Ayende @ Rahien

It's a girl

Hunt the bug

The following code will throw under certain circumstances, what are they?

public class Global : HttpApplication
       public void Application_Start(object sender, EventArgs e)
            HttpUtility.UrlEncode("Error inside!");

Hint, the exception will not be raised because of transient conditions such as low memory.

What are the conditions in which it would throw, and why?

Hint #2, I had to write my own (well, take the one from Mono and modify it) HttpUtility to avoid this bug.



Posted By: Ayende Rahien

Published at

Originally posted at


08/21/2010 09:31 AM by

I guess it has to do with the new HttpEncoder support. I can be disabled by setting Response.DisableCustomHttpEncoder but that is not available in application start. You could have used HttpEncoder.Default however.

08/21/2010 11:16 AM by

I'm guessing that HttpUtility underneath requires an HttpContext and that might not be present at Application_Start.

Victor Kornov
08/21/2010 11:40 AM by
Victor Kornov

Yep, starting .NET 4 HttpUtility requires a HttpContext /HttpResponse wich is not yet available at the time of Application_Start with IIS7 in integrated mode. I guess the stack trace looks like:

[HttpException (0x80004005): Response is not available in this context.]

System.Web.HttpContext.get_Response() +8753496

System.Web.Util.HttpEncoder.get_Current() +39

System.Web.HttpUtility.UrlEncodeToBytes(String str, Encoding e) +31

System.Web.HttpUtility.UrlEncode(String str) +41

Global.Application_Start(Object sender, EventArgs e)

08/21/2010 02:10 PM by

This means that the context is available but it is a fake one - getting the response throws. My workaround that I posted is still valid ;-)

Sebastien Ros
08/21/2010 02:14 PM by
Sebastien Ros

There has been some changes in .NET 4 which introduced this bug. I hit it when we switched our solution to VS2010. So as it's actually forgetting to check HttpContext.Current null values before using it, we hacked this by assigning a mock HttpContext around it.


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