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CS 3870/CS 5870

Note04

Session Variables and Post Back

Static Pages and Dynamic Pages

• Original HTML– Universal Reader– No User input

• Smart HTML– Input controls – User interactive pages

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Windows Programs

• Developers’ Code– Sub Main– EXE file– Class variables to maintain state information– Example: values in textboxes on a form

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ASP.NET Web Applications

• IIS running as interface

• ASP.NET running to generate dynamic pages

• Developers’ code– No Sub Main– Page classes are in DLL files– Pages are created and removed for each request– Page class variables cannot keep state information

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Prog2

• Visit the ordering page

• Calculate result

• Go to page Default

• Come back to the ordering page– Data lost!

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Prog2

After a calculation is completed successfully, if the user goes to the start page then comes back to page OrderingProduct.aspx, then all textboxes should show the same data as when the user left the page.

All textboxes will be blank if the previous calculation is failed.

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Generating Dynamic Pages

• First Visit– No user input– Generating pages based on the DLL file

• Post Back– Requesting the same page– With user input

• Return Visit from other pages– No information on the visited page

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Session Variables

• To maintain state information for each user on each Web site

• Maintained by ASP.NET

• Not by developers’ code

• Defined inside Global.asax

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Session State

ASP.NET session state enables you to store and retrieve values for a user as the user navigates ASP.NET pages in a Web application. HTTP is a stateless protocol. This means that a Web server treats each HTTP request for a page as an independent request. The server retains no knowledge of variable values that were used during previous requests. ASP.NET session state identifies requests from the same browser during a limited time window as a session, and provides a way to persist variable values for the duration of that session. By default, ASP.NET session state is enabled for all ASP.NET applications.

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What is a Session?• Running a Windows program

– The time period between starting a program and terminating the program

• Session of Web Sites– Session start: the first visit of a user to any page of

the site– Cookies to identify the users– Session end: no way to know if the user is still

there or not– Session Time Out

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Session Time Out

• Length of a session– Default value: 30 mins– Set in Web.config file– Discussed later

• Session variables are initialized when session starts and updated in event procedures

• Session variables could be cleared at session end

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Visiting Dynamic Pages

• First Visit– Can use the initial value of session variables

• Post Back– Use user input– Don’t use session variables to overwrite user input

• Return Visit from other pages– Need to restore the value of session variables

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Page.IsPostBack Property

Click Here

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Session Variables

• Don’t need to be declared• We need to remember the type of session variables• Initialized inside Session_Start of Global.asax• Accessed and updated inside event procedures• Restored in Page Load event procedure

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Creating Global.asax

• Must be inside the Web site main folder, not under any sub-folder

• Only one global file each Web site• Right click Solution or Project• Add• Add New• Global Application Class

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Global.asax

• Application_Start

• Application_End

• Application_Error

• Session_Start

• Session_End

Prog2All Session Variables Begin with Prog2_

• 6 Session variables• String

– Prog2_ID– Prog2_Price– Prog2_Quantity– Prog2_SubTotal– Prog2_Tax– Prog2_GrandTotal

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• 4 Session variables• String

– Prog2_ID– Prog2_Price– Prog2_Quantity

• Boolean

– Prog2_Computed

Initialize Session Variables<%@ Application Language="VB" %>

<script runat="server">

Sub Application_Start(. . .)

End Sub

Sub Application_End(. . .)

End Sub

Sub Application_Error(. . .)

End Sub

Sub Session_Start(. . .)

Session("Prog2_ProductID") = ""

Session("Prog2_ProductIPrice") = ""

Session("Prog2_ProductQuantity") = ""

Session("Prog2_Computed") = False

End Sub

Sub Session_End(. . .)

End Sub

</script> 18

Update Session Variables

Protected Sub Button1_Click(…) Handles Button1.Click

‘ Exit if invalid input

‘ Computing result

. . .

Session("Lab2_ProductID") = txtID.Text

Session("Lab2_ProductIPrice") = txtPrice.Text

Session("Prog2_Computed") = True

End Sub

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Page Load Event Procedure

If Not IsPostBack Then

‘ first or return visits

‘ restore Session data

Else

‘ Let event procedures

‘ handle the user input

End If

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Page Load Event Procedure

If Not IsPostBack Then

‘ first or return visits

‘ restore Session data

‘ Else is not needed!Else is not needed!

End If

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Restore Session Variables

Protected Sub Page_Load(…) Handles Me.Load

If Not IsPostBack Then

‘If Session("Prog2_Computed") = True Then

txtID.Text = Session("Lab2_ProductID")

txtPrice.Text = Session("Lab2_ProductIPrice")

‘End If

End If

End Sub

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User Input Will Be Lost!

‘ For each request, the event procedure will be invoked

‘ and we never get the user input.

Protected Sub Page_Load(…) Handles Me.Load

txtID.Text = Session("Lab2_ProductID")

txtPrice.Text = Session("Lab2_ProductIPrice")

End Sub

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Button Reset

•The reset button make it easier

•CauseValidation: False!

•Reset session variables

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Web Configuration File

• Creating Web.config file– Right click the web site

– Add new item

– Web Configuration file

• Many settings

• Different levels– Machine– Site– Folder

Web.config

<?xml version="1.0"?>

<configuration>

<system.web>

<compilation debug=“true" targetFramework="4.0" />

<system.web>

<configuration>

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Web.config

<?xml version="1.0"?>

<configuration>

<system.web>

<compilation debug=“true" targetFramework="4.5" />

<system.web>

<configuration>

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Session TimeOut

• Configure file Web.config

• Short timeout values make development easier

<sessionState timeout="1" />

• Longer timeout values for production Web sites

<sessionState timeout=“30" />

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Web.config

<?xml version="1.0"?>

<configuration>

<system.web>

<sessionState timeout="1" />

<system.web>

<configuration>

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Web.config<?xml version="1.0"?>

<configuration>

<system.web>

<compilation debug="true" targetFramework="4.5"

urlLinePragmas="true“ />

<customErrors mode="Off"/>

<sessionState timeout="1" />

<system.web>

<configuration>

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Compiling Web Pages

• All code files are precompiled into a single assembly (dll file).

• Web pages are compiled at the first request

• Dynamic compilation causes some delay for the first visit

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Page Directives<%@ Page Language="VB" AutoEventWireup="false"

CodeFile="OrderingProduct.aspx.vb"

Inherits="Prog2_OrderingProduct" %>

AutoEventWireupTrue: No “handles” for page event procedures

(ASP.NET uses procedure name and signature)

False: page event procedures must have “handles”

Only for page events, not control events

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