// private members

        private void AddToCache(MemoryCacheEntry entry)
        {
            // add outside of lock
            if (entry == null)
            {
                return;
            }

            if (entry.HasExpiration())
            {
                _expires.Add(entry);
            }

            if (entry.HasUsage() &&
                (!entry.HasExpiration() || entry.UtcAbsExp - DateTime.UtcNow >= CacheUsage.MIN_LIFETIME_FOR_USAGE))
            {
                _usage.Add(entry);
            }

            // One last sanity check to be sure we didn't fall victim to an Add concurrency
            if (!entry.CompareExchangeState(EntryState.AddedToCache, EntryState.AddingToCache))
            {
                if (entry.InExpires())
                {
                    _expires.Remove(entry);
                }

                if (entry.InUsage())
                {
                    _usage.Remove(entry);
                }
            }

            entry.CallNotifyOnChanged();
            if (_perfCounters != null)
            {
                _perfCounters.Increment(PerfCounterName.Entries);
                _perfCounters.Increment(PerfCounterName.Turnover);
            }
        }