Thread-safe, enumeration

Thread-safe enumeration in C#

https://theburningmonk.com/2010/03/thread-safe-enumeration-in-csharp/

https://stackoverflow.com/questions/58541336/thread-safe-cached-enumerator-lock-with-yield

https://learn.microsoft.com/en-us/dotnet/standard/threading/managed-threading-best-practices