build dll and use dll with command-line C# compiler

csc.exe /t:library /out:Math1.dll Addition.cs

using System;
namespace MyMath {

  public class Addition {

    public static int add(int a, int b) {
       return a+b;
    }
  }
}

csc.exe /reference:Math1.dll TestMath.cs
or
csc.exe /reference:Math1.dll /lib:somedirectory_that_have_Math1.dll TestMath.cs

using System;
using MyMath;


public class Hello {

  public static void Main() {
    Console.Write("Result: "+Addition.add(1,3));
	}
}