private void OnCacheItemAdded(CacheItemAddedEventArgs <T> eventArgs)
 {
     CacheItemAdded?.Invoke(
         sender: this,
         e: new CacheItemAddedEventArgs <T>(
             cacheKey: eventArgs.CacheKey,
             originalValue: eventArgs.OriginalValue,
             valid: eventArgs.Valid,
             validation: eventArgs.Validation));
 }
        void IObserver <CacheItemAdded <TValue> > .OnNext(CacheItemAdded <TValue> value)
        {
            TIndex key = _keyProvider(value.Value);

            _index.TryAdd(key, value.Value);
        }