CodeRunner supports AppleScript, C, C++, Java, Objective-C, Perl, PHP, Ruby, and Shell Script. It does not support C#. If you want C# support in
CodeRunner, then you should express your desires to
CodeRunner's developer, Nikolai Krill.
As you are no doubt know, C# is a part of Microsoft's .NET frameworks. I am not aware of the value of C# outside the .NET frameworks. .NET does not exist on MacOS X. However, there is a clone of the .NET frameworks that should be fairly mature now. It is
Mono 2.10.8.
In the best case--the one in which you can actually integrate C# with
CodeRunner--you will need to download and install
Mono.