7: State and Life Cycle in VS .NET

Printer QR Code ISO/IEC18004 in VS .NET 7: State and Life Cycle

If you click the button to cause a postback, the trace information will include the additional events indicated in Figure 7-4, including ProcessPostData Second Try, ChangedEvents, and PostBackEvent. Often when you re trying to track down why your page is behaving a certain way or why some of your code does not seem to work as you would expect, looking at the life cycle behavior can be very illuminating.

State, in the case of a web page, is the current value of all the controls and variables, for the current user, in the current session. The Web is inherently a stateless environment, which means that each time a page is posted to the server and then sent back to the browser, the page is created again from scratch. Unless the state of all the controls is explicitly preserved before the page is posted, the state is lost and all the controls will be created with default values. One of the great strengths of ASP.NET is that it automatically maintains state for server controls both HTML and ASP.NET so you do not have to write any code to accomplish this. This section will explore how this is done and how you can make use of the ASP.NET state management capabilities. ASP.NET manages four types of state: Control state Used to provide features such as paging and sorting of GridView controls. Control state cannot be modified, accessed directly, or disabled. View state The state of all the controls on the page. View state only lasts for that one page display, and is updated every time the page is redrawn. It can be disabled for specific controls, the page, or the entire web site.

Session state Data specifically saved across page posts, for use by all the pages in a web application. Application state Data available to all the users of a web application, even across multiple sessions. Table 7-1 compares the kinds of state management (other than Control state, which is not accessible to the developer).

Table 7-1. Comparison of types of state Feature Uses server resources Uses bandwidth Times out Security exposure Optimized for nonprimitive types Available for arbitrary data Programmatically accessible Scope Survives restart View state No Yes No Yes No Yes Yes Page Yes Session state Yes No Yes Depends Yes Yes Yes Session Depends on configuration Application state Yes No No No Yes Yes Yes Application No

The view state is the state of the page and all its controls. The view state is automatically maintained across posts by the ASP.NET Framework. When a page is posted to the server, the view state is read. Just before the page is sent back to the browser, the view state is restored. The view state is saved in a hidden field on the page. Because the view state is maintained via a form field, this technique works with all browsers. The information saved in the hidden field is Base64 encoded, but not encrypted. As such, any information stored in view state is not immune to prying eyes. If there is no need to maintain the view state for a given page, you can boost performance by disabling view state for that page. For example, if the page does not post back to itself or if the only control on a page that might need to have its state maintained is populated from a database with every round trip to the server, then there will be no need to maintain the view state for that page. To disable view state for a page, add the EnableViewState attribute with a value of false to the Page directive: