新手泛型赋值问题！求教

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.IO;
using System.Xml.Serialization;

namespace MangerFT.FS
{
public partial class WebForm4 : System.Web.UI.Page
{
public static string UCFilePath = "D:\\UserConfig.xml";

protected void Page_Load(object sender, EventArgs e)
{

}

[Serializable]
/// <summary>
/// 用户信息类
/// </summary>
public class Userconfig
{
public List<UserDetail> ListUser;
}


[Serializable]
public class UserDetail
{
private string id;
/// <summary>
/// 用户Id
/// </summary>
public string Id
{
get
{
return id;
}
set
{
id = value;
}
}

private string name;
/// <summary>
/// 用户名
/// </summary>
public string Name
{
get
{
return name;
}
set
{
name = value;
}

}

private string role;
/// <summary>
/// 系统角色
/// </summary>
public string Role
{
get
{
return role;
}
set
{
role = value;
}
}

private bool permission;
/// <summary>
/// 扫描枪使用权限
/// </summary>
public bool Permission
{
get
{
return permission;
}
set
{
permission = value;
}
}

private bool unLock;
/// <summary>
/// 解锁权限
/// </summary>
public bool UnLock
{
get
{
return unLock;
}
set
{
unLock = value;
}
}
}

void Serialize()
{
Userconfig a = new Userconfig();
UserDetail b = new UserDetail();

a.ListUser.Add(b);//这里报错，未将对象引用到实例的对象。。。

a.ListUser[0].Id = "000000";
a.ListUser[0].Name = "admin";
a.ListUser[0].Permission = true;
a.ListUser[0].Role = "admin";
a.ListUser[0].UnLock = true;

a.ListUser.Add(b);

a.ListUser[1].Id = "000001";
a.ListUser[1].Name = "admin1";
a.ListUser[1].Permission = true;
a.ListUser[1].Role = "admin1";
a.ListUser[1].UnLock = true;

XmlSerializer xl = new XmlSerializer(typeof(Userconfig));
xl.Serialize(File.Create(UCFilePath),a);
}

protected void Button1_Click(object sender, EventArgs e)
{
Serialize();
}
}
}

求教