Adding Dynamic Types to C#

Developers using statically typed languages such as C and Java increasingly have to interoperate with APIs and object models defined in dynamic languages. This impedance mismatch results in code that is difficult to understand, awkward to analyze, and expensive to maintain. In this paper we describe new features in C4.0 that support the safe combination of dynamically and statically typed code by deferring type checking of program fragments with static type dynamic until runtime. When executed, these dynamic code fragments are typechecked and resolved using the same rules as statically typed code. We formalize these features in a core fragment of C and prove important safety properties. In particular, we show that subtyping remains transitive.