public override int SaveChanges()
        {
            //判断是用重写的savechanges方法 还是普通的savechange方法
            if (LogChangesDuringSave)
            {
                //过滤所有修改了的实体，包括：增加 / 修改 / 删除
                var entries = from obj in this.ChangeTracker.Entries()
                              where obj.State != EntityState.Unchanged
                              select obj;

                foreach (var item in entries)
                {
                    switch (item.State)
                    {
                        case EntityState.Added:
                            Console.WriteLine("Adding a {0}", item.Entity.GetType());
                            PrintPropertyValues(item.CurrentValues, item.CurrentValues.PropertyNames);
                            break;
                        case EntityState.Deleted:
                            Console.WriteLine("Adding a {0}", item.Entity.GetType());
                            PrintPropertyValues(item.CurrentValues, item.CurrentValues.PropertyNames);
                            break;

                        case EntityState.Modified:
                            Console.WriteLine("Adding a {0}", item.Entity.GetType());
                            PrintPropertyValues(item.CurrentValues, item.CurrentValues.PropertyNames);
                            break;
                        default:
                            break;
                    }
                }
            }

            //返回普通的savechange方法
            return base.SaveChanges();
        }