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Every request into an ASP.NET application is handled by a specialized component known as an HTTP handler. The HTTP handler is the backbone of the ASP.NET request processing framework. ASP .NET uses different HTTP handlers to serve different file types. For example, the handler for web pages creates the page and control objects, runs your code, and renders the final HTML. The handler for web services has a slightly simpler task it simply deserializes the SOAP message and invokes the corresponding code. All HTTP handlers are defined in the <httpHandlers> section of a configuration file. The core set of HTTP handlers is defined in the root web.config file. Here s an excerpt of that file: <httpHandlers> <add verb="*" <add verb="*" <add verb="*" <add verb="*" ... </httpHandlers> path="trace.axd" type="System.Web.Handlers.TraceHandler"/> path="*.config" type="System.Web.HttpForbiddenHandler"/> path="*.cs" type="System.Web.HttpForbiddenHandler"/> path="*.aspx" type="System.Web.UI.PageHandlerFactory"/>

When you deal with an outside resource such as a database, you need to protect your code with a basic amount of error-handling logic Even if you ve avoided every possible coding mistake, you still need to defend against factors outside your control for example, if the database server isn t running or the network connection is broken You can count on the SqlDataSource to properly release any resources (such as connections) if an error occurs However, the underlying exception won t be handled Instead, it will bubble up to the page and derail your processing As with any other unhandled exception, the user will receive a cryptic error message or an error page This design is unavoidable if the SqlDataSource suppressed exceptions, it could hide potential problems and make debugging extremely difficult.

Here s an example: UpdateCommand="UPDATE Employees SET FirstName=@FirstName, LastName=@LastName, Title=@Title, City=@City FROM Employees WHERE EmployeeID=@original_EmployeeID"> This problem has another workaround The @original_ naming convention is configurable You can use a different prefix by changing the SqlDataSourceOldValuesParameterFormatString property This property takes a string of the form @original_{0}, where {0} indicates the unadulterated name If you are sure that your key values are read-only and won t be modified in an update operation, you have no reason to use this convention, because there s no difference between the original name and the current name So, simply change the OldValuesParameterFormatString property to {0}, and your commands will continue to work with the unmodified field names..

Inside the <httpHandlers> section you can place <add> elements that register new handlers and <remove> elements to unregister existing handlers. In this example, four classes are registered. All requests for trace.axd are handed to the TraceHandler, which renders an HTML page with a list of all the recently collected trace output (as described in 3). Requests for files that end in .config or .cs are handled by the HttpForbiddenHandler, which always generates an exception informing the user that these file types are never served. And files ending in .aspx are handled by the PageHandlerFactory. In this case, PageHandlerFactory isn t actually an HTTP handler. Instead, it s a factory class that will create the appropriate HTTP handler. This extra layer allows the factory to create a different handler or configure the handler differently depending on other information about the request. ASP .NET also uses another ingredient in page processing, called HTTP modules. HTTP modules participate in the processing of a request by handling application events, much like the global.asax file. A given request can flow through multiple HTTP modules, but it always ends with a single HTTP handler. Figure 5-11 shows how the two interact.

However, it s a good idea to handle the problem in your web page and show a more suitable error message To do this, you need to handle the data source event that occurs immediately after the error If you re performing a query, that s the Selected event If you re performing an update, delete, or insert operation, you would handle the Updated, Deleted, or Inserted events instead (If you don t want to offer customized error messages, you could handle all these events with the same event handler) In the event handler, you can access the exception object through the SqlDataSource StatusEventArgsException property If you want to prevent the error from spreading any further, simply set the SqlDataSourceStatusEventArgsExceptionHandled property to true Then, make sure you show an appropriate error message on your web page to inform the user that the command was not completed.