Basic Structural Concepts of .NET

1. Basic Structural Concepts of .NET Managing State & Scope Christopher M. Pascucci

2. What is State • State refers to data maintained by an application for a single user. • An application must maintain a separate state for each user. • HTTP is a stateless protocol, which means it doesn’t maintain state between roundtrips. • Since HTTP is stateless the web applications must handle this.

3. How ASP.NET Maintains State • ViewState (Page) • Session • Request • Application • Cookies • Cache

4. How ASP.NET Maintains State • ViewState (Page) • Maintains the values of form controls and properties that are NOT stored in the HTTP header. • HTML form elements post their values to the HTTP header when the form is submitted. • ASP.NET uses the header to repopulate the form on a postback. • Even W3C got this one wrong and said that the ViewState stores this. • ViewState is implemented by default. • When a webform (aspx) is loaded, it can re-post to itself by means of a form submit. • A special state Boolean variable named IsPostBack is set to False when the page is initially loaded, and to True after its first postback. • Page state is maintained across postbacks to a single page by a single user. • Page state is sometimes called ViewState, because there is a ViewState object that contains property values for controls in the webform as well as the user created attribute/value collection; however, only simple data types and serializable objects can be stored in the ViewState object. • ViewState is maintained across postbacks. • It stores the information (encrypted) in a hidden field in the form. • <input type="hidden" name="__VIEWSTATE" value="dDwtMzQzNDc3MjYyO3Q8O2w8aTwyPjs+O2w8dDw7bDxpPDY+O2k8Nz47PjtsPHQ8cDxwPGw8VGV4dDs+O2w8SEVMTE8gTVIuIEEgQTs+Pjs+Ozs+O3Q8cDxwPGw8VGV4dDs” />

5. Using ViewState • There are a few cases that you may want to disable ViewState • When it affects the way you want the form to work • When the size of the ViewState field gets so large that it affects performance. • You can enable or disable view state for a form control by setting its EnableViewState Property to True or False. Example: txtName.EnableViewState = False OR use the Property Window in Design Mode • You can use ViewState for your own data • Adding or Updating a ViewState Item ViewState.Add(“SubTotal”, dblSubTotal) ViewState(“SubTotal”) = 155.79 • Retrieving data from a ViewState Item Dim myTotal As Double = Ctype(ViewState(“SubTotal”), Double) • Removing an item from ViewState ViewState.Remove(“SubTotal”)

6. How ASP.NET Maintains State • Session • Session state is unique to each user of an application but maintained throughout their use of the application.  • A Session is started when a user sends a URL to activate a given web site or Application. • The session can end in one of 3 ways: • If there is no request sent for a time called the Session Inactive Time. There is a default value set within the web server for this time, but it can be overridden by the session.Timeout property for a particular session object. • By the program using the session.Abandon method • If the client is terminated by closing the browser window, but Session Inactive Time will still have to elapse.

7. Using Session State Object • You can use the Session object to store your own data that can be used by other pages or on PostBacks to maintain state. • Adding or Updating a Session Item Session.Add(“Username”, strUsername) Session(“Username”) = “cp2579” • Retrieving data from a ViewState Item Dim strUsername As String = Ctype(Session(“Username”), String) • Removing an item from Session Session.Remove(“Username”)

8. How ASP.NET Maintains State • Request • A Request object is created whenever a URL request is made from the browser to a web server. • Request state is maintained during the life of a single request object. This means from one page to the next only, like in a single redirect. • HTTP is stateless so information from a previous request is lost. • The program can access data from web form controls on the server-side by simply using Request(“FormControlName”) in the code. • Application • Application state is shared among all users (sessions) of an application. • It can be used to maintain values that apply to all users and you can add your own data/values to the application state object. • The Application is all pages contained in a given web site. The application starts when the first user sends a URL to the web server on which the web is located. It ends when the last client (user) of the web ends its session or when the application is stopped / restarted.

9. Using Application State Object • You can use the Application object to store your own data that can be used by other pages or on PostBacks to maintain state. • Used the same way as Session object to store, update, retrieve and remove items. Example: Application(“UsersCount”) = count + 1 • Since the Application state object is used by all users of the application across all pages there are issues about updating items. You should use synchronization, Lock() and Unlock() methods, so that items are updated properly.

10. How ASP.NET Maintains State • Cookies • Cookie state is maintained in the client browser and can persist across page, session and application state. • It can even persist after the application terminates, for any predetermined time period. Its lifetime can be programmatically controlled. • Two types of cookies: • Session cookies – kept in the browser’s memory and exists until the session ends. • Persistent cookies – stored on the user’s hard drive and is retained until the cookie’s expiration date or until the user deletes it. • Cache • Cache state has application scope but variable lifetime determined by the following events: • Specific files or database tables being updated • Another Cache attribute changes value • Time expiration

11. ASPX Browser – Server Interaction

12. Global.asax File • This file contains a CodeBehind file that consists of application level events raised by ASP.NET and that can be used to maintain state in your web application. • If you view the CodeBehind file the stubs for the events are automatically written for you. All you have to do is place your code in the correct event. • Any URL request for this file is rejected. External users cannot download or view the code written within it. • The global.asax file can be placed in the web at the directory level just below the application root. • Events: Runs the first time a user runs any page in your application Sub Session_OnStart    ...End Sub Runs when a user's session terminates Sub Session_OnEnd    ...End Sub Runs when the server loads an application for the first time Sub Application_OnStart    ...End Sub Runs when the application terminates Sub Application_OnEnd    ...End Sub

13. The End… • For more information on the CIS4309 website see the maintaining state & scope pages. • From more information about the Instrinsic HTTP objects covered in this lecture. • These slides discussed a good portion of the Intrinsic HTTP objects (Request, Response, Session, Application, Server) , but there is additional material on this topic not covered in these slides.