Object instance annotation technology is disclosed. This technology is capable of annotating any number of instances for any number of objects. Object instance annotations can be of various types, and annotations can serve as data extensions. Objects are enabled for annotation either by developers, or, alternatively, automatically by a programming language or framework. Furthermore, an interface is provided for annotations. This interface has a variety of operations, some of which allow for adding, retrieving, or removing annotations. Annotations can be reference types or even value types. They may also be private class types. Once object instances are enabled for annotating, and the interface is set up, this interface can annotate a multitude of different kinds of objects. For example, it can annotate generic objects, XML trees, CodeDom trees, and so on.