The process cannot access the file because it is being used by another process.

Question

I have a function to write log to a text file:

using (var mutex = new Mutex(false, fullPath.Replace("\\", "")))
{
    mutex.WaitOne(Timeout.Infinite, false);
    File.AppendAllText(fullPath, content.ToString());
    mutex.ReleaseMutex();
}

The function is called from multiple places, so I use Mutex to make sure only 1 process is accessing the file.

It was working perfectly until I upgraded to .Net 8, I hit the error "The process cannot access the file because it is being used by another process." intermittently. Looks like the file is not being released sometimes.

It only happen when I deploy to Azure App Service, it is working fine in my local. Anyone please help...

Answer 1

@Jimmy Koo You can try using the FileShare enumeration to specify the type of access other FileStream objects can have to the same file. For example, you can use the following code to open the file with read/write access and allow other FileStream objects to read from the file:

using (var fileStream = new FileStream(fullPath, FileMode.Append, FileAccess.Write, FileShare.Read))
{
    using (var streamWriter = new StreamWriter(fileStream))
    {
        streamWriter.Write(content.ToString());
    }
}

This code opens the file with the Append FileMode, which means that new data is written to the end of the file. The FileAccess.Write parameter specifies that the file is opened with write access, and the FileShare.Read parameter specifies that other FileStream objects can read from the file while it is open.

You can also consider using a logging framework like Serilog or NLog, which provide built-in support for file locking and can handle concurrent writes to the same file. These frameworks also offer additional features like log rotation and filtering, which can be useful in a production environment.

Regarding the issue happening only in Azure App Service, it could be related to the way the App Service scales out instances and handles file locking across multiple instances. You can try using a centralized logging service like Azure Monitor, which can aggregate logs from multiple instances and provide a more scalable and reliable solution for logging in a cloud environment.

Let me know if you have any further questions or concerns by replying to this answer. Otherwise, if you find this helpful, please consider accepting it as the answer.