Server cannot set status after HTTP headers have been sent IIS7.5

Question

Sometimes I get exception in my production environment:

• Process information
  • Process ID: 3832
  • Process name: w3wp.exe
  • Account name: NT AUTHORITY\NETWORK SERVICE
• Exception information
  • Exception type: System.Web.HttpException
  • Exception message: Server cannot set status after HTTP headers have been sent.
• Request information
  • Request URL: http://www.myulr.pl/logon
  • Request path: /logon
  • User host address: 10.11.9.1
  • User: user001
  • Is authenticated: True
  • Authentication Type: Forms
  • Thread account name: NT AUTHORITY\NETWORK SERVICE
• Thread information
  • Thread ID: 10
  • Thread account name: NT AUTHORITY\NETWORK SERVICE
  • Is impersonating: False

Stack trace: at System.Web.HttpResponse.set_StatusCode(Int32 value) at  
System.Web.HttpResponseWrapper.set_StatusCode(Int32 value) at  
System.Web.Mvc.HandleErrorAttribute.OnException(ExceptionContext filterContext) at  
System.Web.Mvc.ControllerActionInvoker.InvokeExceptionFilters(ControllerContext controllerContext, IList(1) filters, Exception exception) at  
System.Web.Mvc.ControllerActionInvoker.InvokeAction(ControllerContext controllerContext, String actionName) at System.Web.Mvc.Controller.ExecuteCore() at  
System.Web.Mvc.MvcHandler.<>c__DisplayClass8.<BeginProcessRequest>b__4() at  
System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass1.<MakeVoidDelegate>b__0() at  
System.Web.Mvc.Async.AsyncResultWrapper.<>c__DisplayClass8(1).<BeginSynchronous>b__7(IAsyncResult _) at  
System.Web.Mvc.Async.AsyncResultWrapper.WrappedAsyncResult(1).End() at   
System.Web.Mvc.MvcHandler.EndProcessRequest(IAsyncResult asyncResult) at  
System.Web.HttpApplication.CallHandlerExecutionStep.System.Web.HttpApplication.IExecutionStep.Execute() at  
System.Web.HttpApplication.ExecuteStep(IExecutionStep step, Boolean& ompletedSynchronously) 

I didn't noticed this error on my test environment what should I check?

I am using ASP.NET MVC 2 (Release Candidate 2)

Answer 1

I'll broadly agree with Vagrant on the cause:

1. your action was executing, writing markup to response stream
2. the stream was unbuffered forcing the response headers to get written before the markup writing could begin.
3. Your view encountered a runtime error
4. Exception handler kicks in trying to set the status code to something else non-200
5. Fails because the headers have already been sent.

Where I disagree with Vagrant is the "cause no errors in binding" remedy - you could still encounter runtime errors in View binding e.g. null reference exceptions.

A better solution for this is to ensure that Response.BufferOutput = true; before any bytes are sent to the Response stream. e.g. in your controller action or On_Begin_Request in application. This enables server transfers, cookies/headers to be set etc. right the way up to naturally ending response, or calling end/flush.

Of course also check that buffer isn't being flushed/set to false further down in the stack too.

MSDN Reference: HttpResponse.BufferOutput