Obsolete / historical: preserved for context. ReactiveCompositeCollections is a third-party project, not part of ReactiveUI. Modern ReactiveUI projects should use DynamicData for composing observable collections.

ReactiveCompositeCollections, by Brad Phelan, added an ICompositeCollection<T>. It supported Select, SelectMany and Where, the way IEnumerable<T> and IObservable<T> do. That let a screen compose and filter several source lists into one collection declaratively. The library lives at Reactive Composite Collections (GitHub). It was published on NuGet too.

DynamicData covers the same ground today. It has its own source lists, caches and operators. Collections documents DynamicData.