ASP.NET post訪問外網介面返回結果值

Code
string postData = string.Format("a=1&b=2");   //post傳遞參數
Stream outstream = null;
Stream instream = null;
StreamReader sr = null;
HttpWebResponse response = null;
HttpWebRequest request = null;
Encoding encoding = Encoding.Default;
byte[] data = encoding.GetBytes(postData);
// 準備請求
try
{
    // 設定參數
    request = WebRequest.Create("http://www.bbb.com") as HttpWebRequest;
    CookieContainer cookieContainer = new CookieContainer();
    request.CookieContainer = cookieContainer;
    request.AllowAutoRedirect = true;
    request.Method = "POST";
    request.ContentType = "application/x-www-form-urlencoded";
    request.ContentLength = data.Length;
    outstream = request.GetRequestStream();
    outstream.Write(data, 0, data.Length);
    outstream.Close();
    //發送請求並擷取相應回應資料
    response = request.GetResponse() as HttpWebResponse;
    //直到request.GetResponse()程式才開始向目標網頁發送Post請求
    instream = response.GetResponseStream();
    sr = new StreamReader(instream, encoding);
    //返回結果網頁（html）代碼
    return sr.ReadToEnd();
}
catch
{
    return null;
}