ArgumentException on session Flush

[igorvorona]

I have upgraded existing NHibernate 4.0.4 to 5.3.8. All seem to be working with minor changes, but lots of tests failing with the following error when session is either manually flushed or auto-flushed when executing HQL scripts:

System.ArgumentException : An item with the same key has already been added.
   at System.ThrowHelper.ThrowArgumentException(ExceptionResource resource)
   at System.Collections.Generic.Dictionary`2.Insert(TKey key, TValue value, Boolean add)
   at NHibernate.Collection.Generic.PersistentGenericSet`1.GetSnapshot(ICollectionPersister persister)
   at NHibernate.Engine.CollectionEntry.AfterAction(IPersistentCollection collection)
   at NHibernate.Action.CollectionUpdateAction.Execute()
   at NHibernate.Engine.ActionQueue.InnerExecute(IExecutable executable)
   at NHibernate.Engine.ActionQueue.ExecuteActions[T](List`1 list)
   at NHibernate.Engine.ActionQueue.ExecuteActions()
   at NHibernate.Event.Default.AbstractFlushingEventListener.PerformExecutions(IEventSource session)
   at NHibernate.Event.Default.DefaultAutoFlushEventListener.OnAutoFlush(AutoFlushEvent event)
   at NHibernate.Impl.SessionImpl.AutoFlushIfRequired(ISet`1 querySpaces)
   at NHibernate.Impl.SessionImpl.List(IQueryExpression queryExpression, QueryParameters queryParameters, IList results, Object filterConnection)
   at NHibernate.Impl.SessionImpl.List(IQueryExpression queryExpression, QueryParameters queryParameters, IList results)
   at NHibernate.Impl.AbstractSessionImpl.List(IQueryExpression queryExpression, QueryParameters parameters)
   at NHibernate.Impl.AbstractQueryImpl2.List()

[igorvorona]

It also throws sometimes session.Refresh() in EvictCachedCollections.

[bahusoid]

Looks like regression from #2394

@maca88 It seems SetSnapShot.Add behaves differently for .NET Core and NetFx. .NET Core version allows duplicates and NetFx throws.
It seems NetFx Add method should use dic[key] = value instead of dic.Add

[igorvorona]

Yes, I agree. Btw, this code runs in NET Framework 4.8.
Is there a way I can override code in flush/refresh listeners?

[maca88]

Looks like regression from #2394

You are correct, the indexer should be used instead.

[igorvorona]

@maca88 when can we get it fixed?

[maca88]

After the fix (#2806) will be reviewed and merged you should be able to test it out by using the package from myget.

[igorvorona]

Got it, thanks!