namespace System.Collections.Generic
{
    [DefaultMember("Item")]
    public class Dictionary<TKey, TValue> : ICollection<KeyValuePair<TKey, TValue>>, IDictionary<TKey, TValue>, IEnumerable, IEnumerable<KeyValuePair<TKey, TValue>>, IReadOnlyCollection<KeyValuePair<TKey, TValue>>, IReadOnlyDictionary<TKey, TValue>, ICollection, IDictionary, IDeserializationCallback, ISerializable
        where TKey, notnull
    {
        public sealed class KeyCollection : ICollection<TKey>, IEnumerable, IEnumerable<TKey>, IReadOnlyCollection<TKey>, ICollection
        {
            public bool Contains(TKey item);
        }
    }
}