At least for development code, it is always a good idea to instruct db4o to check for available constructors at storage time. (If you've configured db4o to use constructors at all.)
.NET: configuration.ExceptionsOnNotStorable(true)
If this setting triggers exceptions in your code, or if instances of a class seem to lose members during storage, check the involved classes (especially their constructors) for problems similar to the one shown in the following section.