Server cannot append header after HTTP headers have been sent

I get this exception intermittently in my asp.net c# web application:

Server cannot append header after HTTP headers have been sent.

And it caused by the application appending content to the pages response header after the page has been sent. I am not sure why its intermittent but what I need to do is perform a check prior to the modification for the h开发者_开发问答eaders to check if the page has been sent.

Anyone know how I can achieve such a check?

There are 2 ways to do it:

1. Subscribe to the PreSendRequestHeaders of HttpApplication and assume that the headers have been sent at this point. Set a flag on the context and check for it everywhere

2. The ugly solution: HttpResponse has a internal property called HeadersWritten. Since it's internal you'll have to access it through reflection. I'd recommand only to use this for debugging. Check it before/after all the page lifecyle events and find out where the problem is. Don't leave this in production code