I don't know what C# has for an interactive prompt nowadays, but F#'s commandline environment, via its fsi.exe, was a revelation back then. It prevented having to have entire solutions to contain test projects to explore different areas of the vast .NET framework, especially when just learning how to use specific methods or objects.

For C# you can use LINQpad, a .NET scratchpad. You can even mix and match F# in it.