Here is a Mono CsharpRepl which is what you are looking for.
This documents the features available
in the C# interactive shell that is
part of Mono's C# compiler. An
interactive shell is usually referred
to as a read eval print loop or repl.
The C# interactive shell is built on
top of the Mono.CSharp
(http:/monodoc/N:Mono.CSharp) library,
a library that provides a C# compiler
service that can be used to evaluate
expressions and statements on-the-fly.