asp.net 请求参数 .net post请求的api

/// <summary> 
        /// 调用API POST请求与获取结果  
        /// </summary>  ///URL 与 JSON串
        public static string HttpPost(string Url, string postDataStr)
        {
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url);
            request.Method = "POST";
            request.ContentType = "application/json";
            //request.ContentLength = postDataStr.Length;
            StreamWriter writer = new StreamWriter(request.GetRequestStream(), Encoding.UTF8);
            writer.Write(postDataStr);
            writer.Flush();
            HttpWebResponse response = (HttpWebResponse)request.GetResponse();
            string encoding = response.ContentEncoding;
            if (encoding == null || encoding.Length < 1)
            {
                encoding = "UTF-8"; //默认编码  
            }
            StreamReader reader = new StreamReader(response.GetResponseStream(), Encoding.GetEncoding(encoding));
            string retString = reader.ReadToEnd();            return retString;
        }