Question

I have a file which is an XML representation of some data that is taken from a Web service and cached locally within a Web Application. The idea being is that this data is very static, but just might change. So I have set it up to cache to a file, and stuck a monitor against it to check if it has been deleted. Once deleted, the file will be refreshed from its source and rebuilt.

I am now running in to problems though, because obviously in a multi-threaded environment it falls over as it is trying to access the data when it is still reading/writing the file.

This is confusing me, because I added a object to lock against, and this is always locked during read/write. It was my understanding that attempted access from other threads would be told to ‘wait’ until the lock was released?

Just to let you know, I am real new to multi-threaded development, so I am totally willing to accept this is a screw up on my part 🙂

• Am I missing something?
• What is the best file access strategy in a multi-threaded environment?
  

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Edit

Sorry – I should have said this is using ASP.NET 2.0 🙂

Answer 1

Here is the code that I use to make sure a file is not locked by another process. It’s not 100% foolproof, but it gets the job done most of the time:

    /// <summary>     /// Blocks until the file is not locked any more.     /// </summary>     /// <param name='fullPath'></param>     bool WaitForFile(string fullPath)     {         int numTries = 0;         while (true)         {             ++numTries;             try             {                 // Attempt to open the file exclusively.                 using (FileStream fs = new FileStream(fullPath,                     FileMode.Open, FileAccess.ReadWrite,                      FileShare.None, 100))                 {                     fs.ReadByte();                      // If we got this far the file is ready                     break;                 }             }             catch (Exception ex)             {                 Log.LogWarning(                    'WaitForFile {0} failed to get an exclusive lock: {1}',                      fullPath, ex.ToString());                  if (numTries > 10)                 {                     Log.LogWarning(                         'WaitForFile {0} giving up after 10 tries',                          fullPath);                     return false;                 }                  // Wait for the lock to be released                 System.Threading.Thread.Sleep(500);             }         }          Log.LogTrace('WaitForFile {0} returning true after {1} tries',             fullPath, numTries);         return true;     } 

Obviously you can tweak the timeouts and retries to suit your application. I use this to process huge FTP files that take a while to be written.