ChangeTracker.Entries() throws NullReferenceException

[xrkolovos]

Steps to reproduce

Realy don't know

The issue

I have this code

public async Task<int> SaveChangesAsync()
        {
            try
            {
                ApplyConcepts();
                return await base.SaveChangesAsync();
            } 
            catch (Exception ex) {
                throw ex;
            }
        }
protected virtual void ApplyConcepts()
        {
            foreach (var entry in ChangeTracker.Entries())
            {
                switch (entry.State)
                {
                    case EntityState.Added:
                        SetCreationAuditProperties(entry);
                        //CheckAndSetTenantIdProperty(entry);
                        //EntityChangedEventHelper.TriggerEntityCreatedEvent(entry.Entity);
                        break;
                    case EntityState.Modified:
                        PreventSettingCreationAuditProperties(entry);
                        //CheckAndSetTenantIdProperty(entry);
                        SetModificationAuditProperties(entry);

                        if (entry.Entity is ISoftDelete && entry.Entity.As<ISoftDelete>().IsDeleted)
                        {
                            if (entry.Entity is IDeletionAudited)
                            {
                                SetDeletionAuditProperties(entry.Entity.As<IDeletionAudited>());
                            }
                            //EntityChangedEventHelper.TriggerEntityDeletedEvent(entry.Entity);
                        }
                        else
                        {
                            //EntityChangedEventHelper.TriggerEntityUpdatedEvent(entry.Entity);
                        }

                        break;
                    case EntityState.Deleted:
                        PreventSettingCreationAuditProperties(entry);
                        HandleSoftDelete(entry);
                        //EntityChangedEventHelper.TriggerEntityDeletedEvent(entry.Entity);
                        break;
                }
            }
        }

And when i try to insert something random i get this error. Any clue?

Exception message:
ChangeTracker.Entries() 'ChangeTracker.Entries()' threw an exception of type 'System.NullReferenceException'    System.Collections.Generic.IEnumerable<Microsoft.EntityFrameworkCore.ChangeTracking.EntityEntry> {System.NullReferenceException}

Stack trace:
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.RelationshipsSnapshot.RemoveFromCollection(IPropertyBase propertyBase, Object removedEntity)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntry.RemoveFromCollectionSnapshot(IPropertyBase propertyBase, Object removedEntity)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.RemoveFromCollection(InternalEntityEntry entry, INavigation navigation, IClrCollectionAccessor collectionAccessor, Object value)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.NavigationFixer.NavigationCollectionChanged(InternalEntityEntry entry, INavigation navigation, IEnumerable`1 added, IEnumerable`1 removed)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.InternalEntityEntryNotifier.NavigationCollectionChanged(InternalEntityEntry entry, INavigation navigation, IEnumerable`1 added, IEnumerable`1 removed)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ChangeDetector.DetectNavigationChange(InternalEntityEntry entry, INavigation navigation)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ChangeDetector.DetectChanges(InternalEntityEntry entry)
   at Microsoft.EntityFrameworkCore.ChangeTracking.Internal.ChangeDetector.DetectChanges(IStateManager stateManager)
   at Microsoft.EntityFrameworkCore.ChangeTracking.ChangeTracker.DetectChanges()
   at Microsoft.EntityFrameworkCore.ChangeTracking.ChangeTracker.TryDetectChanges()
   at Microsoft.EntityFrameworkCore.ChangeTracking.ChangeTracker.Entries()

Further technical details

EF Core version: 1.0.0
Operating system: win10
Visual Studio version: VS 2015

[ajcvickers]

@xrkolovos Can you please post code for the entity types that are being used? Also, if the application is adding more than just a single entity, then can you post code that reproduces how the entities are being added?

[xrkolovos]

I actually managed to make it work.
The issue was that before saving my new entity i was retrieving some entities from the db without using AsNotracking(). This code was trying to create many rows automatically for the user from some inputs. After calling Table.Add(entity) for each one of them, the entity that was retrieved without AsNotracking(that was related with the one i was trying to save) was attached to the new entity causing this problem.
The mayor problem i think was poor error information.

[ajcvickers]

@xrkolovos Glad you got it working, but there still should not be any case where this code throws null-ref exception, so if you could help me reproduce the issue that would be great.

[xrkolovos]

@ajcvickers it's complicate to find a way to reproduce it. I must give you all the source code for my model layer and repository layer and add you sample console app to trigger it.
Do you have any other ideas?
We can have a call/meeting if you want to show you.

[rowanmiller]

We're not too concerned about this one as it sounds like a very difficult situation to get into. If you end up supplying a way to reproduce it, then we'll look into it, but we're not too concerned if you can't.

[multiarc]

@ajcvickers looks like I spot the same issue.
Here is the preconditions:

public class Customer
{
   public int Id { get; set; }
   public int Status { get; set; }
   public ICollection<Order> Orders { get; set; }
}

public class Order
{
   public int Id { get; set; }
   public int CustomerId { get; set; }
   public Customer Customer { get; set; }
}

1. Read the collection of Order entity with tracking (do not include Customer)
2. Update the Order.CustomerId to the different one (existing real Id, not sure if matters)
3. Read the collection of primary entities with tracking (Customers), this way NavigationFixer won't connect Orders collections respectively to Customer
4. Update any field in customer (Status)
5. SaveChanges()
6. Profit -> ((HashSet)_values[index]).Remove(removedEntity); will fail in RelationshipsSnapshot because the related collection is null, because wasn't set by NavigationFixer because was updated prior to read.

[ajcvickers]

The problem here is that EF is not aware of the change made to CustomerId. When a change is made to an entity, it is a requirement that the application make EF aware of this change before performing any further operations with the context. In this case there are three main ways of doing this:

• Call context.ChangeTracker.DetectChanges()
• Use EF APIs to make the update--e.g. context.Entry(order).Property(e => e.CustomerId).CurrentValue = 2;
• Implement INotifyPropertyChanged on entity types and configure EF to use these nnotifications with HasChangeTrackingStrategy

Any one of these will result in correct behavior.

The problem with any other approach is that detecting changes can be slow, which means if we do it automatically in lots of places, then EF perf will get much worse. So we choose to call DetectChanges automatically only in the places where it adds most value. Typically before a new query is not one of those places.

The behavior if the context is not made aware of some change to an entity is undefined. However, in this case it is possible to be more resilient to a null relationship snapshot. I will submit a PR for that,

[ajcvickers]

Note for triage: RelationshipsSnapshot can be null when DetectChanges (or something equivalent) has not been used when it was needed. This is an application bug (not calling DetectChanges or similar) and the behavior of EF is undefined. However, in some cases the application may work fine because the FK is correct and the lack of fixed up navigation property is not significant to EF.

Three possible resolution paths come to mind for this case:

• Be resilient to null relationship snapshot. This is a trivial change, and can sometimes make an application work correctly--such as in this case.
• However, even though the application may work, it also may not, and being resilient may mean the application fails later, perhaps with data corruption. Therefore, we could check for null and throw an exception indicating that DetectChanges (or similar) is needed. Of course, there are many other code paths where DetectChanges is needed where this exception would never be thrown.
• Do nothing. Behavior of EF is undefined, so throwing NullRefEx is reasonable.

[rowanmiller]

Consensus is that we go with option 1