DetachedCriteria criteria1 = DetachedCriteria.For<SecurityObjectDTO>("so")
    .Add(Expression.Eq("ObjectCode", "1234"));

XmlSerializer s = new XmlSerializer(typeof(DetachedCriteria));
TextWriter writer = new StringWriter();
s.Serialize(writer, criteria1);
writer.Close();

// Convert the DetachedCriteria to a byte array
MemoryStream ms = new MemoryStream();
IFormatter formatter = new BinaryFormatter();
formatter.Serialize(ms, detachedCriteria);

// Serialize the byte array
XmlSerializer s = new XmlSerializer(typeof(byte[]));
TextWriter writer = new StringWriter();
s.Serialize(writer, ms.Buffer);
writer.Close();