IEnumerable does not protect your list..

Test.cs

using Microsoft.VisualStudio.TestTools.UnitTesting;
using System;
using System.Collections.Generic;

namespace Tests
{
    [TestClass]
    public class Test
    {
        [TestMethod]
        public void MyTestMethod()
        {
            var x = new ListTest();

            var l = (List<string>)x.List;

            l.Add("test");

            foreach (var item in x.List)
            {
                Console.WriteLine(item);
            }
        }
    }

    public class ListTest {

        public IEnumerable<string> List => _list;

        private List<string> _list = new List<string>();
    }
}