public DefaultRazorDynamicFileInfoProvider(DocumentServiceProviderFactory factory)
        {
            if (factory == null)
            {
                throw new ArgumentNullException(nameof(factory));
            }

            _factory = factory;

            _entries          = new ConcurrentDictionary <Key, Entry>();
            _createEmptyEntry = (key) => new Entry(CreateEmptyInfo(key));
        }