using System;

using System.Collections.Generic;

using System.Collections.ObjectModel;

public class Program

{

    public static void Main()

    {

        var list = new List<string>() { "a", "b", "c", "d", "e", "f" };

        var roc = new ReadOnlyCollection<string>(list);

        ICollection<string> evil = (ICollection<string>)roc;

        evil.Add("x");

    }

}