In an object oriented computer system, a domain-neutral object is modified with domain-specific run-time extensions to customize the object to a particular domain. A special factory uses the extension identifier to create the domain-neutral object with appropriate extensions in a collection that corresponds to the domain extension. If the special factory is not available, or if a specialized collection associated with the particular extension type has not been previously created, the domain-neutral object with appropriate extensions is created in a default collection. In this manner, an object that is domain-neutral can be customized to a particular domain and created in an appropriate collection for objects of that type.