• Acts exactly like Dictionary and HashSet, except they don't support any changes after initialization.
• both built just like their .Net counterpart, with an array acting as a hash map. Given a constant set of items, these collections choose a small prime number for the hash map with no collisions. If such a number can't be found, the general purpose Dictionary and HashSet are used.
• These collections are O(1) (not amortized), as there are no collisions by definition.

When you have no writes, and a lot of reads, these collections can shave 10% - 50% of the Dictionary / HashSet performance. Always benchmark your code.