Thursday, March 6, 2008

Exception handling in ASP.Net with Anthem and Web Services

Dorian, our CIO, has been busily hammering the VB.NET based shopping cart selected for a large client into shape.

Anthem.NET is a free, cross-browser AJAX toolkit for the ASP.NET development environment that works with both ASP.NET 1.1 and 2.0.

Please ignore lack of indenting ... blogger keeps eating my directives. Kent Bolton

Before Anthem it was enough to put an error handler in the Global.asax on the Application_Error event to log the error and use the tag in the web.config to redirect errors to a common error page.

Anthem complicates things.

In Anthem it is possible to have Ajax call backs to the web page, controls or even custom methods decorated. Exceptions could be raised on the methods called or even on methods not directly called by these methods such as load events for controls that Anthem causes to load dynamically.

If an error occurs during an Anthem Callback the exception will not be caught by Application_Error and won't follow instructions. On the server side the only thing you can do to catch an Anthem Error is catch it on the page using a Page_Error method. This method will also catch regular page exceptions thus preventing Application_Error from firing. It may look something like this:

Private Sub Page_Error(ByVal sender As Object, ByVal e As EventArgs) Handles MyBase.Error
End Sub

In order to avoid putting this method on every page it must be placed on the base page class for the entire website. There is a small downside to this which is a page can no longer define its own Page_Error method because control would be lost in the event execution chain when a Response.Redirect is started (it aborts the thread). This is however a necessary evil as there doesn’t seem to be another way to catch Anthem exceptions.

Because we are on the topic of Anthem and error handling here is a couple more useful facts. You can catch errors in Anthem call backs on the client by adding a JavaScript method to pages calls Anthem_Error. If a server side error occurs in an Anthem callback any previously registered JavaScript to execute will still be executed. For example if you had an Anthem button which looked like this:

Protected Sub btnNext_Click(ByVal sender As Object, ByVal e As System.Web.UI.ImageClickEventArgs) Handles btnNext.Click

SomeHiddenPanel.Visible = True

Anthem.Manager.AddScriptForClientSideEval("if (typeof document.someform != ""undefined"") { documentsomeform.submit(); }")

End Sub

And after this code executed a server side error occurred (perhaps in rendering SomeHiddenPanel) the JavaScript added would still be executed. It is important to write such JavaScript to be tolerate of an error occurring. The code above checks for the existence of the form which SomeHiddenPanel would make visible before it calls submit().

Web services are another nasty for exception handling. Basically, you need to try catch every web service method and handle all exceptions. The same error logging routine could be use by web services and page errors if it was tolerant of information that may not be present on web service requests. View state is one example. Don’t expect to find that in a web service.

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