ASP.NET State Management in C#

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17
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ASP.NET State Management
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In the beginner s mind there are many possibilities. In the expert s mind there are few. Shunryu Suzuki All real-world applications of any shape and form need to maintain their own state to serve users requests. ASP.NET applications are no exception. However, unlike other types of applications, they need special system-level tools to achieve the result. The reason for this peculiarity lies in the stateless nature of the underlying protocol that Web applications still rely upon. As long as HTTP remains the transportation protocol for the Web, all applications will run into the same problem figuring out the most effective way to persist state information. Application state is a sort of blank container that each application and programmer can fill with whatever piece of information makes sense to persist: from user preferences to global settings, from worker data to hit counters, from lookup tables to shopping carts. This extremely variegated mess of data can be organized and accessed according to a number of different usage patterns. Typically, all the information contributing to the application state is distributed in various layers, each with its own settings for visibility, programmability, and lifetime. ASP.NET provides state management facilities at four levels: application, session, page, and request. Each level has its own special container object, which is a topic we ll cover in this chapter. In this chapter, we ll explore the HttpApplicationState, HttpSessionState, and ViewState objects, which provide for application, session, and page state maintenance, respectively. In the next chapter, we ll dive into the Cache object. Note In this chapter, we won t discuss cookies in detail, but cookies are definitely useful for
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storing small amounts of information on the client. The information is sent with the request to the server and can be manipulated and re-sent through the response. The cookie is a text-based structure with simple key/value pairs, and it consumes no resources on the server. In e-commerce applications, for example, cookies are the preferred way of storing user preferences. In addition, cookies have a configurable expiration policy. The negatives for cookies are their limited size (browser-dependent, but seldom greater than 8 KB) and the fact that the user can disable them.
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Part IV
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Infrastructure of the Application
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The Application s State
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Table 17-1 summarizes the main features of the various state objects.
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TABLE 17-1
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State Management Objects at a Glance
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Lifetime
Implements an automatic scavenging mechanism, and periodically clears less frequently used contents Created when the first request hits the Web server, and released when the application shuts down Spans the entire lifetime of the individual request
Object
Cache
Data Visibility
Global to all sessions
Location
Does not support Web farm or Web garden scenarios Same as for Cache
HttpApplicationState
Same as for Cache
HttpContext
Global to the objects involved with the request
Same as for Cache
HttpSessionState
Created when the user makes Global to all requests the first request, and lasts un- issued by the user who til the user closes the session started the session Represents the calling context of each page being generated Limited to all requests queued for the same page
Configurable to work on Web farms and gardens Configurable to work on Web farms and gardens
ViewState
The HttpApplicationState object makes a dictionary available for storage to all request handlers invoked within an application. An instance of the HttpApplicationState class is created the first time a client requests any resource from within a particular virtual directory. Each running application holds its own global state object. The most common way to access application state is by means of the Application property of the Page object. Application state is not shared across either a Web farm or Web garden. Important Application state exists today mostly for compatibility reasons, and I don t know of any application where Application is used instead of the more powerful and built-in Cache object or external distributed cache engines.
Properties of the HttpApplicationState Class
The HttpApplicationState class is sealed and inherits from a class named NameObjectCollectionBase. In practice, the HttpApplicationState class is a collection of pairs, each made of a string key and an object value. Such pairs can be accessed either using the key string or the index. Internally, the base class employs a hashtable with an initial capacity of zero that is automatically increased as required. Table 17-2 lists the properties of the HttpApplicationState class.
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