namespace System.Collections.Concurrent
{
    [DefaultMember("Item")]
    public class ConcurrentDictionary<TKey, TValue> : ICollection<KeyValuePair<TKey, TValue>>, IDictionary<TKey, TValue>, IEnumerable, IEnumerable<KeyValuePair<TKey, TValue>>, IReadOnlyCollection<KeyValuePair<TKey, TValue>>, IReadOnlyDictionary<TKey, TValue>, ICollection, IDictionary
        where TKey, notnull
    {
        [DefaultMember("Item")]
        public readonly struct AlternateLookup<TAlternateKey>
            where TAlternateKey, notnull
        {
            public TValue this[TAlternateKey key] { get; set; }
        }
    }
}