InvalidOperationException: "Collection was modified" inside NavigationFixer

[joshmouch]

I have some code that is adding an entity to the DbContext if it hasn't previously been added. This works in most cases, however, in one case I've found, there's an exception being throw in the ChangeTracking.Internal.NavigationFixer.

The code is:

public void Add<TEntity>(TEntity entity) where TEntity : class
		{
			var entry = this.DataContext.Entry(entity);
			if (entry.State == EntityState.Detached)
			{
				this.DataContext.Add((object)entity);
			}
		}

The exact exception is:

System.InvalidOperationException occurred
  HResult=0x80131509
  Message=Collection was modified; enumeration operation may not execute.
  Source=System.Core
  StackTrace:
   at System.Collections.Generic.HashSet`1.Enumerator.MoveNext()
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.InitialFixup(InternalEntityEntry entry, ISet`1 handledForeignKeys, Boolean fromQuery)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.StateChanged(InternalEntityEntry entry, EntityState oldState, Boolean fromQuery)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.StateChanged(InternalEntityEntry entry, EntityState oldState, Boolean fromQuery)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(EntityState oldState, EntityState newState, Boolean acceptChanges)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(EntityState entityState, Boolean acceptChanges)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.PaintAction(EntityEntryGraphNode node)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.TraverseGraph(EntityEntryGraphNode node, Func`2 handleNode)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.AttachGraph(InternalEntityEntry rootEntry, EntityState entityState)
   at Microsoft.EntityFrameworkCore.DbContext.SetEntityState(InternalEntityEntry entry, EntityState entityState)
   at Microsoft.EntityFrameworkCore.DbContext.SetEntityState(Object entity, EntityState entityState)
   at Microsoft.EntityFrameworkCore.DbContext.Add(Object entity)

The stack trace is:

 	System.Core.dll!System.Collections.Generic.HashSet<MyEntity>.Enumerator.MoveNext()	Unknown
>	Microsoft.EntityFrameworkCore.dll!Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.InitialFixup(Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry entry, System.Collections.Generic.ISet<Microsoft.EntityFrameworkCore.Metadata.IForeignKey> handledForeignKeys, bool fromQuery)	Unknown
 	Microsoft.EntityFrameworkCore.dll!Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.StateChanged(Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry entry, Microsoft.EntityFrameworkCore.EntityState oldState, bool fromQuery)	Unknown
 	Microsoft.EntityFrameworkCore.dll!Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.StateChanged(Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry entry, Microsoft.EntityFrameworkCore.EntityState oldState, bool fromQuery)	Unknown
 	Microsoft.EntityFrameworkCore.dll!Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(Microsoft.EntityFrameworkCore.EntityState oldState, Microsoft.EntityFrameworkCore.EntityState newState, bool acceptChanges)	Unknown
 	Microsoft.EntityFrameworkCore.dll!Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.SetEntityState(Microsoft.EntityFrameworkCore.EntityState entityState, bool acceptChanges)	Unknown
 	Microsoft.EntityFrameworkCore.dll!Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.PaintAction(Microsoft.EntityFrameworkCore.ChangeTracking.EntityEntryGraphNode node)	Unknown
 	Microsoft.EntityFrameworkCore.dll!Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityEntryGraphIterator.TraverseGraph(Microsoft.EntityFrameworkCore.ChangeTracking.EntityEntryGraphNode node, System.Func<Microsoft.EntityFrameworkCore.ChangeTracking.EntityEntryGraphNode, bool> handleNode)	Unknown
 	Microsoft.EntityFrameworkCore.dll!Microsoft.EntityFrameworkCore.ChangeTracking.Internal.EntityGraphAttacher.AttachGraph(Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry rootEntry, Microsoft.EntityFrameworkCore.EntityState entityState)	Unknown
 	Microsoft.EntityFrameworkCore.dll!Microsoft.EntityFrameworkCore.DbContext.SetEntityState(Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry entry, Microsoft.EntityFrameworkCore.EntityState entityState)	Unknown
 	Microsoft.EntityFrameworkCore.dll!Microsoft.EntityFrameworkCore.DbContext.SetEntityState(object entity, Microsoft.EntityFrameworkCore.EntityState entityState)	Unknown
 	Microsoft.EntityFrameworkCore.dll!Microsoft.EntityFrameworkCore.DbContext.Add(object entity)	Unknown

I went through the source code in that class and I only see some "foreach" loops, but I'm not immediately aware of anything I may be doing to affect these Enumerables.

I'm still investigating.

The only thing of note so far is that the TEntity type in the HashSet in the stack trace is a child collection of the TEntity type passed into the Add method in the first code snippet.