immutable collection
A collection that can't be changed; "changes" produce a new one.
System.Collections.Immutable types (ImmutableList, ImmutableDictionary, …) return new instances from every modifying operation; frozen collections (FrozenSet, FrozenDictionary) are read-only and optimized for fast lookups.
Examples
var more = prices.Add(4.99m);on anImmutableList<decimal>leavespricesas it was and returns a new list.