System.Collections.Hashtable.SyncHashtable.ContainsKey C# (CSharp) Метод

            public override bool ContainsKey(Object key)
            {
                if (key == null)
                {
                    throw new ArgumentNullException(nameof(key), SR.ArgumentNull_Key);
                }
                Contract.EndContractBlock();
                return _table.ContainsKey(key);
            }