Elmah App_Start

ElmahPackage.cs

[assembly: WebActivator.PreApplicationStartMethod(typeof(YourNamespace.App_Start.ElmahPackage), "PreStart")]

namespace YourNamespace.App_Start
{
    using System;
    using System.Web;
    using System.Web.Mvc;
    using System.Web.Routing;
    using Elmah;
    using Microsoft.Web.Infrastructure.DynamicModuleHelper;

    public static class ElmahPackage
    {
        public static void PreStart() 
        {
            DynamicModuleUtility.RegisterModule(typeof(ErrorLogModule));

            RouteTable.Routes.Add("elmah", 
                new Route(
                    "elmah/{*pathInfo}",
                    new RouteValueDictionary(new { controller = string.Empty, action = string.Empty }),
                    new ElmahRouteHandler()));

            GlobalFilters.Filters.Add(new ElmahHandleErrorAttribute());
        }
    }

    public class ElmahRouteHandler : IRouteHandler
    {
        private static ErrorLogPageFactory factory = new ErrorLogPageFactory();

        public IHttpHandler GetHttpHandler(RequestContext requestContext)
        {
            var application = (HttpApplication)requestContext.HttpContext.GetService(typeof(HttpApplication));

            var pathInfo = string.Concat("/", requestContext.RouteData.Values["pathInfo"]);
            
            application.Context.RewritePath("/elmah", pathInfo, application.Request.QueryString.ToString(), false);

            return factory.GetHandler(application.Context, null, null, null);
        }
    }

    public class ElmahHandleErrorAttribute : HandleErrorAttribute
    {
        public override void OnException(ExceptionContext context)
        {
            base.OnException(context);
            if (context.ExceptionHandled)
                RaiseErrorSignal(context.Exception);
        }

        private static void RaiseErrorSignal(Exception e)
        {
            var context = HttpContext.Current;
            ErrorSignal.FromContext(context).Raise(e, context);
        }
    }
}