EF6: Collection was modified, enumeration operation may not execute

Question

I have been trying to add multiple objects to a DbSet, but everything I try results in a 'System.InvalidOperationException: Collection was modified; enumeration operation may not execute.' Stacktrace:

at System.ThrowHelper.ThrowInvalidOperationException(ExceptionResource resource) at System.Collections.Generic.List1.Enumerator.MoveNextRare() at System.Collections.Generic.List1.Enumerator.MoveNext() at System.Data.Entity.Core.Objects.EntityEntry.TakeSnapshotOfRelationships() at System.Data.Entity.Core.Objects.Internal.EntityWrapperWithoutRelationships1.TakeSnapshotOfRelationships(EntityEntry entry) at System.Data.Entity.Core.Objects.ObjectContext.AddSingleObject(EntitySet entitySet, IEntityWrapper wrappedEntity, String argumentName) at System.Data.Entity.Core.Objects.ObjectContext.AddObject(String entitySetName, Object entity) at System.Data.Entity.Internal.Linq.InternalSet1.<>c__DisplayClassd.b__c() at System.Data.Entity.Internal.Linq.InternalSet1.ActOnSet(Action action, EntityState newState, Object entity, String methodName) at System.Data.Entity.Internal.Linq.InternalSet1.Add(Object entity) at System.Data.Entity.DbSet`1.Add(TEntity entity)

This only occurs the second time a 'Mail' object is being added to the context The code:

            foreach (var folderId in folderIds)
            {
                db.Mails.Add(new Mail
                {
                    From = from,
                    To = to,
                    CC = cc,
                    Attachments = attachments,
                    BodyHtml = bodyHtml,
                    BodyPlain = bodyPlain,
                    Subject = subject,
                    Incoming = true,
                    Priority = receivedMail.Priority,
                    ReceivedOn = DateTime.UtcNow,
                    Status = MailStatus.Open,
                    Read = false,
                    Folder = db.Folders.Find(folderId)
                });
            }

The only enumeration I am doing is on folderIds which is a array of ints which as you can see in the stack trace isn't the cause. I've been stuck on this for a while and tried all kinds of approaches like a temp collection and using AddRange or saving the context after every add.

Any help with where the problem lies will be much appreciated.

Your error must be coming from statement Folder = db.Folders.Find(folderId) — Satpal
Folder is a virtual property in your entities Am I right? if yes then you no need to assign value for that. — Chandrasekar Kesavan
@Setpal I also think that is the source. But I haven't got a clue how to fix it :P JenishRabadiya I don't think that is the source of the issues. I tried to list, to array, or puttting it in a for loop which all had the same result. Chandru Yes it is a virtual property. But it has to be assigned somehow otherwise it will add a null, which is not what I want. I have tried db.Folders.Find(folderId).Mails.Add(new Mail....) — Nick
It also throws the same exception when I set FolderId manually using a foreign key attribute in the model. Which should rule out the .find issue. — Nick

Dmitriy Shapar Dmitriy Shapar · Accepted Answer · 2016-04-05T11:11:55

Attachments also collection? If it comes from any db object try to call ToList() when assigning.

 foreach (var folderId in folderIds)
        {
            db.Mails.Add(new Mail
            {
                From = from,
                To = to,
                CC = cc,
                Attachments = attachments.ToList(),
                BodyHtml = bodyHtml,
                BodyPlain = bodyPlain,
                Subject = subject,
                Incoming = true,
                Priority = receivedMail.Priority,
                ReceivedOn = DateTime.UtcNow,
                Status = MailStatus.Open,
                Read = false,
                Folder = db.Folders.Find(folderId)
            });
        }

EF6: Collection was modified, enumeration operation may not execute

1 Answers