An object instance is serialized to a serial format, such as an eXtensible Markup Language (XML) document, based on a mapping between an arbitrary annotated source code file and a schema. The arbitrary annotated source code contains at least one programming type that describes a shape of an object instance and the schema describes a format of a document having a serial format. The mapping defines a correspondence between the shape of the object instance and the format of the document having the serial format. Subsequently, an object instance is converted to the serial format by converting public properties, public fields and method parameters of the object instance to a document having the serial format based on the mapping. Once the mapping is defined, an XML document can also be converted to an object instance based on the mapping.