C# is sometimes written as CSharp.
If you have Visual Studio 2015 Update 1 or later then you can use an interactive C# interface and almost write scripts with it! See New C# REPL and scripting capabilities « Bret Stateham's Blog for details. It is Visual Studio that has this C# scripting in, so, csi.exe is only supplied with VS as far as I can tell, at the moment. A good introduction to this is C# Scripting - Part 1 | raw tech. | Channel 9 and C# Scripting - Part 2 | raw tech. | Channel 9