Good in-memory database for .net/PowerShell?
For small data ( less then few GBs ) you can use a RAM disk, and use any usual storage ( SQ DB, NoSQL DB, or even a text file), to store data, as usual.
Now in memory database is available in Microsoft.Data.Sqlite:
https://docs.microsoft.com/en-us/dotnet/standard/data/sqlite/in-memory-databases