title: How to know if the request is ajax in asp.net in Application_Error()
body: <p>How to know if the request is ajax in asp.net  in  Application_Error()</p>
  
  <p>I want to handle app error in Application_Error().If the request is ajax  and  some exception is thrown,then write the error in  log file and return a json data that contains error tips for client .
  Else if the request is synchronism and some exception is thrown ,write the error in log file and then redirect to a error page.</p>
  
  <p>but now i cant judge which kind  the request is . I want to get "X-Requested-With" from header ,unfortunately    keys of  headers don't contain "X-Requested-With" key ,why?</p>