Mechanisms that allow frameworks significant flexibility in varying the library of common base classes in a manner that better suits the domain of applications served by the framework. Instead of providing the base class library, the runtime provides a data contract for the data structure of each base class. The frameworks can then define each base class in a custom way so long as the data contract is honored. Thus, for example, the framework may provide custom framework-specific methods and/or properties as is appropriate for the framework. Another framework might define the base classes in a different way.