How to observe the Add action of DbSet<T>?

Question

I has two classes named Contact and ContactField as following. When the ContactField is added into Contact, I hope to assign SortOrder to ContactField automatically. Do I need to inherit DbSet and customize the Add method ? How to achieve it ?

public class Foo {
        private MyDbContext _db = new MyDbContext();

        public void HelloWorld() {
            Contact contact = ....; //< A contact from database.

            ContactField field = ....; ///< A new field 
            .... ///< assign other properties into this `field`
            field.FieldType = FieldType.Phone;

            // How to automatically update `SortOrder` 
            // when adding field into `ContactFields`
            contact.ContactFields.Add(field);

            _db.SaveChanges();
        }
}

public class Contact {
        public long ContactID { get; set; }

        public string DisplayName { get; set; }
        public string DisplayCompany { get; set; }
        public DateTime CreatedTime { get; set; }
        public DateTime ModifiedTime { get; set; }

        // Original codes    
        //public virtual ICollection<ContactField> ContactFields { get; set; }
        public virtual MyList<ContactField> ContactFields { get; set; }
}

 public class ContactField {
        public long ContactFieldID { get; set; }
        public int SortOrder { get; set; }
        public int FieldType { get; set; }

        public string Value { get; set; }
        public string Label { get; set; }

        [Column("ContactID")]
        public int ContactID { get; set; }
        public virtual Contact Contact { get; set; }
 }

Edit: I found what I need is to monitor the changes of ICollection<ContactField> ContactFields. And the List<T> is an implementation of ICollection<T>. So, I create a custom MyList and ask it notifies the changes of MyList container. I will test it works or not later.

public class MyList<TEntity> : List<TEntity> {
        public delegate OnAddHandler(object sender, TEntity entry);
        public event OnAddHandler OnAddEvent;

        public new void Add(TEntity entity) {
             OnAddEvent(this, entity); 
             base.Add(entity);
        }
 }

Answer 1

The DbSet has a Local property which is an ObservableCollection. You can subscribe to the CollectionChanged event and update the sort order there.

public class Foo {
        private MyDbContext _db = new MyDbContext();

        public void HelloWorld() {

            _db.Contacts.Local.CollectionChanged += ContactsChanged;

            Contact contact = ....; //< A contact from database.

            ContactField field = ....; ///< A new field 
            .... ///< assign other properties into this `field`
            field.FieldType = FieldType.Phone;

            // How to automatically update `SortOrder` 
            // when adding field into `ContactFields`
            contact.ContactFields.Add(field);

            _db.SaveChanges();
        }

        public void ContactsChanged(object sender, NotifyCollectionChangedEventArgs args) {

            if (args.Action == NotifyCollectionChangedAction.Add)
            {

                // sort
            }    
        }
}

Answer 2

Alternatively override the SaveChanges method on the DbContext, and use the local ChangeTracker property to find new entities of a particular type and set their sort order property. Works great for setting things like last updated date in 1 place.