将派生类序列化为XML

我试图序列化一个派生类，并序列化派生的属性和基础的属性。目前，派生属性是序列化的，但基属性不是。我错过了什么?基类型和派生类型都标有[Serializable]。

String SaveBody<T>(String bodyFile, T body) where T : FreeBody
{
    XmlSerializer serializer = new XmlSerializer(typeof(T));
    using (var xmlStream = new MemoryStream())
    using (TextWriter fileStream = new StreamWriter(xmlStream))
    {
        serializer.Serialize(fileStream, body);
        fileStream.Flush();
        xmlStream.Seek(0, SeekOrigin.Begin);
        String text = new StreamReader(xmlStream).ReadToEnd();
        File.WriteAllText(bodyFile, text);
        return text;
    }
}