using System;

public class Program

{

    public static void Main()

    {

        string s = "hello";

        Update(ref s);

        Console.WriteLine(s);

        var b = new Box() {

            s = "hello2"

        };

        Update2(b);

        Console.WriteLine(b);

    }

    public static void Update(ref string s)

    {

        s = "world";

    }

    public static void Update2(Box b){

        b.s = "world";

    }

​

}

​

public class Box 

{

    public string s;    

}