C# detect page redirect

I am trying to determine if a qualification exists on http://www.accreditedqualifications.org.uk in the form:

http://www.accreditedqualifications.org.uk/qualification/50084811.seo.aspx

50084811 being a qualification aim entered by the end user.

If they enter an invalid one e.g.

http://www.accreditedqualifications.org.uk/qualification/50084911.seo.aspx

They are redirected t开发者_如何学Co an error page (with incorrect http headers as far as I can see). Is there a way to detect the redirect in C#. I would hope to be able to detect the redirect in http headers (thinking it will issue 2) or similar as oppose to having to download the whole page. This could be happening a lot so I would like to minimize traffic.

Edit

Used this to have a look at the headers looks like two are issued for an invalid page:

http://pageheaders.com/display-http-headers.php?url=http%3A%2F%2Fwww.accreditedqualifications.org.uk%2Fqualification%2F50084911.seo.aspx&agent=ie6

There are a number of different codes that could be returned. You could check the various codes a la:

response.StatusCode == HttpStatusCode.Redirect

You can view all the possibilities at http://msdn.microsoft.com/en-us/library/system.net.httpstatuscode.aspx

Alternatively, you might find it sufficient to check whether the Location in the response is different.

var request = (HttpWebRequest)WebRequest.Create(uri);
request.Method = "HEAD";
request.AllowAutoRedirect = false;

string location;
using (var response = request.GetResponse() as HttpWebResponse)
{
  location = response.GetResponseHeader("Location");
}
return (location != uri.OriginalString);

The simplest way is probably to fetch the content using a HEAD request (set Method to "HEAD") in an HttpWebRequest having set AllowAutoRedirect to false. I can't remember offhand whether that will cause an exception or not, but either way it should be easy to handle.

There are two ways to detect a page redirect:

1. check if response.StatusCode == HttpStatusCode.Redirect is set in your HttpWebResponse
2. compare request.RequestUri and response.ResponseUri

Please note that 1) depends on the implementation the server, not all servers set this status code, so option 2) might be more reliable:

HttpWebRequest request = CreateWebRequest(requestString);
HttpWebResponse response = (HttpWebResponse)request.GetResponse();
bool redirected = request.RequestUri.ToString() != response.ResponseUri.ToString();

You can only do that using a webclient from your server. If you give the link to your client, you will not be in part of the communication between him and the accredited qualificationsweb server afterwards, and tehrefore you will not be able to get the information that the link has been redirected to an error.

As far as I can understand your project, I would make some call using Webclient ( or what ever) from my server to be sure the qualification exists and store the result in the database with a buffering time. It would allow not to make too many calls and to get reliable information nonetheless.

Try this:

try
{
     HttpWebRequest myHttpWebRequest = (HttpWebRequest)WebRequest.Create(line);
     myHttpWebRequest.Timeout = 20000;
     myHttpWebRequest.MaximumAutomaticRedirections = 1;
     myHttpWebRequest.AllowAutoRedirect = true;
     HttpWebResponse myHttpWebResponse = (HttpWebResponse)myHttpWebRequest.GetResponse();
     if (myHttpWebResponse.ResponseUri.ToString() == "Some website")
         {
              //your logic
         }

     myHttpWebResponse.Close();
}
catch (WebException)
{
     // record exception
}

It is based on the HEAD request of httpwebrequest having set AllowAutoRedirect to false.