In C#, sometimes you would like to define a base class, or a library, that uses an enumeration, and you would like to allow derived classes or users of your library to define new values for it. Trouble is, enums are not extensible: a derived class or user code cannot define new values.



For instance, one time, I wrote a library for serializing and deserializing "shapes" with geographic coordinates to a text file. In this library, several different shapes were supported: circles, rectangles, lines, polygons, and so on.