text: public virtual Stream ObtainFile(string sessionId, string source, string fileName){lock (padlock){EnsureOpen();ReplicationSession session;if (sessions.TryGetValue(sessionId, out session) && session != null && session.IsExpired(ExpirationThreshold)){ReleaseSession(sessionId);session = null;}if (session == null){throw new SessionExpiredException(string.Format("session ({0}) expired while obtaining file: source={1} file={2}", sessionId, source, fileName));}sessions[sessionId].MarkAccessed();return session.Revision.Revision.Open(source, fileName);}}