Hello folks, we've found a performance bottleneck searching for items in a large in-memory collection. Most of the code was converted from 15+ year old C++ code into C# so a lot of weird looking stuff came across. There are lots of sequential searches for items in arrays and List<> which have complexity O(n) and get really slow for large collections. I previously converted one bottleneck List<> into a Dictionary<> and it was a miracle cure, but it was a simple case with one property value being a unique key. The next bottleneck collection needs to be searched efficiently on different property values, and I'm wondering if anyone knows of a collection which supports multiple indexes (possibly non-unique). An in-memory SQLite table with multiple indexes would do the job, but it's a bit heavyweight and like hammering a screw. Using multiple Dictionaries or Sets to index the same objects won't work due to non-unique values. Maybe there is some lightweight package around for this sort of thing? It's a classic computer science problem. *Greg Keogh*