C#

private void myFileSystemWatcher_Changed(object sender, FileSystemEventArgs e)
{
  ThreadPool.QueueUserWorkItem(new WaitCallback(DoFileDiff), e.FullPath);
}
private void DoFileDiff(object state)
{
  string path = state as string;
 
  // Diff the file contents with another in a cache or DB - just an exmaple.
  int changedLines = Diff(path);
 
  // Now lets say you need to sync in order to write to a log file, you
  // can use the lock keyword in C# against an object. So long as the object
  // is the same object that you lock against in other methods you want
  // synchronized with each other, it will work (it is a Moniter).
  lock (this)
    WriteToLog(path, "Changed", changedLines);
}

// Declare the delegate as a nested delegate or stand-alone.
// In our case, the <code>WaitCallback</code> that we used earlier is
// already defined, so we'll just use that. The line below is only an
// example of how the WaitCallback is defined:
// public delegate void WaitCallback(object state);
 
private void myFileSystemWatcher_Changed(object sender, FileSystemEventArgs e)
{
  WaitCallback callback = new WaitCallback(DoFileDiff);
  callback.BeginInvoke(e.FullPath, null, null);
}

Monitor.Enter(syncRoot);
try
{
  // The body in the "lock" block would go here.
}
finally
{
  // This always executes whether or not an exception is thrown and
  // makes sure that resources are not locked indefinitely.
  Monitor.Exit();
}