Christopher M. Pascucci

Basic Structural Concepts of .NET

Basic Structural Concepts of .NET

Managing State & ScopeManaging State & Scope

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.

ServerFirst HTTP request:The browser requests apage.

Client

Web server

First HTTP response:The server returns therequested page and theapplication ends.

Next HTTP request:The browser requests anotherpage. The server has no wayto associate the browser withits previous request.

Web server

Web serverBrowser

Browser

Browser

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

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” />

Using ViewState There are a few cases that you may want to disable ViewState

1. When it affects the way you want the form to work

2. 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”)

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:

1. 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.

2. By the program using the session.Abandon method

3. If the client is terminated by closing the browser window, but Session Inactive Time will still have to elapse.

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”)

Property Description

SessionID The unique ID of the session.

Item(name) The value of the session state item with the specified name. If you set a value for an item that doesn’t exist,that item is automatically created.

Count The number of items in the session state collection.

TimeOut Time in minutes until the session ends after no requests are made by the user. Property is R/W.

Abandon Destroys the session object and releases its resources.

Method Description

Add(name, value) Adds an item to the session state collection.

Clear Removes all items from the session state collection.

Remove(name) Removes the item with the specified name from the session collection.

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.

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.

Property Description

Item(name) The value of the application state item with the specified name. If you set a value for an item that doesn’t exist,that item is automatically created.

Count The number of items in the application state collection.

Method Description

Add(name, value) Adds an item to the application state collection.

Clear Removes all items from the application state collection.

Remove(name) Removes the item with the specified name from the application state collection.

Lock() Locks the application state collection so only the current user can access it.

Unlock() Unlocks the application state collection so other users can access it.

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

ASPX Browser – Server Interaction

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 applicationSub Session_OnStart    ...End Sub Runs when a user's session terminatesSub Session_OnEnd    ...End Sub Runs when the server loads an application for the first timeSub Application_OnStart    ...End Sub Runs when the application terminatesSub Application_OnEnd    ...End Sub

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.