Entity Framework 6 实体某些字段根据模型状态进行自动更新内容

public class BaseEntity
{
　　public int Id { get; set; }
　　public DateTime? CreateTime { get; set; }
　　public DateTime? UpdateTime { get; set; }
　　public BaseEntity()
　　{
　　　　CreateTime = DateTime.Now;
　　}
}

public class DictEntity:BaseEntity
{
      /// <summary>
      /// 分组名称
      /// </summary>
      [StringLength(20)]
      public string GroupName { get; set; }
}

public class DBContext : DbContext
{
        public DBContext() : base("Name=ElectronicPrescriptionDB")
        {
            Database.SetInitializer(new DBInitializer());
        }
      　public DbSet<DictEntity> Dicts { get; set; }

　　　　 public override int SaveChanges()
        {
            var selectedEntityList = ChangeTracker.Entries().Where(x => x.Entity is BaseEntity && x.State == EntityState.Modified);
            //可以加入cookie里面的username,比如：操作人，需扩展DateTimeEntity的属性。
            //var userName = "MUKESH";
            foreach (var entity in selectedEntityList)
            {
                ((BaseEntity)entity.Entity).UpdateTime = DateTime.Now;
            }
            return base.SaveChanges();
        }
}

public RepositoryContext() : base("ApiConn")
{
    ((IObjectContextAdapter)this).ObjectContext.ObjectStateManager.ObjectStateManagerChanged += this.OnSaveChanges;
}