ASP.NET Session State

ASP.NET session state enables you to store and retrieve values for a user as the user navigates the different ASP.NET pages that make up a Web application. HTTP is a stateless protocol, your Web server treats each HTTP request for a page as an independent request; by default, the server retains no knowledge of variable values used during previous requests. ASP.NET session state identifies requests received from the same browser during a limited period of time as a session, and provides the ability to persist variable values for the duration of that session.

ASP.NET session state is enabled by default for all ASP.NET applications.

Session["FirstName"] = FirstNameTextBox.Text;
Session["LastName"] = LastNameTextBox.Text;

By default, session variables can be any valid .NET type. For example, the following code example stores an ArrayList of values in a session variable named “StockPicks.” Note that the value returned by the “StockPicks” session variable must be cast as the appropriate type upon retrieval from the SessionStateItemCollection.

// When retrieving an object from session state, cast it as 
// the appropriate type.
ArrayList stockPicks = (ArrayList)Session["StockPicks"];

// Write the modified stock picks list back to session state.
Session["StockPicks"] = stockPicks;

Session Identifiers

Sessions are identified by a unique session identifier that can be read using the SessionID property. When session state is enabled for an ASP.NET application, each request for a page in the application is examined for a SessionID value sent from the browser. If no SessionID value is supplied, ASP.NET starts a new session and the SessionID for that session is sent to the browser with the response.

SessionID values are stored in a cookie, by default, but you can also configure your application to store SessionID values in the URL for a “cookieless” session.

A session is considered active as long as requests continue to be made with the same SessionID value. If the time between requests for a particular session exceeds the specified time-out value in minutes, then the session is considered expired. Requests made with an expired SessionID value result in a new session being started.

Concurrent Requests and Session State
Access to ASP.NET session state is exclusive per session, which means that if two different users make concurrent requests, access to each separate session is granted concurrently. However, if two concurrent requests are made for the same session (that is, using the same SessionID value), then the first request received gains exclusive access to the session information and the second request will execute once the first request completes, or until the exclusive lock on the information is freed due to the first request exceeding the lock timeout. If the EnableSessionState page directive is set to ReadOnly, then a request for the read-only session information does not result in an exclusive lock on the session data. Read-only requests for session data may still have to wait for a lock gained by a read-write request for session data to clear.

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