System.Collections.Immutable by Microsoft

<PackageReference Include="System.Collections.Immutable" Version="8.0.0-preview.1.23110.8" />

.NET API 249,008 bytes

ItemsFrozenSet<T, TThisWrapper>

abstract class ItemsFrozenSet<T, TThisWrapper> : FrozenSetInternalBase<T, TThisWrapper> where TThisWrapper : struct, IGenericSpecializedWrapper<T, TThisWrapper>

protected ItemsFrozenSet(HashSet<T> source, bool optimizeForReading = true)