[magicalhippo]

Which collection APIs were you missing? Between the standard interfaces and their default implementations[1] and the extension methods to IEnumerable[2] you can do quite a lot in my experience.

[1]: https://docs.microsoft.com/en-us/dotnet/api/system.collectio...

[2]: https://docs.microsoft.com/en-us/dotnet/api/system.collectio...