Want to write to a file, but may have potentially multiple writers at once, need to lock

lock(this)
{
  // perform the write.
}

// try enter will return false if another thread owns the lock
   if (Monitor.TryEnter(lockObj))
   {
      try
      {
         // check last write time here, return if too soon; otherwise, write
      }
      finally
      {
         Monitor.Exit(lockobj);
      }
   }

FileInfo fi = new FileInfo(path);
if (fi.Exists
    && (DateTime.UtcNow - fi.LastWriteTimeUtc < TimeSpan.FromMinutes(15)) {
  // file is fresh
  return;
}

FileStream fs;
try {
  fs = new FileStream(
    path, FileMode.Create, FileAccess.Write, FileShare.Read);
} catch (IOException) {
  // file is locked
  return;
}

using (fs) {
  // write to file
}