C#调用返回值为字符串的dll api,调试模式下崩溃

pass, string softId);
        [DllImport("dc.dll")]
        private static extern IntPtr RecYZM_A(string imgPath, string user, string pass,

string softId);
        [DllImport("dc.dll")]
        private static extern void ReportError(string user, string worker);
        static void Main(string[] args)
        {
            //账号配置
            string user = "user";
            string pass = "pass";
            string softId = "0";
 
            //查询剩余点数
            IntPtr info = GetUserInfo(user, pass);
            Console.WriteLine("剩余点数{0}",Marshal.PtrToStringAnsi(info));   
         
            //按图片数据识别验证码
            FileStream fs = new FileStream("code.jpg", FileMode.Open,FileAccess.Read);
            byte[] bFile = new byte[fs.Length];
            BinaryReader r = new BinaryReader(fs);
            bFile=r.ReadBytes((int)fs.Length);
            r.Close();
            r=null;
            fs.Close();
 
            Console.WriteLine("RecByte_A验证码远程识别中");
            IntPtr result = RecByte_A(bFile, bFile.Length, user, pass, softId);
            Console.WriteLine("返回的验证码识别结果:{0}", Marshal.PtrToStringAnsi(result));
             
            //按图片路径识别验证码
            Console.WriteLine("RecYZM_A验证码远程识别中");
            IntPtr result2 = RecYZM_A("code.jpg", user, pass, softId);
            Console.WriteLine("返回的验证码识别结果:{0}", Marshal.PtrToStringAnsi

(result2));
 
            /*
            //报告错误 只有在确定验证码错误的情况下才调用该函数
            string worker = "F36B1D3A30474A75B497D82FC6342CFC";
            ReportError(user,worker);
            */
            Console.Read();
        }
    }
}