Mono is available for Mac OS X, Windows and Linux.

Install Mono by following installation instructions.

Createt HelloWorld.cs file with the following content:

public class Program
{
    public static void Main()
    {
        System.Console.WriteLine("Hello, world!");
        System.Console.WriteLine("Press any key to exit..");
        System.Console.Read();
    }
}

If you are using Windows, run the Mono Command Prompt which is included in the Mono installation and ensures that the necessary environment variables are set. If on Mac or Linux, open a new terminal.

To compile the newly created file, run the following command in the directory containing HelloWorld.cs:

mcs -out:HelloWorld.exe HelloWorld.cs

The resulting HelloWorld.exe can then be executed with:

mono HelloWorld.exe

which will produce the output:

Hello, world!   
Press any key to exit..