using System;

using System.Collections.Generic;

public class Program

{

    static readonly List<int> readonlyList = new List<int>();

    public static void Main()

    {

        var test = new Test();

        test.ListReadOnly.Add(1);

        //not valid

        //test.ListReadOnly = new List<int>();

        //not valid

        //test.ListReadOnly2.Add(1);

    }

    public class Test

    {

        public readonly List<int> ListReadOnly;

        public IReadOnlyList<int> ListReadOnly2;

        private List<int> ListRegular;

        public Test()

        {

            ListReadOnly = new List<int>(); 

            ListRegular = new List<int>();

            ListReadOnly2 = ListRegular.AsReadOnly();

        }

    }

}