public TValue this[TKey key] { get { // key could be null if (key == null) { // ConcurrentDictionary hates null throw new ArgumentNullException(nameof(key)); } return(_wrapped[key]); } set { // key could be null if (key == null) { // ConcurrentDictionary hates null throw new ArgumentNullException(nameof(key)); } _wrapped.Set(key, value); } }