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Working with ASP Pages. The <form> Tag (1). Remember that most ASP.NET pages contain a single <form> tag with the runat attribute set It’s possible to have multiple <form> tags but only one can be visible at a time Visible property is true

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the form tag 1
The <form> Tag (1)
  • Remember that most ASP.NET pages contain a single <form> tag with the runat attribute set
  • It’s possible to have multiple <form> tags but only one can be visible at a time
    • Visible property is true
    • This capability is great for implementing wizards (sequential dialog boxes)
    • The MultiView and Wizard control can also be used
the form tag 2
The <form> Tag (2)
  • Generally <form> tags are the outermost tag but can be children of container controls <table>, <div>, etc…
  • Non-container (server) controls must appear inside of a <form> tag or exceptions will be thrown at run-time
    • No compile error occurs
the htmlform class 1
The HtmlForm Class (1)
  • A form is accessed on the server using the HtmlForm class
  • The DefaultButton property sets the form’s default button
    • The button clicked when enter is pressed
  • The InnerHtml property contains the markup between the opening and closing tags
the htmlform class 2
The HtmlForm Class (2)
  • The HasControls method indicates whether a from has child controls
  • The FindControl finds the control instance with a particular ID
    • Only direct children are searched
  • The Focus method sets input focus to a particular control
cross page posting introduction 1
Cross-Page Posting (Introduction 1)
  • You can use a HyperLink control to create a link to another page
  • Any Button control can be used for cross-page postbacks
    • Set the PostBackURL property to the desired Web page
cross page posting introduction 2
Cross-Page Posting (Introduction 2)
  • Use the Server.Transfer method
    • The target page is passed as an argument
    • Any code following Server.Transfer is not executed


cross page posting getting data from the previous page
Cross-Page Posting (Getting Data from the Previous Page)
  • The PreviousPage property of the Page object contains a reference to the page that caused the postback
  • This property allows you to get at the controls from the previous form and their values
    • This is magically accomplished through viewstate
cross page posting example
Cross-Page Posting (Example)
  • Get a reference to a control on another page and print the text

TextBox txt;

txt =

(TextBox)PreviousPage.FindControl( "txtPreserve");


detecting cross page postbasks 1
Detecting Cross-Page Postbasks (1)
  • Test the PreviousPage property to see if it is null
    • If it is, the page was not loaded as a cross-page postback
  • Test the IsCrossPagePostBack property of the ‘posting’ page
detecting cross page postbacks 2
Detecting Cross-Page Postbacks (2)
  • The @PreviousPageType directive allows you to use strongly typed syntax if page “b” will only be posted from page “a”

<%@ PreviousPageTypevirtualPath=“a.aspx” %>

cross page postback vs server transfer
Cross page Postback vs. Server.Transfer
  • Cross page postbacks are client-based
  • The Transfer method is server-based
  • Both allow you to reference the PreviousPage property
introduction to error handling
Introduction to Error Handling
  • Just like desktop applications, exceptions are thrown in various cases
    • Divide by 0, IO, Database, etc.
  • Unhandled, we call these the yellow page of death
  • What happens depends on the following Web.config setting:

<compilation debug="false">

handling errors introduction
Handling Errors (Introduction)
  • Exception handlers are created (as usual) using try, catch blocks
  • The Page_Error event handler gives you a global exception handler for the page
  • The Application_Error event applies to the entire application itself
handling errors introduction1
Handling Errors (Introduction)
  • Use Server.GetLasError() to get information about the most recent error
  • Use Server.ClearError() to clear the exception
  • Create other exception handlers, as necessary
configuring custom errors
Configuring Custom Errors
  • It’s all controlled by the customErrors section of the Web.config file
  • Attributes
    • defaultRedirect attribute causes a redirect to the specified page
    • Mode attribute enables / disables custom errors
      • On – Enables custom for remote and local hosts
      • Off – Disables custom errors
      • RemoteOnly – (default) Custom errors are displayed to the remote client
configuring custom errors example
Configuring Custom Errors (Example)

<customErrors mode="On" defaultRedirect="GenericErrorPage.htm">

<error statusCode ="403" redirect="NoAccess.htm" />

<error statusCode="404" redirect="FileNotFound.htm" />


tracing introduction
Tracing (Introduction)
  • It’s possible to trace execution by enabling tracing in the Web.config file
  • @Page directives can also be used
  • Example

<trace enabled="true" pageOutput="true"/>

  • See handout