Show a custom error page for a custom exception. For example, when trying to access a non-existent database record (app/people/details/9999999).

EntityNotFoundException.cs

// You could put more information and functionality here if you want.
// I just needed an exception distinguishable from other exceptions.
// I implemented a "FindOrDie" method in my repository code, from which
// I throw this exception whenever a query comes up empty.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;

namespace Inspect.Models {

    [Serializable]
    public class EntityNotFoundException : Exception {

        public EntityNotFoundException(string message) : base(message) { }

    }
}

EntityNotFoundHandleErrorAttribute.cs

using Inspect.Models;
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.Mvc;

namespace Inspect.Utilities {

    public class EntityNotFoundHandleErrorAttribute : HandleErrorAttribute {

        public override void OnException(ExceptionContext filterContext) {
            if (!filterContext.IsChildAction && !filterContext.ExceptionHandled && filterContext.HttpContext.IsCustomErrorEnabled && filterContext.Exception is EntityNotFoundException) {
                string controllerName = (string)filterContext.RouteData.Values["controller"];
                string actionName = (string)filterContext.RouteData.Values["action"];
                HandleErrorInfo model = new HandleErrorInfo(filterContext.Exception, controllerName, actionName);
                ViewResult result = new ViewResult {
                    ViewName = this.View,
                    ViewData = new ViewDataDictionary<HandleErrorInfo>(model),
                    TempData = filterContext.Controller.TempData
                };
                filterContext.Result = result;
                filterContext.ExceptionHandled = true;
                filterContext.HttpContext.Response.Clear();
                filterContext.HttpContext.Response.StatusCode = 500;
                filterContext.HttpContext.Response.TrySkipIisCustomErrors = true;
            }
        }

    }
}

FilterConfig.cs

// This registers the error handler application-wide.
// If you only want it to take effect on certain controllers or actions,
// you can decorate the class definition with [EntityNotFoundHandleError(View = "~/Views/Errors/EntityNotFound")]
// In that case it would probably make more sense to move that string inside the HandleErrorAttribute definition though.

using System.Web;
using System.Web.Mvc;
using Inspect.Utilities;

namespace Inspect {

    public class FilterConfig {
        
        public static void RegisterGlobalFilters(GlobalFilterCollection filters) {
            
            filters.Add(new EntityNotFoundHandleErrorAttribute() {
                ExceptionType = typeof(Inspect.Models.EntityNotFoundException),
                View = "~/Views/Errors/EntityNotFound.aspx",
                Order = 2
            });
            
            // Or the default error handler. Your custom handler should not replace
            // the default handler unless because it's only written to intercept a specific exception type
            filters.Add(new Utilities.ElmahHandleErrorAttribute(), 1);
        }
    }
}