Question

As an exercise in idle curiosity more than anything else, consider the following simple logging class:

internal static class Logging
{
    private static object threadlock;

    static Logging()
    {
        threadlock = new object(); 
    }

    internal static void WriteLog(string message)
    {
        try
        {
            lock (threadlock)
            {
                File.AppendAllText(@"C:\logfile.log", message);
            }
        }
        catch
        {
            ...handle logging errors...
        }
    }
}

Is the lock needed around File.AppendAllText(...) or is the method inherently thread-safe by its own implementation ?

Searching for information on this yields a lot of contradictory information, some say yes, some say no. MSDN says nothing.

Answer 1

File.AppendAllText is going to acquire an exclusive write-lock on the log file, which would cause any concurrent thread attempting to access the file to throw an exception. So yes, you need a static lock object to prevent multiple threads from trying to write to the log file at the same time and raising an IOException.

If this is going to be an issue, I’d really suggest logging to a database table which will do a better job of handling concurrent log writers.

Alternatively, you can use TextWriterTraceListener which is thread-safe (well, it’s going to do the locking for you; I’d rather write as little of my own multithreaded code as possible).