Handling Errors in ASP.NET

 The other day, I was working through some code, adding error handling. We use the Enterprise Instrumentation Framework to do this (oddly only available via MSDN Subscriber download at the Universal level). When I got into the ASP.NET web pages I’d written, I had a bit of a problem. I didn’t really want to add try/catch blocks around every single method in the page’s base class – that would be a little redundant, and would mean that I’d have to change code in a bunch of places if I decided to change the style of errors I was reporting.

This is where it pays to be friends with Fritz Onion. :) I pinged him, and he suggested a clever little hack (in the good sense of that word) to put all my error handling code in one place.

The trick is to re-implement IHttpHandler on the page class itself. Since IHttpHandler::ProcessRequest is the one entry point for all requests into the page – whether that’s a callback handler for an event from the client or the initial Page_Load call – I could simply delegate back to the Page class’s ProcessRequest, but catch any errors that came out of it. One place to put all my high-level error handling. And I could even rethrow the exception if I wanted to still use ASP.NET’s built-in error handling.

It goes something like this:

using System;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;

public class Default : Page, IHttpHandler
{
  public void ProcessRequest(HttpContext ctx)
  {
    try {
      // Let the Page class itself handle the processing, as usual
      base.ProcessRequest(ctx);
    }
    catch (Exception e) {
      // Log it here – event log, EIF, whatever

      // Use our own error page or simply re-throw
      ctx.Response.Redirect("myerror.aspx");
    }
  }

  public void Page_Load(object o, EventArgs e)
  {
    throw new ApplicationException("Whoops");
  }

  public void Button_OnClick(object o, EventArgs e)
  {
    throw new ApplicationException("Aiieeeee!!!");
  }
}