Computer-implemented methods, systems and apparatus implement techniques for generating custom coclasses. A runtime environment includes a client application implementing an object model, which includes a hierarchy of object classes that can be instantiated in the client application. The object model includes class templates, each having an associated class type, and includes class functionality information—including required class functionality and elective class functionality—for custom classes that can be implemented in the client application. When a user requests the creation of a custom class of a specified type, a design-time environment is launched and class functionality information is retrieved for a class template associated with the type. A custom class is created, based on a class definition defining a class having the required class functionality and the specified elective class functionality. Objects belonging to the custom class can be instantiated from the runtime environment independent of the design-time environment.