(For the longer version, go here: https://github.com/scriptcs/scriptcs)
At a command prompt, run this:
@powershell -NoProfile -ExecutionPolicy Unrestricted -Command "iex ((New-Object Net.WebClient).DownloadString('https://chocolatey.org/install.ps1'))" && SET PATH=%PATH%;%systemdrive%\chocolatey\bin
Then run this:cinst scriptcs
That's it. Really. Now you can write C# at a command prompt:
C:\>scriptcs
scriptcs (ctrl-c or blank to exit)
> var message = "Hello, world!";
> Console.WriteLine(message);
Hello, world!
Want to save some C# and run it any time? Do this:
C:\Temp>copy con helloworld.csx
var message = "Hello, world!";
Console.WriteLine(message);
1 file(s) copied.
C:\Temp>scriptcs helloworld.csx
Hello, world!
Note: I got an error when trying to run this at the root of the C: drive, so I just switched to my temp folder and it worked.
Enjoy!