为什么要将XML反序列化为返回空值的对象?

我有一个这样的XML字符串:

<?xml version="1.0" ?>
<result>
<vmeet_id>7121</vmeet_id>
<username>MT_Hue_QuangBinh_QuangTri</username>
<email></email>
<begin_date>2010-04-21 08:53</begin_date>
<expiry_date>2010-12-21 00:00</expiry_date>
<point></point>
<info>OK</info>
</result>

我想将它反序列化为一个对象,因此我创建了这个类:

[Serializable] 
[XmlRoot(ElementName = "result", IsNullable = false)]
public class UserInfo
{
    [XmlAttribute("vmeet_id")]
    public int UserID { get; set; }
    [XmlAttribute("username")]
    public string Username { get; set; } 
    [XmlAttribute("email")]
    public string Email { get; set; }
    [XmlAttribute("begin_date")]
    public DateTime BeginDate { get; set; }
    [XmlAttribute("expiry_date")]
    public DateTime ExpiryDate { get; set; }
    [XmlAttribute("point")]
    public string Point { get; set; }
    [XmlAttribute("info")]
    public string Info { get; set; }
}

然后使用此代码反序列化:

var deserializer = new XmlSerializer(typeof(UserInfo));
        using (var stream = new StringReader(result))
        {
            UserInfo userInfo = (UserInfo)deserializer.Deserialize(stream);
            return userInfo;
        }

返回值不是空值,但其所有属性都是空值:

<result vmeet_id="0" begin_date="0001-01-01T00:00:00" expiry_date="0001-01-01T00:00:00"/>

这里怎么了?我忘记什么了吗?

谢谢您。