https://learn.microsoft.com/en-us/dotnet/api/system.collections.immutable.immutablehashset-1.builder?view=dotnet-uwp-10.0
ImmutableHashSetT.Builder Class (System.Collections.Immutable) | Microsoft Learn
Represents a hash set that mutates with little or no memory allocations and that can produce or build on immutable hash set instances efficiently.
class systembuildercollectionsimmutablemicrosoft