.net实体类与json相互转换时，注意要点：
1.jsonhelp编写时候添加的引用。System.Runtime.Serialization.Json; 
2.实体类需声明为public 

jsonhelp代码： 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Runtime.Serialization.Json;
using System.IO;
namespace JsonTest
{
 class JsonHelp
 {
 public JsonHelp()
 {

 //

 // TODO: Add constructor logic here

 //

 }
 /// <summary>
 /// 把对象序列化 JSON 字符串 
 /// </summary>
 /// <typeparam name="T">对象类型</typeparam>
 /// <param name="obj">对象实体</param>
 /// <returns>JSON字符串</returns>
 public static string GetJson<T>(T obj)
 {
 //记住 添加引用 System.ServiceModel.Web 
 /**
 * 如果不添加上面的引用,System.Runtime.Serialization.Json; Json是出不来的哦
 * */
 DataContractJsonSerializer json = new DataContractJsonSerializer(typeof(T));
 using (MemoryStream ms = new MemoryStream())
 {
 json.WriteObject(ms, obj);
 string szJson = Encoding.UTF8.GetString(ms.ToArray());
 return szJson;

 }

 }

 /// <summary>
 /// 把JSON字符串还原为对象
 /// </summary>
 /// <typeparam name="T">对象类型</typeparam>
 /// <param name="szJson">JSON字符串</param>
 /// <returns>对象实体</returns>
 public static T ParseFormJson<T>(string szJson)
 {
 T obj = Activator.CreateInstance<T>();
 using (MemoryStream ms = new MemoryStream (Encoding.UTF8.GetBytes(szJson)))
 {
 DataContractJsonSerializer dcj = new DataContractJsonSerializer(typeof(T));
 return (T)dcj.ReadObject(ms);
 }
 }
 

 }

} 

实体类代码： 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace JsonTest
{
 public class testData
 {
 public testData()

 {

 }
 public int Id { get; set; }
 public string Name { get; set; }
 public string Sex { get; set; }
 }
} 

控制台应用程序测试代码： 

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
namespace JsonTest

{
 class Program

 {
 static void Main(string[] args)
 {

 //实体类转json
 testData t1 = new testData();
 t1.Id = 1;
 t1.Name = "001姓名";
 t1.Sex = "男";
 testData t2 = new testData();
 t2.Id = 2;
 t2.Name = "002姓名";
 t2.Sex = "男";
 testData t3 = new testData();
 t3.Id = 3;
 t3.Name = "003姓名";
 t3.Sex = "男";
 List<testData> tlist = new List<testData>();
 tlist.Add(t1);
 tlist.Add(t2);
 tlist.Add(t3);
 Console.WriteLine(JsonHelp.GetJson<List<testData>>(tlist));

 // Console.ReadKey();
 //json转实体类

 List<testData> tl = JsonHelp.ParseFormJson <List<testData>>(JsonHelp.GetJson<List<testData>>(tlist));
 Console.WriteLine(tl.Count);
 Console.WriteLine(tl[0].Name);
 Console.ReadKey();
 }

 }
}