Understanding Cookie, Cache, Session, Application and Viewstate Objects

Understanding Cookie, Cache, Session, Application and Viewstate Objects

Developers always get confused how and where to store their data when working with web applications. This posting would help people to understand the difference between each data holders in ASP.NET

Cookies

A cookie is a file created by an Internet site to store information on your computer, such as your preferences when visiting that site. For example, if you inquire about a flight schedule at an airline's Web site, the site might create a cookie that contains your itinerary. Or it might only contain a record of the pages you looked at within the site you visited, to help the site customize the view for you the next time you visit.

A persistent cookie is one stored as a file on your computer, and it remains there even when you close Internet Explorer. The cookie can be read by the Web site that created it when you visit that site again.

In .NET: HttpCookie (HttpCookieCollection), found as a property of the Request (Collection of cookies sent by the client) and Response class (Response cookie collection).

A temporary or session cookie is stored only for your current browsing session, and is deleted from your computer when you close Internet Explorer.

Cache

An application's Cache object allows you to store and retrieve arbitrary data on subsequent requests . The cache is not specifically associated with a page or user session. It is used primarily to enhance application performance and it remains valid as long as the application domain remains active.

In .NET: Cache, found as a property of the Page class (is the actual cache object associated with the application in which the page resides)

Session

This provides information about the current request's session. A Session object is maintained for each user that requests a page or document from an ASP.NET application. Variables stored in the Session object are not discarded when the user moves from page to page in the application; instead, these variables persist as long as the user is accessing pages in your application.

In .NET: HttpSessionState, found as a property of the Page class

Application

Defines the methods, properties, and events common to all application objects within an ASP.NET application. This class is the base class for applications defined by the user in the global.asax file. Enables sharing of global information across multiple sessions and requests within an ASP.NET application.

Note: Application state is not shared across either a Web farm (in which an application is hosted across multiple servers) or a Web garden (in which an application is hosted across multiple processes on the same computer).

In .NET: HttpApplication, found as a property of the Page class

Viewstate

View state for a page or control is the cumulative propery values, or view, of that page or control. This class is the primary storage mechanism for all HTML and Web server controls. It stores attribute/value pairs as strings associated with the control. It tracks changes to these attributes only after the OnInit method is executed for a page request, and save the changes to the page's or control's view state.

You can read from this class during any stage of the control processing lifecycle, but you should not write to it while the control is rendering.

In .NET: StateBag, found as a property of the Page class

The following code would help:

private void Page_Load(object sender, System.EventArgs e)
{
string strDateVal = System.DateTime.Now.ToString();

//This Application value would remain the same for all sessions
if (Application.Get("ApplicationValue") == null)
    Application.Add("ApplicationValue", "Application Value updated at " + strDateVal);

//This cookie value once set would remain the same for 7 days
if (Request.Cookies.Get("CookieValue") == null) 
{ 
    HttpCookie MyCookie = new HttpCookie("CookieValue", "Cookie Value updated at " + strDateVal);
    //Expires after 7 days from the date of creation
    MyCookie.Expires = DateTime.Now.Add(new TimeSpan(7,0,0));
    Response.Cookies.Add(MyCookie);
}

//Check the same cache value by opening another web page instance in IE
if (Cache["CacheValue"] == null) 
    Cache["CacheValue"] = "Cache Value updated at " + strDateVal;

//Check the same session value in another page in the application and another web page instance in IE
if (Session["SessionValue"] ==null) 
    Session["SessionValue"] = "Session Value updated at" + strDateVal;

//Put a breakpoint and check here during PostBack
if (this.ViewSate["ViewStateValue"] ==null) 
    this.ViewState.Add("SessionValue", "Session Value updated at" + strDateVal);
}
protected override void Render(HtmlTextWriter writer)
{
    writer.Write(Cache["CacheValue"] as string + "<br>");
    writer.Write(Session["SessionValue"] as string + "<br>");
    writer.Write(Request.Cookies.Get("CookieValue").Value + "<br>");
    writer.Write(Application.Get("ApplicationValue").ToString() + "<br>");
    writer.Write(this.ViewSate["ViewStateValue"] as string + "<br>");
    base.Render (writer);
}