Tested on Versions 4.4.2 -> 4.5.3_RC

Code Example 1 (PersonCategory is already an item in PersonCategories EntitySet):

• Person.PersonCategories.Clear(); (same with Remove)
• Person.PersonCategories.Add( PersonCategory );

System.InvalidOperationException: Message = "State with key 'Person-PersonCategories-PersonCategory, (957, 921)' is already exists."

StackTrace:

• at Xtensive.Orm.Disconnected.StateRegistry.Create(Key key, Tuple tuple, Boolean isLoaded)
• at Xtensive.Orm.DisconnectedState.Persist(EntityState entityState, PersistActionKind persistAction)
• at Xtensive.Orm.Disconnected.DisconnectedSessionHandler.<persist>b__0(EntityState item)
• at Xtensive.Core.EnumerableExtensions.ForEachT
• at Xtensive.Orm.Disconnected.DisconnectedSessionHandler.Persist(EntityChangeRegistry registry, Boolean allowPartialExecution)
• at Xtensive.Orm.Session.Persist(PersistReason reason)
• at Xtensive.Orm.Session.CommitTransaction(Transaction transaction)
• at Xtensive.Orm.Transaction.Commit()
• at Xtensive.Orm.TransactionScope.Dispose()
• at Xtensive.Core.DisposableExtensions.DisposeSafely(IDisposable disposable)
• at Xtensive.Orm.TransactionalAttribute.OnExit(MethodExecutionArgs args)
• at Xtensive.Orm.EntitySetBase.Add(Entity item)
• at Xtensive.Orm.EntitySetBase.Add(IEntity item)
• at Xtensive.Orm.EntitySet`1.Add(TItem item)

Code Example 2 (No item in PersonCategories EntitySet):

• Person.PersonCategories.Add( PersonCategory );
• Person.PersonCategories.Clear();
• Person.PersonCategories.Add( PersonCategory );
• Person.PersonCategories.Clear();

System.NullReferenceException: Message = "Object reference not set to an instance of an object."

StackTrace:

• at Xtensive.Orm.Session.RemoveOrCreateRemovedEntity(Type type, Key key)
• at Xtensive.Orm.EntitySetBase.<>cDisplayClass10.<remove>b9()
• at Xtensive.Orm.EntitySetBase.Remove(Entity item, SyncContext syncContext, RemovalContext removalContext)
• at Xtensive.Orm.EntitySetBase.Remove(Entity item)
• at Xtensive.Orm.EntitySetBase.Remove(IEntity item)
• at Xtensive.Orm.EntitySetBase.Clear()