Global Variables In ASP.NET? ( Application, Session, Cache, Cookie )

How to store global variables? (Application, Session, Cache, Cookie)

Global variables should always be used with caution. They are the best means of storing data that has to be accessed anywhere. The most common ways of accessing global variables in ASP.NET are by using Application, Cache, and Session objects.

Application - Application objects are application level global variables, that need to be shared for all user sessions. Thus, data specific to a user should'nt be saved in application objects. While using application objects, the objects are locked so that multiple page requests cannot access a specific application object. Below is a code example for usage of application object...

Application.Lock();
Application("UserData") = "dotnetuncle";
Application.UnLock();
Response.Redirect("DestinationPage.aspx");

//DestinationPage.aspx gets the value from the Application State
String sString = Application("UserData").ToString();

Cache - The cache object is similar to the application object in scope, however, it does not need any explicit locking and unlocking. Code below shows usage of Cache object...

Cache("Userdata") = "dotnetuncle";
Response.Redirect("DestinationPage.aspx");

//Destination.aspx retrieves the value from Cache object
String sString = Cache("Userdate").ToString();

The cache object also shares data across all user sessions. The cache object has features like it can automatically expire cached content after specified time periods or once memory consumption has reached a maximum.

Session - The session object is used to store the data specific to a user for the entire length of a user's visit to a website. Below is a code that shows usage of the session object in ASP.NET ...

//InitialPage.aspx stores the user’s credentials in Session state
Session("UserName") = txtUserName.Text;
Server.Transfer("DestinationPage.aspx");

//DestinationPage.aspx gets the user’s name from Session state
String sString = Session("UserName").ToString();

ASP।NET stores session values in the server memory. If there are plenty of active user's of a website, then the memory consumption on the server increases by leaps. Because of this reason, large websites use very less Session Variables. Session state can be configured to be automatically stored in a SQL Server database, or it can be configured to be stored centrally in a state server within a server farm. By default, a user’s session ends 20 minutes after their last page request and their data goes out of scope, freeing it from memory. In case user information is to be tracked by a large website, then a oookie is preferred.


Cookie

What is a cookie? Limitations of cookie? Permanent cookie?

Cookie - A cookie is a piece of data that is stored on a user's browser. Thus, a cookie does not use any server memory. It is actually a small text file which is created by the broswer on the hard disk of the user. It is actually a piece of information in the form of text strings. A web server sends a cookie to a user (client browser) and then the browser stores it.

A cookie is used to store information of a user & information about a user's preferences. How does the cookie works? - When a user visits a site, say www.amazon.com, and creates a profile out there, the server sends an ID (basically an ID to track this user) and saves the ID through the user's browser in the form of a cookie on the user's system. When the user revisits this site, the website tracks the user's system for the existence of any cookie, and in case it finds a cookie, it customizes the site based on the user's settings and preferences.

Now lets talk about how to create a cookie in ASP.NET. It is pretty simple. There is a class in the System.Web namespace by the name HttpCookie. This class may be used to easily create a cookie on the user's system. Below is a code sample on how to use a cookie in ASP.NET ...

//Creating a cookie HttpCookie sampleCookie = new HttpCookie("UserColorSetting");
sampleCookie.Values.Add("Background", txtBackgroundColor.Text);
sampleCookie.Expires = #12/31/2010#; Response.Cookies.Add(sampleCookie);

//Getting a cookie value from the user's computer
String sGetCookie;
sGetCookie = Request.Cookies("UserColorSetting")("Background").ToString();

Limitations of Cookies - Cookies are meant for infrequent storage of small pieces of information. They are not meant as a normal communication or mechanism. Note that web browsers are not required to save more than 300 cookies total, nor more than 20 cookies per web server (for the entire server, not just for the page or site on the server), nor to retain more than 4 kilobytes of data per cookie (both name and value count towards this 4 kilobyte limit). The biggest limitation of these is the 20 cookies per server limit, and so it is not a good idea to use a different cookie for each variable that has to be saved. Rather save a single cookie containing a lot of information.