.NET 2.0 : File.AppendAllText(…) - Thread safe implementation

Question

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

internal static class Logging
{
    private static object          


        

Answer 1

It is thread safe in the sense that it opens the file with Read sharing, so assuming your filesystem honors file locks, only one thread will be allowed to write to the file at a time. Other threads may, however, get dirty reads if they are attempting to read the same file.

Answer 2

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).

Answer 3

Testing parallel writes shows that you would get a System.IO.IOException if you were to comment out your lock statement.

[Test]
public void Answer_Question()
{
    var ex = Assert.Throws<AggregateException>(() => Parallel.Invoke(
        () => Logging.WriteLog("abc"),
        () => Logging.WriteLog("123")
    ));

    // System.IO.IOException: The process cannot access the file 'C:\Logs\thread-safety-test.txt' because it is being used by another process.
    Console.Write(ex);
}