Working with ASP Pages

Slide 2 The Tag (1) Remember that most ASP.NET pages contain a single tag with the runat attribute set It’s possible to have multiple 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

Slide 3 The Tag (2) Generally tags are the outermost tag but can be children of container controls,, etc… Non-container (server) controls must appear inside of a tag or exceptions will be thrown at run-time No compile error occurs

Slide 4 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

Slide 5 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

Slide 6 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

Slide 7 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 Server.Transfer(“DestinationPage.aspx”);

Slide 8 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

Slide 9 Cross-Page Posting (Example) Get a reference to a control on another page and print the text TextBox txt; txt = (TextBox)PreviousPage.FindControl( "txtPreserve"); Response.Write(txt.Text);

Slide 10 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

Slide 11 Detecting Cross-Page Postbacks (2) directive allows you to use strongly typed syntax if page “b” will only be posted from page “a”

Slide 12 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

Slide 13 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:

Slide 14 Error Message ( Debug=true )

Slide 15 Error Message ( Debug=false )

Slide 16 Error Message – Enabling Debugging

Slide 17 Error Message – Enabling Script Debugging

Slide 18 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

Slide 19 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

Slide 20 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

Slide 21 Configuring Custom Errors (Example)

Slide 22 Tracing (Introduction) It’s possible to trace execution by enabling tracing in the Web.config directives can also be used Example See handout

Slide 23 Tracing (Output)