ASP.NET post访问外网接口返回结果值

string postData = string.Format("a=1&b=2");   //post传递参数
Stream outstream = null;
Stream instream = null;
StreamReader sr = null;
HttpWebResponse response = null;
HttpWebRequest request = null;
Encoding encoding = Encoding.Default;
byte[] data = encoding.GetBytes(postData);
// 准备请求
try
{
    // 设置参数
    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();
    //发送请求并获取相应回应数据
    response = request.GetResponse() as HttpWebResponse;
    //直到request.GetResponse()程序才开始向目标网页发送Post请求
    instream = response.GetResponseStream();
    sr = new StreamReader(instream, encoding);
    //返回结果网页（html）代码
    return sr.ReadToEnd();
}
catch
{
    return null;
}