Understanding ASP.NET View State continued...

The Role of View State

View state's purpose in life is simple: it's there to persist state across postbacks. (For an ASP.NET Web page, its state is the property values of the controls that make up its control hierarchy.) This begs the question, "What sort of state needs to be persisted?" To answer that question, let's start by looking at what state doesn't need to be persisted across postbacks. Recall that in the instantiation stage of the page life cycle, the control hierarchy is created and those properties that are specified in the declarative syntax are assigned. Since these declarative properties are automatically reassigned on each postback when the control hierarchy is constructed, there's no need to store these property values in the view state.

For example, imagine we have a Label Web control in the HTML portion with the following declarative syntax:

  Text="Hello, World!">

When the control hierarchy is built in the instantiation stage, the Label's Text property will be set to "Hello, World!" and its Font property will have its Name property set to Verdana. Since these properties will be set each and every page visit during the instantiation stage, there's no need to persist this information in the view state.

Understanding ASP.NET View State continued...

Stage 0 - Instantiation

The life cycle of the ASP.NET page begins with instantiation of the class that represents the requested ASP.NET Web page, but how is this class created? Where is it stored?

ASP.NET Web pages, as you know, are made up of both an HTML portion and a code portion, with the HTML portion containing HTML markup and Web control syntax. The ASP.NET engine converts the HTML portion from its free-form text representation into a series of programmatically-created Web controls.

When an ASP.NET Web page is visited for the first time after a change has been made to the HTML markup or Web control syntax in the .aspx page, the ASP.NET engine auto-generates a class. If you created your ASP.NET Web page using the code-behind technique, this autogenerated class is derived from the page's associated code-behind class (note that the code-behind class must be derived itself, either directly or indirectly, from the System.Web.UI.Page class); if you created your page with an in-line, server-side "script" block, the class derives directly from System.Web.UI.Page. In either case, this autogenerated class, along with a compiled instance of the class, is stored in the WINDOWS\Microsoft.NET\Framework\version\Temporary ASP.NET Files folder, in part so that it doesn't need to be recreated for each page request.

Understanding ASP.NET View State

Introduction

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.