Anonymous

using System;
using System.Collections.ObjectModel;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

namespace WindowsApplication2
{
    public interface IReadOnlyList<T>
        : IEnumerable<T>, System.Collections.ICollection
    {
        T this[int Index] { get; }
    }

    public class MyList<T>: List<T>, IReadOnlyList<T>
    {
    }

    public partial class Form1 : Form
    {
        MyList<int> list = new MyList<int>();

        public Form1()
        {
            for(int i=0; i<20;++i) 
                list.Add(i);
            InitializeComponent();
        }
        public IReadOnlyList<int> GetList { get { return list; } }

        public ReadOnlyCollection<int> GetList2 { get { return list.AsReadOnly(); } }

        private void button1_Click(object sender, EventArgs e)
        {
            IReadOnlyList<int> ro = this.GetList;
            ro.Add(20);

            ReadOnlyCollection<int> ro2 = this.GetList2;
            ro2.Add(20);
        }
    }
}