The system for instance customization functions to place code behind documents in a multi-program environment in a manner that eliminates the object instance management problems of existing systems. The system for instance customization consists of the process whereby the behavior of an object is changed by the user without changing its identity. In particular, the system for instance customization combines a customizable object and a language engine object to form the customized instance of the object. Instance customization is enabled by the customized object aggregating the user defined customizations into a standard object that is supplied by Visual Basic. The Visual Basic object is given a pointer to the standard part of the document object which implements the base functionality of the document.