error_handling.cs

void Application_Error(object sender, EventArgs e)
{
    Exception ex = Server.GetLastError();
    if (ex.GetType() != typeof(HttpException))
    {
        if (ex.GetType() == typeof(HttpUnhandledException) && ex.InnerException != null)
            ex = ex.InnerException;
        CoreManager.ExceptionService.HandleException(ex);
    }
    else if (ex.GetType() == typeof(HttpException))
    {
        HttpException httpEx = ex as HttpException;
        if (httpEx != null && httpEx.GetHttpCode() == 404)
        {
            RouteData routeData = new RouteData();
            routeData.Values.Add("controller", "Error");
            routeData.Values.Add("action", "HttpError404");
            routeData.Values.Add("exception", ex);
            Server.ClearError();
            Response.TrySkipIisCustomErrors = true;
            IController errorController = new ErrorController();
            errorController.Execute(new RequestContext(new HttpContextWrapper(Context), routeData));
        }
    }
}

//... In the ErrorController file...

public class ErrorController : Controller
{
    public ActionResult HttpError404()
    {
        Response.StatusCode = 404;
        return View();
    }
}