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Unhandled .Exceptions
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In the last example page that threw an exception, ASP .NET responded by redirecting to the default error page . In ASP .NET, you also can trap exceptions by setting up a handler for Error events fired by HttpApplication so that you can handle them more appropriately . The easiest way to accomplish this is to define a handler in your HttpApplication-derived class in Global .asax .cs . With the handler connected to the event, your application will receive notifications whenever something bad happens, and you can deal with it gracefully . For example,
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Part IV
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Diagnostics and Plumbing
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you might log the error or show it on the debug console before redirecting the user to an error page . The following example redirects the exception to an error page:
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public class Global : System.Web.HttpApplication { protected void Application_Error(object sender, EventArgs e) { Exception ex = Server.GetLastError(); // display the exception System.Diagnostics.Debug.WriteLine("Error in app: " + ex); // display the exception System.Diagnostics.Debug.WriteLine("Error in app: " + ex); if (ex is HttpUnhandledException) { Context.ClearError(); Server.Transfer("somethingbadhappened.htm"); } else {} } }
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The preceding code traps the exception before the redirection happens . This gives you the opportunity to log the exception (or, as in this example, to show it in the System.Diagnostics.Debug context) . The following graphic shows the exception details listed in the Output tab in Visual Studio .
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You can also redirect users to a different page if you want to hijack the exception handling before ASP .NET redirects to the page specified in web .config . Be sure to call Context.ClearError first to clear the error so that ASP .NET won t generate its standard error page .
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17 Diagnostics and Debugging
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.17 .Quick .Reference
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Prepare a Web site for debugging
Do This
Include the following in web .config:
<system.web> <compilation debug="true"/> </system.web>
Enable tracing for an entire application
Include the following in web .config:
<system.web> <trace enabled="true"/> </system.web>
Enable tracing for your page Debug a Web application in Visual Studio
Set the Page class s trace attribute to true either by using the Properties pane in Visual Studio or declaring Trace= true in the page directive . Ensure that the debug attribute is turned on in web .config . Start the program running in debug mode by 1 . Clicking Debug, Start Debugging on the main menu or 2 . Pressing the F5 key .
Set up breakpoints in an application in Visual Studio
Place the cursor on the line where you d like to stop execution and 1 . Click Debug, Toggle Breakpoint or 2 . Press the F9 key or 3 . Toggle the breakpoint by clicking the gray ribbon to the left of the text in the code editor .
Execute a line of source code in the Visual Studio debugger
While the debugger is running and execution has stopped at the line you d like to execute, 1 . Click Debug, Step Over on the main menu or 2 . Press the F10 key .
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Instruct ASP .NET to show a particular page when a specific HTTP error occurs Trap specific .NET exceptions or deal with general unhandled exceptions in ASP .NET
Assign the error-handling page to the specific error in the <customErrors> section of web .config . Handle exceptions, including otherwise uncaught exceptions, within the Application_Error handler in Global .asax . Usually, you can then redirect to a specific page . (Note that specific errors are assigned as the InnerException of the HttpUnhandledException!)
18
The HttpApplication Class and HTTP Modules
After completing this chapter, you will be able to
n n n n
Use HttpApplication as a rendezvous point for your application . Manage data in the HttpApplication object . Manage events in the HttpApplication object . Work with HTTP modules .
This chapter covers working with application state and application-wide events in your ASP .NET application . In typical desktop applications, the notion of a global meeting place for various parts of an application is well understood . For example, Microsoft Foundation Class (MFC), a C++ class library supporting low-level Windows development, includes a class named CWinApp that holds state that is useful throughout the program . This state includes such items as a handle to the current instance of the application, a handle to the main window, and the parameters that were passed in when the application started . The CWinApp class also runs the message loop something that can be done only within the global scope of a Windows application . A running Windows application contains one and only one instance of the CWinApp class, and it is universally available from anywhere in the application . Both Windows Forms and the Windows Presentation Foundation the .NET libraries that support Windows desktop applications have a similar class named Application . Each includes the same sort of state: access to command-line parameters, a top-level window, other state required by the program . Their Application classes also run the message loop . In fact, Microsoft Silverlight employs a model to WPF by including an Application object . Web development also requires the same sort of global space that a desktop application requires . Having a global space in a Web application makes implementing features such as data caching and session state possible . This chapter looks at how ASP .NET implements a global space for Web applications .
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