如何在.net中模拟post方式提交json数据

最近看见bing的网页上有一个机器人可以根据回答猜人物
http://renlifang.msra.cn/Q20/index.aspx
我想通过代码模拟提交的方式去交互（纯属个人试着玩），我在JS端试了用$.post()的方式去提交数据是可以交互的（必需制定json类型，否则返回错误）。现在我想在.net后台模拟这个动作，不知道该怎么实现。

$.post(

      "/Q20/Handlers/HandlerQ20.ashx",

      { uid: “507937844”, sid: ”809662793“, choice: "yes", a: "GenerateQuestionHandler", questfd: "0"},

      function(data){ },

      "json"

)

我自己了用System.Net.HttpWebRequest对象来模拟提交，结果什么也没返回，但是将 request.ContentType = "application/json;charset=UTF-8";改为request.ContentType = "application/x-www-form-urlencoded"可以接受到返回数据，返回的是出错提示。用WebClient我也试过，结果一样。请大神看看是什么问题。

string url = "http://renlifang.msra.cn/Q20/Handlers/HandlerQ20.ashx";

string param = "{\"uid\":\"246534824\",\"sid\":\"906508525\",\"choice\":\"yes\",\"a\":\"GenerateQuestionHandler\",\"questfd\":\"0\"}";

string callback = PostMoths(url,param);

public static string PostMoths(string url, string param)

{

      string strURL = url;

      System.Net.HttpWebRequest request;

      request = (System.Net.HttpWebRequest)WebRequest.Create(strURL);

      request.Method = "POST";

      request.ContentType = "application/json;charset=UTF-8";

      string paraUrlCoded = param;

      byte[] payload;

      payload = System.Text.Encoding.UTF8.GetBytes(paraUrlCoded);

      request.ContentLength = payload.Length;

      Stream writer = request.GetRequestStream();

      writer.Write(payload, 0, payload.Length);

      writer.Close();

      System.Net.HttpWebResponse response;

      response = (System.Net.HttpWebResponse)request.GetResponse();

      System.IO.Stream s;

      s = response.GetResponseStream();

      string StrDate = "";

      string strValue = "";

      StreamReader Reader = new StreamReader(s, Encoding.UTF8);

      while ((StrDate = Reader.ReadLine()) != null)

      {

          strValue += StrDate + "\r\n";

      }

      return strValue;

}