如何调用外部的Web API

Uri uri = new Uri(url + "?" + postData);
            System.Net.HttpWebRequest request = (System.Net.HttpWebRequest)System.Net.WebRequest.Create(uri);
            request.Method = "GET";
            request.ContentType = "application/x-www-form-urlencoded";
            request.AllowAutoRedirect = false;
            request.Timeout = 5000;
            System.Net.HttpWebResponse response = (System.Net.HttpWebResponse)request.GetResponse();
            Stream responseStream = response.GetResponseStream();
            StreamReader readStream = new StreamReader(responseStream, System.Text.Encoding.UTF8);
            string retext = readStream.ReadToEnd().ToString();
            readStream.Close();
            return retext;

}