如何使用OnSerializing和OnDeserializing属性

我曾尝试在xml中实现自动加密和解密，但它不仅有效，即数据未加密。原因可能是什么？我的代码如下所示。我正在使用XmlSerializer类。感谢

[Serializable]
public class User
{
    public string _username;
    public string _password;
    public string[] _roles;
    [XmlIgnore]
    public string Username
    {
        get { return _username; }
        set { _username = value; }
    }
    [XmlIgnore]
    public string Password
    {
        get { return _password; }
        set { _password = value; }
    }
    [XmlIgnore]
    public string[] Roles
    {
        get { return _roles; }
        set { _roles = value; }
    }
    [OnDeserializingAttribute]
    internal void DecryptPersonalData(StreamingContext context)
    {
        _username = Crypto.Decrypt(_username);
        _password = Crypto.Decrypt(_password);
        for (int i = 0; i < _roles.Length; i++)
        {
            _roles[i] = Crypto.Decrypt(_roles[i]);
        }
    }
    [OnSerializingAttribute]
    internal void EncryptPersonalData(StreamingContext context)
    {
        _username = Crypto.Encrypt(_username);
        _password = Crypto.Encrypt(_password);
        for (int i = 0; i < _roles.Length; i++)
        {
            _roles[i] = Crypto.Encrypt(_roles[i]);
        }
    }
}