Asp.net 如何接收post過來的 json 數據

<%@ WebHandler Language="C#" Class="Handler" %>
using System;
using System.Web;
public class Handler : IHttpHandler 
{
    //服務端
    public void ProcessRequest (HttpContext context) {
        context.Response.ContentType = "application/json";
        context.Response.Cache.SetCacheability(HttpCacheability.NoCache);
        using (var reader = new System.IO.StreamReader(context.Request.InputStream))
        {
            String xmlData = reader.ReadToEnd();

            if (!string.IsNullOrEmpty(xmlData))
            {
                         //業務處理
             }
        }
    }
    public bool IsReusable {
        get {
            return false;
        }
    }
}

     //客戶端
private string HttpPost(string Url, string postDataStr)
{
    HttpWebRequest request = (HttpWebRequest)WebRequest.Create(Url);
    request.Method = "POST";
    request.ContentType = "application/json";
    request.ContentLength = Encoding.UTF8.GetByteCount(postDataStr);
    Stream myRequestStream = request.GetRequestStream();
    StreamWriter myStreamWriter = new StreamWriter(myRequestStream, Encoding.GetEncoding("gb2312"));
    myStreamWriter.Write(postDataStr);
    myStreamWriter.Close();
    HttpWebResponse response = (HttpWebResponse)request.GetResponse();
    Stream myResponseStream = response.GetResponseStream();
    StreamReader myStreamReader = new StreamReader(myResponseStream, Encoding.GetEncoding("utf-8"));
    string retString = myStreamReader.ReadToEnd();
    myStreamReader.Close();
    myResponseStream.Close();
    return retString;
}