System.Collections.SortedList.SyncSortedList.IndexOfKey C# (CSharp) Method

            public override int IndexOfKey(Object key)
            {
                if (key == null)
                    throw new ArgumentNullException(nameof(key), SR.ArgumentNull_Key);
                Contract.EndContractBlock();

                lock (_root)
                {
                    return _list.IndexOfKey(key);
                }
            }