C#中类的字段或属性不被序列化成JSON或XML

using Newtonsoft.Json;
using System;
using System.IO;
using System.Web.Script.Serialization;
using System.Xml.Serialization;

namespace QRCoderTest
{
    class Program
    {
        static void Main(string[] args)
        {
            //创建用户信息
            UserInfo user = new UserInfo();
            user.Name = "张三";
            user.Age = 25;
            user.Email = "[email?protected]";

            //1、使用Newtonsoft转JSON
            string newtonStr = JsonConvert.SerializeObject(user);

            //2、使用JavaScriptSerializer类转JSON
            JavaScriptSerializer serializer = new JavaScriptSerializer();
            string serializedStr = serializer.Serialize(user);

            //3、XML序列化
            string xmlStr = XmlSerialize<UserInfo>(user);
            Console.WriteLine(newtonStr);
            Console.WriteLine(serializedStr);
            Console.WriteLine(xmlStr);
            System.Console.ReadLine();
        }
        //XML序列化方法
        public static string XmlSerialize<T>(T obj)
        {
            using (StringWriter sw = new StringWriter())
            {
                Type t = obj.GetType();
                XmlSerializer serializer = new XmlSerializer(obj.GetType());
                serializer.Serialize(sw,obj);
                sw.Close();
                return sw.ToString();
            }
        }

    }

    /// <summary>
    /// 用户信息类
    /// </summary>
    public class UserInfo
    {
        /// <summary>
        /// 名称
        /// </summary>
        public string Name { get; set; }

        /// <summary>
        /// 年龄
        /// </summary>
        public int Age { get; set; }

        /// <summary>
        /// 电子邮件
        /// </summary>
        [Newtonsoft.Json.JsonIgnore]
        [System.Web.Script.Serialization.ScriptIgnore]
        [System.Xml.Serialization.XmlIgnore]
        public string Email { get; set; }
    }
}