Why do I get "Cannot redirect after HTTP headers have been sent" when I call Response.Redirect(url)?

Question

When I call Response.Redirect(someUrl) I get an HttpException: "Cannot redirect after HTTP headers have been sent".

Answer 1

A Redirect can only happen if the first line in an HTTP message is "HTTP/1.x 3xx Redirect Reason".

If you already called Response.Write() or set some headers, it'll be too late for a redirect. You can try calling Response.Headers.Clear() before the Redirect to see if that helps.

Answer 2

According to the MSDN documentation for Response.Redirect(string url), it will throw an HttpException when "a redirection is attempted after the HTTP headers have been sent". Since Response.Redirect(string url) uses the Http "Location" response header (http://en.wikipedia.org/wiki/HTTP_headers#Responses), calling it will cause the headers to be sent to the client. This means that if you call it a second time, or if you call it after you've caused the headers to be sent in some other way, you'll get the HttpException.

One way to guard against calling Response.Redirect() multiple times is to check the Response.IsRequestBeingRedirected property (bool) before calling it.

// Causes headers to be sent to the client (Http "Location" response header)
Response.Redirect("http://www.stackoverflow.com");
if (!Response.IsRequestBeingRedirected)
 // Will not be called
 Response.Redirect("http://www.google.com");

Answer 3

Once you send any content at all to the client, the HTTP headers have already been sent. A "Response.Redirect()" call works by sending special information in the headers that make the browser ask for a different URL.

Since the headers were already sent, asp.net can't do what you want (modify the headers)

You can get around this by a) either doing the Redirect before you do anything else, or b) try using Response.Buffer = true before you do anything else, to make sure that no output is sent to the client until the whole page is done executing.

Answer 4

The redirect function probably works by using the 'refresh' http header (and maybe using a 30X code as well). Once the headers have been sent to the client, there is not way for the server to append that redirect command, its too late.

Answer 5

Just check if you have set the buffering option to false (by default its true). For response.redirect to work,

1. Buffering should be true,
2. you should not have sent more data using response.write which exceeds the default buffer size (in which case it will flush itself causing the headers to be sent) therefore disallowing you to redirect.

Answer 6

There is one simple answer for this: You have been output something else, like text, or anything related to output from your page before you send your header. This affect why you get that error.

Just check your code for posible output or you can put the header on top of your method so it will be send first.

Answer 7

If you are trying to redirect after the headers have been sent (if, for instance, you are doing an error redirect from a partially-generated page), you can send some client Javascript (location.replace or location.href, etc.) to redirect to whatever URL you want. Of course, that depends on what HTML has already been sent down.