如何观察DbSet <T>的Add动作？

Question

我有一个名为两个类Contact和ContactField如下.当ContactField被加入Contact,我希望能分配SortOrder到ContactField自动.我是否需要继承DbSet并自定义Add方法？怎么实现呢？

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: 我发现我需要的是监控变化ICollection<ContactField> ContactFields.而且List<T>是一个实现ICollection<T>.所以,我创建一个自定义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);
        }
 }

Answer 1

DbSet有一个Local属性ObservableCollection.您可以订阅该CollectionChanged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
            }    
        }
}