For the second time I've wasted time trying to work out why the serialized output from the XmlSerializer was missing elements so it's time to write the answer here as a reminder for next time.

Basically the XmlSerializer will not output content that it can't read back in which means that any read only property will not be serialized.

This is not the case for properties that implement ICollection or IEnumerable as collections have their own serialization rules.