FunqCollections (nuget): High performance persistent and immutable collections for .NET, supporting many operations efficiently.  Includes a catenable/indexed deque, a high performance vector, sets, and maps/dictionaries.  Strictly better than anything released for .NET previously, including Microsoft's new immutable collections namespace. Check out the comparisons in the project website. Currently only core operations are fully tested. Not related to the IoC library also called Funq.
Keywords: collection|persistent|immutable|performance|functional|list|vector|set|dictionary|map|queue|stack|deque|funq|data-structure|confluent|tree|concat|concatenate|split|join|catenable