当我尝试从我的c＃代码访问时，Webservice返回401

Question

在我的邮递员工作正常的网络服务中。但是当我尝试使用我的c＃代码时无法正常工作。

  ServicePointManager.ServerCertificateValidationCallback += (sender, certificate, chain, sslPolicyErrors) => true;
                HttpWebRequest webrequest = (HttpWebRequest)WebRequest.Create(url);
                webrequest.Method = "GET";
                webrequest.ContentType = "application/json; charset=utf-8";
                webrequest.Headers.Add("Authorization", "Basic xxxxxxxx==");
                webrequest.Headers.Add("Username", "zzz");
                webrequest.Headers.Add("Password", "aaaa");
                HttpWebResponse webresponse = (HttpWebResponse)webrequest.GetResponse();
                Encoding enc = System.Text.Encoding.GetEncoding("utf-8");
                StreamReader responseStream = new StreamReader(webresponse.GetResponseStream(), enc);
                string result = string.Empty;
                result = responseStream.ReadToEnd();
                webresponse.Close();

它总是返回401未授权。我该如何解决这个问题。