Application_Error is not fired on 404 error

Question

I use an Application_Error handler to log all unhandled exceptions; generally it works well, but 404 errors always are skipped. Any ideas what might be the reason?

Answer 1

Actually, that is because that's an http protocol error returned by your web server. It's not a .net framework exception and thus it can't be handled by asp.net. However, you can configure IIS to serve a custom page when it returns a 404 error and this can be done through your web.config file. Needless to say that this custom page could be an aspx page where you can add some custom processing such as logging the error ;)

Answer 2

404's are not always caused by exceptions. You can just set the Response.Status* fields to generate a 404 response.

Use Application_EndRequest to examine the response and act on it.