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Thursday, March 04, 2010

Understanding ASP.NET View State


Microsoft® ASP.NET view state, in a nutshell, is the technique used by an ASP.NET Web page to persist changes to the state of a Web Form across postbacks. In my experiences as a trainer and consultant, view state has caused the most confusion among ASP.NET developers. When creating custom server controls or doing more advanced page techniques, not having a solid grasp of what view state is and how it works can come back to bite you. Web designers who are focused on creating low-bandwidth, streamlined pages oftentimes find themselves frustrated with view state, as well. The view state of a page is, by default, placed in a hidden form field named __VIEWSTATE. This hidden form field can easily get very large, on the order of tens of kilobytes. Not only does the __VIEWSTATE form field cause slower downloads, but, whenever the user posts back the Web page, the contents of this hidden form field must be posted back in the HTTP request, thereby lengthening the request time, as well.
This article aims to be an in-depth examination of the ASP.NET view state. We'll look at exactly what view state is storing, and how the view state is serialized to the hidden form field and deserialized back on postback. We'll also discuss techniques for reducing the bandwidth required by the view state.
Note   This article is geared toward the ASP.NET page developer rather than the ASP.NET server control developer. This article therefore does not include a discussion on how a control developer would implement saving state. For an in-depth discussion on that issue, refer to the book Developing Microsoft ASP.NET Server Controls and Components.
Before we can dive into our examination of view state, it is important that we first take a quick moment to discuss the ASP.NET page life cycle. That is, what exactly happens when a request comes in from a browser for an ASP.NET Web page? We'll step through this process in the next section.

The ASP.NET Page Life Cycle

Each time a request arrives at a Web server for an ASP.NET Web page, the first thing the Web server does is hand off the request to the ASP.NET engine. The ASP.NET engine then takes the request through a pipeline composed of numerous stages, which includes verifying file access rights for the ASP.NET Web page, resurrecting the user's session state, and so on. At the end of the pipeline, a class corresponding to the requested ASP.NET Web page is instantiated and the ProcessRequest() method is invoked (see Figure 1).
Click here for larger image.

Figure 1. ASP.NET Page Handling
This life cycle of the ASP.NET page starts with a call to the ProcessRequest() method. This method begins by initializing the page's control hierarchy. Next, the page and its server controls proceed lock-step through various phases that are essential to executing an ASP.NET Web page. These steps include managing view state, handling postback events, and rendering the page's HTML markup. Figure 2 provides a graphical representation of the ASP.NET page life cycle. The life cycle ends by handing off the Web page's HTML markup to the Web server, which sends it back to the client that requested the page.
Note   A detailed discussion of the steps leading up to the ASP.NET page life cycle is beyond the scope of this article. For more information read Michele Leroux-Bustamante's Inside IIS & ASP.NET. For a more detailed look at HTTP handlers, which are the endpoints of the ASP.NET pipeline, check out my previous article on HTTP Handlers.
What is important to realize is that each and every time an ASP.NET Web page is requested it goes through these same life cycle stages (shown in Figure 2).

Figure 2. Events in the Page Life Cycle

continued in the next post....

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