ReadLine();
        }