How to have synchronous writing to a file (Threads) ?

Question

Hi all. I created and started some Threads that each one writes something to a common text file. but the following error appears to me: "The process cannot access the file 'C:\hello.txt' because it is being used by another process."

void AccessFile()

    {

        int num = 5;

        Thread[] trds = new Thread[5];

        for (int i = 0; i < num; i++)

        {

            trds[i] = new Thread(new ParameterizedThreadStart(WriteToFile));

        }

        for (int i = 0; i < num; i++)

        {

            trds[i].Start(String.Format("{0}: Hello from thread id:#{1}", i, trds[i].ManagedThreadId));

        }

    }





    void WriteToFile(object message)

    {

        string FileName = "C:\\hello.txt";

        string mess = (string)message;

        System.IO.StreamWriter sw = null;

        FileStream objStream = null;

        sw = File.AppendText(FileName);

        if (sw == null)

        {

            objStream = new FileStream(FileName, FileMode.OpenOrCreate, FileAccess.ReadWrite, FileShare.ReadWrite);

            sw = new StreamWriter(objStream);

        }

        sw.WriteLine(mess);

        sw.Close();

        sw.Dispose();

    }

the AccessFile() method is the starting point. could any one tell me what should i do?

Answer 1

What do you expect to happen? Even if you did write to the same file from a number of different threads, what would you acheive? The results would be, at best, undefined.

What are you really trying to acheive?

Answer 2

You could write to that file using a Singleton object with a synchronized lock on it. I'm not at home in c#, but you can only have one instance of the file itself, and you should synchronize access to that file.

Answer 3

At the very least you need to put a lock at the start of WriteToFile and release it at the end after you've closed it.

Writing to a file should be an atomic operation and requires exclusive access to the resource for the duration of the write.

What you have here is several threads all trying to write to the file at the same time. If this were not the cast at best you'd get the last thread overwriting everything that had come before, at worst you'd get a corrupted file.

Answer 4

If you're writing to a file, it's only safe to have it open in one place, since the various threads would overwrite eachother's work.

Instead, if multiple threads need to make changes to a single file (such as a database file), you need to force the threads to take turns writing to the file, either by using synchronization objects, or by setting up a single "management" thread that mediates access to the file on behalf of the other threads.