深入理解 ViewState_Asp.Net开发技巧

时间：2010-09-15 来源：缘分星空

上个星期写了一篇《控件 ViewState 属性的值保存去哪里了》，解释了Control.ViewState最终还是通过Control.SaveViewState和 Control.LoadViewState这两个方法存取的。文章中有一句话可能会让大家感到疑惑的：“我们在OnInit之后使用 this.ViewState[key]读写时该属性都为true”，其中“该属性”指StateItem.IsDirty。到底为什么IsDirty属性在OnInit之后总是为true呢？参考了TRULY Understanding ViewState，我终于明白到其实它并非总是为true，详细原因请听我慢慢说。

首先要让大家来看的是StateBag.TrackViewState方法，这个方法在控件OnInit时就会被调用，而它的作用就是让 StateBag开始跟踪StateItem的变化，任何变化都将导致该StateItem的IsDirty属性变为true。也就是说，在OnInit 之前，IsDirty属性是false的，并且无论你如何设置Value属性的值都不会改变IsDirty属性。在OnInit之后，IsDirty属性也保持着false，直到你第一次改变Value属性的值（指通过this.ViewState[key]的方法改变）。到了SaveViewState 的阶段，只有IsDirty属性为true的StateItem才会被保存下来。

为什么要如此设计呢？例如一个通过声明性定义的Label的Text属性，在ASPX中它被赋了初值，然后该初值自然通过ViewState ["Text"]来持久。在下一个页面生命周期，首先OnInit时这个Label的Text属性会加载ASPX中声明性定义的初值，然后 LoadViewState时会用ViewState中读取到的ViewState["Text"]来覆盖它。然而除非你在上一个页面生命周期以编程的方式改变了Text属性，否则ViewState["Text"]还是初值，那么你就是用ViewState["Text"]保存初值去覆盖声明性定义的初值，同一个值这样覆盖完全没意义，而且还浪费了ViewState的空间。为了解决这个资源浪费的问题，凡是声明性定义之后没改变到的值就不应该使用 ViewState来持久，而详细的实现就是上面说的TrackViewState机制了。

说到这里，Control.ViewState已经解释完毕，如果你是控件设计者你可以放心地按以下方式把控件属性存放到ViewState中：
public string Text
{
get {return this.ViewState["Text"] as string;}
set {this.ViewState["Text"] = value;}
}
它的内部机制会懂得区分你存进去的值是不是ASPX上声明性定义的初值，然后决定是否持久该值。同时，如果你在任何阶段想改变一个ViewState值是否持久的决定，可以通过ViewState.SetItemDirty(key, dirty)来改变，这基本上已经满足了所有控件开发人员的需求。

http://www.cnblogs.com/cathsfz/archive/2006/10/29/543695.html

转自：动态网站制作指南 | www.knowsky.com
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