/// <summary>
/// WebService1 的摘要说明
/// </summary>
[WebService(Namespace = "http://tempuri.org/")]
[WebServiceBinding(ConformsTo = WsiProfiles.BasicProfile1_1)]
[ToolboxItem(false)]
// 若要允许使用 ASP.NET AJAX 从脚本中调用此 Web 服务，请取消对下行的注释。
[System.Web.Script.Services.ScriptService]
public class WebService1 : System.Web.Services.WebService
{

 [WebMethod]
 public string HelloWorld()
 {
  CommonData.Json.ObjectSerialization ser = new CommonData.Json.ObjectSerialization();
  Student stu = new Student();
  stu.Id = 1;
  stu.Name = "hechen";
  string json = ser.EntityToJson(stu);
  return json;
 }
}

/**
 * 
 * 2009-5-26
 * 贺  臣
 * 
 * 将某个对象转化为Json数据格式
 * */
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.IO;
using System.Data;
using System.ServiceModel.Web;
using System.Runtime.Serialization;
using System.Runtime.Serialization.Json;

namespace CommonData.Json
{
 public class ObjectSerialization
 {
  private object _entity;

  /// <summary>
  /// 被序列化得实体对象
  /// </summary>
  public object Entity
  {
   get { return _entity; }
   set { _entity = value; }
  }

  private string _jsonData;

  /// <summary>
  /// 被转化为json格式数据的对象
  /// </summary>
  public string JsonData
  {
   get { return _jsonData; }
   set { _jsonData = value; }
  }

  /// <summary>
  /// 无参数构造方法
  /// </summary>
  public ObjectSerialization()
  { 
  }

  /// <summary>
  /// 有参数构造方法
  /// </summary>
  /// <param name="entity">要被序列化得实体对象</param>
  public ObjectSerialization(object entity)
  {
   this._entity = entity;
  }


  /// <summary>
  /// 序列化实体对象
  /// </summary>
  /// <returns></returns>
  public string EntityToJson()
  {
   var serializer = new DataContractJsonSerializer(Entity.GetType());
   MemoryStream ms = new MemoryStream();
   serializer.WriteObject(ms, Entity);
   byte[] myByte = new byte[ms.Length];
   ms.Position = 0;
   ms.Read(myByte, 0, (int)ms.Length);
   string dataString = Encoding.UTF8.GetString(myByte);
   return dataString;
  }


  /// <summary>
  /// 序列化实体对象
  /// </summary>
  /// <param name="entity">要被序列化得实体对象</param>
  /// <returns></returns>
  public string EntityToJson(object entity)
  {
   this._entity = entity;
   return EntityToJson();
  }

  /// <summary>
  /// 将Json格式数据转换为对象
  /// </summary>
  /// <returns></returns>
  public T GetObjectJson<T>()
  {
   MemoryStream ms = new MemoryStream(Encoding.UTF8.GetBytes(JsonData));
   var serializer = new DataContractJsonSerializer(typeof(T));
   T t = (T)serializer.ReadObject(ms);
   return t;
  }

  /// <summary>
  /// 将Json格式数据转换为对象
  /// </summary>
  /// <param name="jsonData">json数据格式</param>
  /// <returns></returns>
  public T GetObjectJson<T>(string jsonData)
  {
   this._jsonData = jsonData;
   return GetObjectJson<T>();
  }
 }
}

<script src="jquery-1[1].2.3.min.js" type="text/javascript"></script>
<script src="json2.js" type="text/javascript"></script>
<script type="text/javascript">
 $(document).ready(function() {
 $("#btnClick").click(function() {
   $.ajax({
    url:"http://localhost:10168/WebService1.asmx/HelloWorld",
    beforeSend:　function(x)　{　
　　　　　　　　   x.setRequestHeader("Content-Type",　"application/json;　charset=utf-8");　
　　　　　　   },
    data:{},
    dataType:"json",
    type:"POST",
    error:　function(x,　e)　{　
　　　　　　　　  alert(x.responseText);　
　　　　　　  },　
　　　　　　 complete:　function(x)　{　
　　　　　　　　   //alert(x.responseText);　
　　　　　　    }　,
    success:function(data){
     var msg=data.d;
     var json=JSON2.parse(msg);
     alert(json.id);
    }
   });
   
  });
 });
</script>

<webServices>
 <protocols>
 <add name="HttpGet"/>
 <add name="HttpPost"/>
 </protocols>
</webServices>