Detect that asp.net http headers already sent

Question

I am adding headers to a page as follows: Page.Response.AddHeader("foo", "bar");

Depending upon previous processing, sometimes this fails with "Server cannot append header after HTTP headers have been sent." I am dealing with this by enclosing Page.Response.AddHeader("foo", "bar"); within a try-catch construct. However, to keep things cleaner and avoid generating an exception is there any way to detect that the headers have already been sent? (btw if I try evaluating Page.Response.Headers then I get the following error: "This operation requires IIS integrated pipeline mode")

Thanks

I have seen this before. You need to explain what technologies you are using: caching, AJAX, MVC, ... and where in the code you are adding this header so we can help. — Aliostad
– Aliostad, Commented Sep 21, 2010 at 8:34

ChaseMedallion · Accepted Answer · 2016-03-24 17:40:22Z

15

As of .NET 4.5.2, you can do this using the now-public HeadersWritten property of HttpResponse (see the msdn docs):

if (HttpContext.Current.Response.HeadersWritten) { ... }

answered Mar 24, 2016 at 17:40

ChaseMedallion

22k18 gold badges97 silver badges164 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Scott Hanselman · Accepted Answer · 2012-04-04 05:30:13Z

13

You can use an HttpModule to register for the PreSendRequestHeaders event. When it gets called, write a value to HttpContext.Current.Items indicating that the headers are being sent – and then everywhere else in your code you check the value in HttpContext.Current.Items to see if its been sent yet.

answered Apr 4, 2012 at 5:30

Scott Hanselman

17.7k6 gold badges77 silver badges89 bronze badges

2 Comments

anthonyv Over a year ago

Looks like this might have solved the same problem we are having. Thanks!

drphrozen Over a year ago

If you are able to switch to .NET 4.5.2 you should use HttpResponseBase.AddOnSendingHeaders instead. link

Diederik · Accepted Answer · 2021-02-03 08:01:27Z

7

UPDATE: the HeadersWritten property is now available on the HttpResponse object.

Unfortunately, whilst the HttpResponse object has a property called HeadersWritten, and a backing field called _headersWritten, neither of these are accessible from outside of the System.Web assembly - unless you use Reflection. I'm not clear what you think you'll be able to obtain from the Headers collection, it may or may not exist, independently of whether the headers have been sent yet.

If you want to use Reflection, it may have it's own performance penalties, and it will require your application to run in full trust.

All of the publicly accessible methods on HttpResponse that involve the _headersWritten field seem to use it to throw an exception.

edited Feb 3, 2021 at 8:01

Diederik

791 silver badge12 bronze badges

answered Sep 21, 2010 at 10:04

Damien_The_Unbeliever

241k28 gold badges358 silver badges470 bronze badges

2 Comments

Aliostad Over a year ago

+1 good observation. The work around I think is the one I suggested.

OneManBand Over a year ago

In the meantime .Net has made HeadersWritten public since Framework 4.5.2

Aliostad · Accepted Answer · 2010-09-21 09:31:28Z

0

Trying setting buffer to false:

http://msdn.microsoft.com/en-us/library/950xf363.aspx

This will alleviate your first problem but your perfromance and user experience can suffer. Also "This operation requires IIS integrated pipeline mode" is related to non-IIS 7 server processing that line of code. You can find more info on it here:

http://forums.asp.net/p/1253457/2323117.aspx

answered Sep 21, 2010 at 9:31

Aliostad

82k21 gold badges164 silver badges209 bronze badges

Collectives™ on Stack Overflow

Detect that asp.net http headers already sent

4 Answers 4

Comments

2 Comments

2 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

4 Answers 4

Comments

2 Comments

2 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related