1. Session Variables and Post Back
CS 3870/CS 5870
Note04
Session Variables and Post Back

2. Static Pages and Dynamic Pages
Original HTML
Universal Reader
No User input
Smart HTML
Input controls
User interactive pages

3. Windows Programs Developers’ Code Sub Main EXE file
Class variables to maintain state information
Example: values in textboxes on a form

4. 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

5. Prog2 Visit the ordering page Calculate result Go to page Default
Come back to the ordering page
Data lost!

6. 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.

7. 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

8. 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

9. 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.

10. What is a Session? Running a Windows program Session of Web Sites
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

11. 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

12. 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

13. Page.IsPostBack Property
Click Here

14. 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

15. 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

16. Global.asax Application_Start Application_End Application_Error
Session_Start
Session_End

17. Prog2 All Session Variables Begin with Prog2_
String
Prog2_ID
Prog2_Price
Prog2_Quantity
Prog2_SubTotal
Prog2_Tax
Prog2_GrandTotal
4 Session variables
String
Prog2_ID
Prog2_Price
Prog2_Quantity
Boolean
Prog2_Computed

18. Initialize Session Variables
Application Language="VB" %> <script runat="server"> Sub Application_Start(. . .) End Sub Sub Application_End(. . .) Sub Application_Error(. . .) Sub Session_Start(. . .) Session("Prog2_ProductID") = "" Session("Prog2_ProductIPrice") = "" Session("Prog2_ProductQuantity") = "" Session("Prog2_Computed") = False Sub Session_End(. . .) </script>

19. 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

20. 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

21. Page Load Event Procedure
If Not IsPostBack Then ‘ first or return visits ‘ restore Session data ‘ Else is not needed! End If

22. 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

23. 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

24. Button Reset The reset button make it easier CauseValidation: False!
Reset session variables

25. 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

26. Web.config
<?xml version="1.0"?> <configuration> <system.web> <compilation debug=“true" targetFramework="4.0" />

27. Web.config
<?xml version="1.0"?> <configuration> <system.web> <compilation debug=“true" targetFramework="4.5" />

28. 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" />

29. Web.config
<?xml version="1.0"?> <configuration> <system.web> <sessionState timeout="1" />

30. Web.config
<?xml version="1.0"?> <configuration> <system.web> <compilation debug="true" targetFramework="4.5" urlLinePragmas="true“ /> <customErrors mode="Off"/> <sessionState timeout="1" />

31. 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

32. Page Directives AutoEventWireup
Page Language="VB" AutoEventWireup="false"
CodeFile="OrderingProduct.aspx.vb"
Inherits="Prog2_OrderingProduct" %>
AutoEventWireup
True: 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