Question

Let’s say I have the following code within another method:

Parallel.For(0, pageCount, 
             page =>
             {
                 var request = (HttpWebRequest)WebRequest.Create(
                                string.Format(url, 
                                    selectedRegion["id"].InnerText, page));
                 request.BeginGetResponse(
                     new AsyncCallback(FinishRequestRegion), request);
              });

And this is the callback for the expected response:

void FinishRequestRegion(IAsyncResult res)
        {
            try
            {
                var response = (res.AsyncState as HttpWebRequest).EndGetResponse(res) as HttpWebResponse;
                var docCitizens = new XmlDataDocument();
                var readerCitizens = new XmlTextReader(response.GetResponseStream());

                readerCitizens.MoveToContent();
                docCitizens.Load(readerCitizens);
                readerCitizens.Close();

                if (docCitizens["error"]!=null)
                    return;

                var selectedRegion = docCitizens["region"];
                var regionCitizens = selectedRegion["citizens"];

                var result = (regionCitizens.Cast<XmlNode>().Select(citizen => citizen["id"].InnerText)).ToList();

                File.AppendAllLines("list.txt",  result);

            }
            catch (Exception ex)
            {
                return;
            }

        }

I’m getting an error when 2 responses are trying to write to the file at the same time (classic message “another process is accessing the file”).
How can I do to make the concurrent processes to wait for the file to be unlocked so it can then record the desired information there?

Answer 1

You need to lock something:

object fileLock = new object();

void FinishRequestRegion(IAsyncResult res)
        {
            try
            {
                ...
                lock(fileLock)
                   File.AppendAllLines("list.txt",  result);

            }
            catch (Exception ex)
            {
                return;
            }

        }