Try it live here: http://csharp-repl.apphb.com/
At the heart of the OWASP O2 Platform is an C# REPL environment which provides a real powerful and fast way to write C# Scripts.
This environment, when combined with the O2's Fluent# just about creates a new language which feels a lot like a managed/strongly-typed version of Pyhton.
You can get this REPL environment from the main O2 distribution or get it as a stand-alone exe that you can DOWNLOAD from: http://tiny.cc/REPL
Requiring .NET 4.0, when executed (just double-click on the exe file) this is what the current version looks like:
The Panel (top panel) is a WinForms panel that is provided to the script (in the middle) so that it can be used while writing the scripts (note that most O2 Platform GUIs where developed using this REPL environment)
The Scripting environment is pure C#, so what ever you can do with C# you can do in there (Lambdas, Linq, Dynamic, foreach loops, etc...)
Take a look at:
Note that the download above (12Mb in size) contains the entire O2.Platform.Scripts folder (which has tons of script examples and tools) and the Roslyn dlls (which are used to provide better AST error information).
If you want an even smaller version (for example to embed into your project), try the O2.FluentSharp.REPL (.NET 3.5 assembly), which you can get from: http://tiny.cc/O2_FluentSharp_REPL
This version will look almost exactly the same, but it doesn't have the O2 Scripts and the AST error's details. The only visual diference is that the title bar text will say ...CLR 3.5... instead of ...CLR 4.0...).