直接下载安装包：https://www.mono-project.com/download/stable/

sudo apt install mono-complete

using System;
using Math;
 
namespace Application
{
    class Application
    {
        static void Main()
        {
            System.Console.WriteLine("Hello: " + Common.Add(1, 1).ToString());
        }
    }
}

namespace Math
{
    static class Common
    {
        public static int Add(int x,int y)
        {
            return x - y;
        }
    }
}

编译命令（需要编译器在PATH里面）

mcs Main.cs Math.cs