My program frequently stops with a deadlock. When I do a break-all and look at the threads I see that three threads are stuck in our logging function:

public class Logging
{
    public static void WriteClientLog(LogLevel logLevel, string message)
    {
      #if DEBUG
      System.Diagnostics.Debug.WriteLine(String.Format("{0} {1}", DateTime.Now.ToString("HH:mm:ss"), message)); //LOCK
      #endif
      //...Log4net logging
    }
}

If I let the program continue the threads are still stuck on that line.

I can't see where this can lock. The debug class, string class & datetime class seem to be thread safe.

The error goes away when I remove the "#if DEBUG System... #endif" code but I'm curious why this behavior happens.

Thread one:

public void CleanCache()
{
  Logging.WriteClientLog(LogLevel.Debug, "Start clean cache.");//Stuck
}

Thread two:

 private void AliveThread()
 {
   Logging.WriteClientLog(LogLevel.Debug, "Check connection");//Stuck
 }