ELMAH and custom errors

ELMAH and ASP.NET (MVC) custom errors aren't exactly known to be best friends. Question after question has been posted on forums like Stack Overflow, from people having problems with ELMAH, when custom errors are configured. These problems make perfect sense since both ELMAH and custom errors are designed to catch errors and do something about them.

Before looking at some code, we recommend you to read Web.config customErrors element with ASP.NET explained and Demystifying ASP.NET MVC 5 Error Pages and Error Logging. Together, the posts are a great introduction to different ways of implementing custom error pages in ASP.NET MVC.

Back to ELMAH. In most implementations of custom error pages, ASP.NET swallows any uncaught exceptions, putting ELMAH out of play. To overcome this issue, you can utilize MVC's IExceptionFilter to log all exceptions, whether or not it is handled by a custom error page:

public class ElmahExceptionLogger : IExceptionFilter
{
    public void OnException (ExceptionContext context)
    {
        if (context.ExceptionHandled)
        {
            ErrorSignal.FromCurrentContext().Raise(context.Exception);
        }
    }
 }

The OnException method on ElmahExceptionLogger is executed every time an error is happening, by registering it in Application_Start:

protected void Application_Start()
{
    // ...
    GlobalConfiguration.Configuration.Filters.Add(new ElmahExceptionLogger());
    // ...
}

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