one value per key 一键一值
SortedList<TKey, TValue>
multiple values per key 一键多值
Lookup<TKey, TElement>
SortedList与SortedDictionary
1)SortedList<TKey, TValue> uses less memory than SortedDictionary<TKey, TValue>.
2)SortedDictionary<TKey, TValue> has faster insertion and removal of elements.
3)When populating the collection with already sorted data, SortedList<TKey, TValue> is faster if capacity changes are not needed.
