Question: why do you not allow `IImmutableList<T>` for collection navigational properties with backing fields?

Start with the failing test TestSaveBookOneReviewAndReadOk in Test/UnitTests/TestDataLayer/Ch08_RelationshipBackingFields.cs at line 151. Read the relationship backing-field behavior and the RelationshipsSnapshot stack trace to determine why a HashSet is cast to IImmutableList. This issue is a question rather than a defined change, so a maintainer would need to clarify the expected behavior before implementation.

This is a question, rather than a request for a change!

I want to create entity classes where no properties can be changed outside the class (DDD-styled class). For instance:

public class Book
{
     private readonly List<Review> _reviews;
     public IEnumerable<Review> Reviews => _reviews.ToList();
     //... other properties left out
}

From my testing I know that collection navigational properties can use the following types -

• IEnumerable<T>
• IReadOnlyCollection<T>

I want to move away from using IEnumerable<T>, as it has (small) performance problems when you call it multiple times. I would like to use IImmutableList<T> as that sounds like a better type because it ensures that a) it takes a copy (AsReadOnly doesn't take a copy) and, b) makes parallel operations work better. But I get I use IImmutableList<T>.

   TestSaveBookOneReviewAndReadOk [0:00.749] Failed: System.InvalidCastException : Unable to cast object of type 'System.Collections.Generic.HashSet`1[DataLayer.EfClasses.Review]' to type 'System.Collections.Immutable.IImmutableList`1[DataLayer.EfClasses.Review]'.
System.InvalidCastException : Unable to cast object of type 'System.Collections.Generic.HashSet`1[DataLayer.EfClasses.Review]' to type 'System.Collections.Immutable.IImmutableList`1[DataLayer.EfClasses.Review]'.
   at lambda_method(Closure , InternalEntityEntry )
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.RelationshipsSnapshot..ctor(InternalEntityEntry entry)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.EnsureRelationshipSnapshot()
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntrySubscriber.SnapshotAndSubscribe(InternalEntityEntry entry)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.StateManager.StartTracking(InternalEntityEntry entry)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(EntityState oldState, EntityState newState, Boolean acceptChanges, Boolean modifyProperties)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(EntityState entityState, Boolean acceptChanges, Boolean modifyProperties, Nullable`1 forceStateWhenUnknownKey)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.PaintAction(EntityEntryGraphNode`1 node)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.TraverseGraph[TState](EntityEntryGraphNode`1 node, Func`2 handleNode)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.AttachGraph(InternalEntityEntry rootEntry, EntityState targetState, EntityState storeGeneratedWithKeySetTargetState, Boolean forceStateWhenUnknownKey)
   at Microsoft.EntityFrameworkCore.DbContext.SetEntityState(InternalEntityEntry entry, EntityState entityState)
   at Microsoft.EntityFrameworkCore.DbContext.SetEntityState[TEntity](TEntity entity, EntityState entityState)
   at Microsoft.EntityFrameworkCore.DbContext.Add[TEntity](TEntity entity)
   at Test.UnitTests.TestDataLayer.Ch08_RelationshipBackingFields.TestSaveBookOneReviewAndReadOk() in C:\Users\JonPSmith\source\repos\EfCoreinAction-SecondEdition\Test\UnitTests\TestDataLayer\Ch08_RelationshipBackingFields.cs:line 151

IImmutableList<T> seems to tick all the boxes by supporting IEnumerable<T>, ICollection<T>, IList<T>. Also AsReadOnly() doesn't work with a HashSet, but ToImmutableList() does. so why does EF Core not support it? Is there something fundamental or just the way it is written?

NOTE: I have tried this on EF Core 3.1 and 5-preview4