using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        void ModList(List<int> listToMod) {

          listToMod.Add(12);    

        }

        void ModListByRef(ref List<int> listToMod) {

          listToMod.Add(12);    

        }

        var x = new MyClass();

        x.MyList = new List<int>();

        ModList(x.MyList);

        Console.WriteLine(string.Join(';', x.MyList.Select(y => y.ToString())));   //this outputs 12

        var y = new MyClass();

        y.MyList = new List<int>();

        var listAsVar = y.MyList;

        ModListByRef(ref listAsVar);

        Console.WriteLine(string.Join(';', x.MyList.Select(y => y.ToString())));  //this also outputs 12

    }

    public class MyClass{

        public List<int> MyList {get; set;}

    }

}