C# – Server cannot append header after HTTP headers have been sent

asp.netchttp-headershttpresponse

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 headers to check if the page has been sent.

Anyone know how I can achieve such a check?

Best Answer

There are 2 ways to do it:

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
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

Related Solutions

ASP.NET MVC 3: Server cannot append header after HTTP headers have been sent

This can happen if buffering has been switched off on the page. Buffering means that asp.net waits for the whole request to be completed before sending the response. This means the header can be changed at any time. When buffering is off, the output is sent to the client as it's generated. You can't therefore change the headers at will, as they've already been sent.

From your stacktrace, it appears to be an async controller & I'm wondering if this has anything to do with it. I'm only guessing from what you've posted though.

Update

Correction, the async mention is actually framework code & nothing to do with your code. However from your code above, is SearchResultOverview an action on a controller? If so, then using the methods you are using to transfer execution is I think the cause of your problems.

It causing 2 mvchandlers to execute & they're interfering with each other. Routing would be a better way to redirect the request.

C# – Server cannot append header after HTTP headers have been sent c# pdf inline

You must clear the header first before adding new ones

if (inline)
{
   Response.ClearHeaders();

Response.AppendHeader("Content-Disposition", string.Format("inline; filename={0}", PDFFile));
Response.BufferOutput = true;
return File(PDFStream, MediaTypeNames.Application.Pdf);
}
else
return File(PDFStream, MediaTypeNames.Application.Pdf, PDFFile);

Best Answer

Related Solutions

ASP.NET MVC 3: Server cannot append header after HTTP headers have been sent

C# – Server cannot append header after HTTP headers have been sent c# pdf inline

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