```csharp


 public string Post(string Url, string jsonParas)
        {
            string strURL = Url;
            //创建一个HTTP请求  
            HttpWebRequest request = (HttpWebRequest)WebRequest.Create(strURL);
            //Post请求方式  
            request.Method = "POST";
            //内容类型
            request.ContentType = " application/x-www-form-urlencoded";
            request.UserAgent = "app";
            //设置参数，并进行URL编码 
            string paraUrlCoded = jsonParas;//System.Web.HttpUtility.UrlEncode(jsonParas);   
            byte[] payload;
            //将Json字符串转化为字节  
            ASCIIEncoding encoding = new ASCIIEncoding();
            payload = encoding.GetBytes(paraUrlCoded);
            //设置请求的ContentLength   
            request.ContentLength = payload.Length;
            //发送请求，获得请求流 

            Stream writer;
            try
            {
                writer = request.GetRequestStream();//获取用于写入请求数据的Stream对象
            }
            catch (Exception)
            {
                writer = null;
                Console.Write("连接服务器失败!");
            }
            //将请求参数写入流
            writer.Write(payload, 0, payload.Length);
            writer.Close();//关闭请求流
                           // String strValue = "";//strValue为http响应所返回的字符流
            HttpWebResponse response;
            try
            {
                //获得响应流
                response = (HttpWebResponse)request.GetResponse();
            }
            catch (WebException ex)
            {
                response = ex.Response as HttpWebResponse;
            }
            Stream s = response.GetResponseStream();
            //  Stream postData = Request.InputStream;
            StreamReader sRead = new StreamReader(s);
            string postContent = sRead.ReadToEnd();
            sRead.Close();
            return postContent;//返回Json数据
        }


    }
    参数使用 key=value&key=value