Login with c# HttpWebRequest

I would like to automatically log into a site 'petwholesaler.com' and remain in the site using the HttpWebRequest browser class. I tried the following:

  CookieContainer _siteContainer;
             string _login = "myUserName";
             string _password = "myPassword";
           

             string strPostData = String.Format("username={0}&password={1}", _login, _password);

             // Setup the http request.
             HttpWebRequest wrWebRequest = WebRequest.Create("http://www.petwholesaler.com/login.php?ref=express_order_form") as HttpWebRequest;
             wrWebRequest.Method = "POST";
             wrWebRequest.ContentLength = strPostData.Length;
             wrWebRequest.ContentType = "application/x-www-form-urlencoded";
             _siteContainer = new CookieContainer();
             wrWebRequest.CookieContainer = _yahooContainer;

             // Post to the login form.
             using (StreamWriter swRequestWriter = new StreamWriter(wrWebRequest.GetRequestStream()))
             {
                 swRequestWriter.Write(strPostData);
                 swRequestWriter.Close();
             }

             // Get the response.
             HttpWebResponse response = (HttpWebResponse)wrWebRequest.GetResponse();
              StreamReader myStreamReader = new StreamReader(response.GetResponseStream());
             
             string responseString = myStreamReader.ReadToEnd();
             StreamWriter swrite = new StreamWriter(@"PetWholesaler.html");
              swrite.Write(responseString);
              swrite.Close();

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It doesn't work. Does anyone know what's the exact code I need to login?