using System;

public class Program

{   

    public void Main()

    {

        var a = new Test();

        a.Field = 1;

        Change(a);

        Console.WriteLine(a.Field);

        Change(ref a);

        Console.WriteLine(a.Field);

    }

    public static void Change(Test a)

    {

        a.Field = 2;

    }

    public static void Change(ref Test a)

    {

        a.Field = 3;

    }

}

​

public class Test

{

    public int Field;

}