Attaching and Detaching Entities

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原文链接：http://www.cnblogs.com/fast-michael/archive/2011/06/16/2082725.html

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#数据库

本文介绍了如何在ObjectContext中管理实体对象状态，包括更新、断开连接和应用当前值的方法。通过实例演示了如何在对象上下文中操作Racer实体，并在断开连接后修改其属性。

If the entity object is already inside the object context, the existing one is used; otherwise it is fetched newly from the database.Invoking the method ApplyCurrentValues() passes the modified entity

object to the object context,and if there are changes, then the changes are done within the existing entity with the same key inside the object context, and the EntityState is set to EntityState.Modified. Remember that the method ApplyCurrentValues() requires the object to exist within the object context; otherwise the new entity object is added with EntityState.

private static void DetachDemo()
        {
            using (var data = new Formula1Entities())
            {
                data.ObjectStateManager.ObjectStateManagerChanged +=
                      ObjectStateManager_ObjectStateManagerChanged;
                ObjectQuery<Racer> racers = data.Racers.Where("it.Lastname='Alonso'");
                Racer fernando = racers.First();
                EntityKey key = fernando.EntityKey;
                data.Racers.Detach(fernando);
                // Racer is now detached and can be changed independent of the object context
                fernando.Starts++;
                Racer originalObject = data.GetObjectByKey(key) as Racer;
                data.Racers.ApplyCurrentValues(fernando);

                
            }

        }

static void ObjectStateManager_ObjectStateManagerChanged(object sender,
             CollectionChangeEventArgs e)
       {
           Console.WriteLine("Object State change—action: {0}", e.Action);
           Racer r = e.Element as Racer;
           if (r != null)
               Console.WriteLine("Racer {0}", r.Lastname);
       }