parallel.for confusion(收集失败的订单)

Question

我有一个问题,我试图使用parallel.for()将一些文件加载​​到数据库中.我的问题是,传递给数据库函数的文件ID在某种程度上是不正确的.也就是说,数据库返回错误的数据.我试图通过使用并发字典来验证这一点,以添加有和没有并行的id/name对.在我看来,循环结束后列表应该是相同的.但他们不是.这模拟了我正在以非常简单的方式做的事情.

这有意义吗？:

class Program
    {
       ConcurrentDictionary<int, string> _cd = new ConcurrentDictionary<int, string>();
        static void Main()
        {
            //simulate the situation
            int[] idList = new int[] {1, 8, 12, 19, 25, 99};
            string[] fileList = new string[] {"file1", "file8", "file12", "file19", "file25", "file99"};

            //run in serial first
            ProcessFiles(idList, fileList); 

            //write out pairs to text file
            foreach (var item in _cd)
            {
                var key = _cd.key;
                var val = _cd.value;
                string line = string.Format("fileId is {0} and fileName is {1}", key, val);

                File.AppendAllText(@"c:\serial.txt", line + Environment.NewLine);
            }
            //results of text file (all good): 
            //fileId is 1 and fileName is file1
            //fileId is 8 and fileName is file8
            //fileId is 12 and fileName is file12
            //fileId is 19 and fileName is file19
            //fileId is 25 and fileName is file25
            //fileId is 99 and fileName is file99

            _cd.Clear();

            //now run in parallel
            ProcessFilesInParallel(idList, fileList); 

            //write out pairs to text file  
            foreach (var item in _cd)
            {
                var key = _cd.key;
                var val = _cd.value;
                string line = string.Format("fileId is {0} and fileName is {1}", key, val);

                File.AppendAllText(@"c:\parallel.txt", line + Environment.NewLine);
            }

            //results of text file (1. some, not all, are mismatched and 2. not all elements got added): 
            //fileId is 8 and fileName is file8
            //fileId is 12 and fileName is file19
            //fileId is 19 and fileName is file12
            //fileId is 25 and fileName is file25
        }

        private void static ProcessFiles(int[]Ids, string[] files)
        {
            int fileId = 0;
            string fileName = string.Empty;

            for(var i=0, i<Ids.Count; i++) 
            {
                fileId = Ids[i];
                fileName = GetControlFileMetaDataFromDB(fileId);

                _cd.TryAdd(fileId, fileName);
            }
        }

        private void static ProcessFilesInParallel(int[]Ids, string[] files)
        {
            int fileId = 0;
            string fileName = string.Empty;

            Parallel.For(0, Ids.Count, i => 
            {
                fileId = Ids[i];

                //this is returning the wrong fileName 
                fileName = GetControlFileMetaDataFromDB(fileId);

                _cd.TryAdd(fileId, fileName);
            }

            );
        }

        private void static GetControlFileMetaDataFromDB(int fileId)
        {
            //removed for brevity:
            //1. connect to oracle
            //2. call function, passing file id
            //3. iterate over data reader and look for the filename 

            while (reader.Read())
            {
                //strip out filename, add it to collection
                int endPos = reader[0].ToString().IndexOf("txt");
                if (endPos != -1)
                {
                    endPos += 3;
                    int startPos = reader[0].ToString().IndexOf(":\\") - 1; 
                    string path = reader[0].ToString().Substring(startPos, endPos - startPos);
                    sring fileName = Path.GetFileName(path);

                    _cd.TryAdd(fileId, fileName);
                    break;
                }
            }
        }
    }

Answer 1

你已经声明fileId并且fileName 在 Parallel.For 之外,这意味着每次迭代都会共享同一个变量.

由于迭代可能在不同的线程上并行运行,因此您可以重新分配变量,而另一个同时迭代可能正在使用它们.

什么,你需要做的就是将你的变量声明内循环,所以他们每次迭代的地方;

Parallel.For(0, Ids.Count, i => 
{
    int fileId = Ids[i];

    //this is returning the wrong fileName 
    string fileName = GetControlFileMetaDataFromDB(fileId);

    _cd.TryAdd(fileId, fileName);
}