HttpContext.Current.Items cleared via responseMode="ExecuteURL"?

Question

I avoid the default ASP.NET approach of redirecting on errors (as many people do). Clean AJAX code and SEO are among the reasons.

However, I'm using the following method to do it, and it seems that I may lose HttpContext.Current.Items in the transfer?

<httpErrors errorMode="Custom" existingResponse="Replace">
    <remove statusCode="401" />
    <remove statusCode="403" />
    <remove statusCode="404" />
    <remove statusCode="500" />
    <error statusCode="401" responseMode="ExecuteURL" path="/Account/SignIn" />
    <error statusCode="403" responseMode="ExecuteURL" path="/Site/Forbidden" />
    <error statusCode="404" responseMode="ExecuteURL" path="/Site/NotFound" />
    <error statusCode="500" responseMode="ExecuteURL" path="/Site/Error" />
</httpErrors>

I assumed it just performed a Server.Transfer() under the covers, which I understand preserves Items. (See: Scope of HttpContext.Current.Items and http://weblog.west-wind.com/posts/2010/Jan/20/HttpContextItems-and-ServerTransferExecute )

But I'm also capturing something in Items before the "ExecuteURL", and retrieving/outputting it after the transfer (or whatever it is), and it seems to disappear. I've watched it go into the Items collection, I see the Count raise to 5, and then when the value is retrieved there are only 2 items in the collection.

What is going on?

---

If you'd like to understand more about what I'm doing and recommend an alternate implementation, I'm open to it. I'm using this to push the ELMAH Error Id into a ViewModel in a way that is free from race conditions. (i.e. a common workaround for this that I'm replacing is to merely display the most recent error.) Here's my code:

Global.asax

protected void ErrorLog_Logged(object sender, ErrorLoggedEventArgs args) {
    ElmahSupplement.CurrentId = args.Entry.Id;
}

void ErrorLog_Filtering(object sender, ExceptionFilterEventArgs e) {
    if (ElmahSupplement.IsNotFound(e.Exception)) {
        ElmahSupplement.LogNotFound((e.Context as HttpContext).Request);
        e.Dismiss();
    }
}

SiteController.cs

public virtual ActionResult Error() {
    Response.StatusCode = 500;
    return View(MVC.Site.Views.Error, ElmahSupplement.CurrentId);
}

ElmahSupplement.cs

public class ElmahSupplement {
    // TODO: This is a rather fragile way to access this info
    private static readonly Guid contextId = new Guid("A41A67AA-8966-4205-B6C1-14128A653F21");

    public static string CurrentId {
        get { 
            return
                // Elmah 1.2 will fail to log when enumerating form values that raise RequestValidationException (angle brackets)
                // https://code.google.com/p/elmah/issues/detail?id=217
                // So this id could technically be empty here
                (HttpContext.Current.Items[contextId] as string);
        }
        set {
            HttpContext.Current.Items[contextId] = value;
        }
    }

    public static void LogNotFound(HttpRequest request) {
        var context = RepositoryProxy.Context;
        context.NotFoundErrors.Add(new NotFoundError {
            RecordedOn = DateTime.UtcNow,
            Url = request.Url.ToString(),
            ClientAddress = request.UserHostAddress,
            Referrer = request.UrlReferrer == null ? "" : request.UrlReferrer.ToString()
        });
        context.SaveChanges();
    }

    public static bool IsNotFound(Exception e) {
        HttpException he = e as HttpException;
        return he != null && he.GetHttpCode() == 404;
    }
}

Answer 1

As explained here, the ExecuteURL generates two requests: the first one throws the exception and the second one generates the error response.

Since Context.Items is cleared between requests, your code always see the 2nd request generated hence the diff between the Items.

Try the sugestion in the post: use system.web > customErrors with redirectMode="ResponseRewrite" instead.