c# - 如何观察DbSet <T>的Add Action ？

我有两个名为Contact和ContactField的类，如下所示。当将ContactField添加到Contact时，我希望自动将SortOrder分配给ContactField。我是否需要继承DbSet并自定义Add方法？如何实现呢？

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; }
 }

编辑:
我发现我需要监视ICollection<ContactField> ContactFields的更改。 List<T>是ICollection<T>的实现。因此，我创建了一个自定义的MyList并要求它通知MyList容器的更改。我会测试它是否有效。

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);
        }
 }

最佳答案

DbSet具有Local属性ojit_a。您可以订阅ObservableCollection事件并在那里更新排序顺序。

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
            }
        }
}