ASP.NET 2.0: Learning the Basics, Part III

Page Load Event

Page Load event is fired when the page is loaded and accessed for the first time and when it is loaded in response to a client postback.  But if you need to initialize variables or want something to happen only once during the first load of the page, you can use the IsPostBack property of the Page object to determine if the page is loaded for the first time or due to postback:

    Protected Sub Page_Load(ByVal sender As Object, ByVal e As System.EventArgs) Handles Me.Load
        If Not Page.IsPostBack Then
            ' do some initialization here...
        End If
    End Sub

Application and Session Events

If you need to code on the application and/or session events, add the Global.asax file (Add New Item… and select Global Application Class).  You will find 3 application events (Start, End, and Error) and 2 session events (Start and End).

Application and Session State

As always, you can save your application and session states using the Application and Session objects respectively:

Application("MyAppVariable") = "MyAppValue"

and

Session("MySessionVariable") = "MySessionValue"

Session Identifiers

To identify each browser session, ASP.NET assigns a unique ID to a session, the SessionID property of the Session object.

ViewState Object

ViewState object is useful in persisting data on the page between postbacks.  It makes use of the __VIEWSTATE input hidden field that is inserted into the generated page.  __VIEWSTATE contains the state of all the controls on the page in encrypted form.

Profile Object

New in ASP.NET 2.0 is the Profile object.  This is now the preferred way to store user information across multiple visits to a web application.  It’s like the Session object but better.  It is strongly typed and you have the option to save it in the database, in an XML file, or in memory. 

To add properties to your Profile object, you need to add them to your web.config file, inside the <system.web> element:

<configuration>
   <appSettings/>
   <connectionStrings/>
   <system.web>

      ...

      <anonymousIdentification enabled="true" />
      <profile>
         <properties>
            <add name="MyProfileVariable" allowAnonymous="true" />
         </properties>
      </profile>
    
  </system.web>
</configuration>

In your code you can access or set the property like this:

    Profile.MyProfileVariable = "MyValue"

Other ASP.NET Objects

Other objects that are also available in ASP.NET are the following: Request, Form, Trace, User, Server, and PreviousPage.

Query String

To pass data from one page to another, you can use the name/value pair in the query string of the URL:

MyWebPage.aspx?MyVariable=MyValue

On the target page, you can access the value in the query string using the Request object:

Request.QueryString("MyVariable")

Cross Page PostBack

New in ASP.NET 2.0 is the ability to postback to a different page.  To use this, set the PostBackUrl property of the Button control to the page that you want to postback to.  To get information from the source page, you can use the PreviousPage object or the Form dictionary of the Request object.  If you want to check if the target page is running as a result of a cross page postback, use the IsCrossPagePostBack property of the PreviousPage object.  Do not use the target page’s IsPostBack property as it is set to false.

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