string username = "fanjiaxing@hotmail.com";
            string password = "XXXXXX";
            string usernamePassword = username + ":" + password;

            //准备调用的URL及需要POST的数据：

            string url = "http://api.t.sina.com.cn/statuses/update.xml?source = 1776XXXXXX&status= test！";
            //string content = "新的一天新的开始！";
            //准备用于发起请求的HttpWebRequest对象
            WebRequest webRequest = WebRequest.Create(url);
            HttpWebRequest httpRequest = webRequest as HttpWebRequest;
            //准备用于用户验证的凭据
            CredentialCache myCache = new CredentialCache();
            myCache.Add(new Uri(url), "Basic", new NetworkCredential(username, password));
            httpRequest.Credentials = myCache;
            httpRequest.Headers.Add("Authorization", "Basic " + Convert.ToBase64String(new System.Text.ASCIIEncoding().GetBytes(usernamePassword)));
            //发起POST请求
            httpRequest.Method = "POST";
            httpRequest.ContentType = "application/x-www-form-urlencoded";
            System.Text.Encoding encoding = System.Text.Encoding.ASCII;

            WebResponse wr = httpRequest.GetResponse();
            Stream receiveStream = wr.GetResponseStream();
            using (StreamReader reader = new StreamReader(receiveStream, System.Text.Encoding.UTF8))
            {
                string responseContent = reader.ReadToEnd();
            }

为什么WebResponse wr = httpRequest.GetResponse();
返回远程服务器返回错误: (400) 错误的请求。在线等！