.NET 2.0 introduced Version Tolerant Serialisation. This allows a Serializable class to have fields marked with an OptionalFieldAttribute. This means you can change the class by adding optional fields (or removing optional fields) and still deserialise data from the previous class versions.

I have a Data Transfer Object class that is only used to pass query criteria to a data access class. The idea is that by serialising the object we can let users can save their queries, but I was worried that changes to the class would render previously saved queries unusable. As every field is nullable (users only define a subset of the available criteria) I have been able to mark them all as optional. This means we can add and remove fields and still recreate the object from a previously serialised version. Fields dropped from the previous version will be ignored (they presumably no longer have meaning if they are dropped), and newly-added fields will get their default value.

There is at least one situation it won’t handle: changing the type of a field will break the deserialisation. We’ll just have to catch that and let the user know their query no longer works, possibly while begging forgiveness :-).