具有多个命名空间的 XML 反序列化

我正在尝试将以下xml反序列化为对象。Xml 有多个命名空间。我试图将 Xml 反序列化为一个对象。对象（数据）具有对 LastChannel 对象的引用。但是当我要求 data.channel 应该给我 LastChannel 时，我得到了一个空指针。

Xml：

<rdf:RDF xmlns:rdf="http://www.w3.org/1999/02/22-rdf-syntax-ns#"
         xmlns="http://purl.org/rss/1.0/"
         xmlns:dc="http://purl.org/dc/elements/1.1/"
         xmlns:mp="http://www.tagesschau.de/rss/1.0/modules/metaplus/"
         xmlns:sy="http://purl.org/rss/1.0/modules/syndication/"
         xmlns:content="http://purl.org/rss/1.0/modules/content/">
<channel>
<title>title</title>
<description>Default description</description>
<dc:date>2013-04-15 13:27:06</dc:date>
<sy:updateBase>2013-04-15 13:27:06</sy:updateBase>
<sy:updatePeriod>hourly</sy:updatePeriod>
<sy:updateFrequency>12</sy:updateFrequency>
</channel>
</rdf:RDF>

对象如下所示：

[XmlRoot("RDF", Namespace = "http://www.w3.org/1999/02/22-rdf-syntax-ns#")]
public class LastRss
{
   [XmlElement("channel")]
   public LastChannel channel { get; set; }
} 

和

public class LastChannel
{
    [XmlElement("title")]
    public string title { get; set; }
    [XmlElement("description")]
    public string description { get; set; }
    [XmlElement("date", Namespace = "http://purl.org/dc/elements/1.1/")]
    public DateTime date { get; set; }
    [XmlElement("updateBase", Namespace = "http://purl.org/rss/1.0/modules/syndication/")]
    public DateTime updateBase { get; set; }
    [XmlElement("updatePeriod", Namespace = "http://purl.org/rss/1.0/modules/syndication/")]
    public string updatePeriod { get; set; }
    [XmlElement("updateFrequency", Namespace = "http://purl.org/rss/1.0/modules/syndication/")]
    public int updateFrequency { get; set; }
}

有人看到为什么data.channel ist null吗？

序列化程序：

LastRss data = new LastRss();
XmlSerializer serializer = new XmlSerializer(typeof(LastRss));
System.IO.TextReader reader = new System.IO.StringReader(xml);
try
{
    object o = serializer.Deserialize(reader);
    data = (LastRss)o;
}

  [XmlElement("channel", Namespace = "http://purl.org/rss/1.0/")]
  public LastChannel channel { get; set; }