The problem is that a class property with a private setter can be modified if the property is of reference type.

Program.cs

using System;
using System.Collections.Generic;

namespace DilemmaWithReference
{
    class Program
    {
        static void Main(string[] args)
        {
            Person p = new Person();
            p.Numbers.Add(3); // I don't want this to be possible
            Console.WriteLine(p.Numbers.Count.ToString());
        }
    }

    public class Person
    {
        public List<int> Numbers 
        {
            get { return numbers; }
            private set { numbers = value; }
        }
        private List<int> numbers;

        public Person()
        {
            numbers = new List<int>();
            numbers.Add(0); // This is okay
        }
    } 
}

This is a classic problem.

Ask yourself two things.

• Do you only need to iterate the collection from outside? Then just cast it to a IEnumerable<int> and expose it.
• Do you need random access outside? Then go for a ReadOnlyCollection<T>. It is quite strict, dont forget that.

The situation you need to be aware of is :

ICollection<Foo> evil = (ICollection<Foo>) bar.Foos;
evil.Add(...);

You can also yield return to make things simpler. I personally avoid the skip(0) hack.

Suggested reading:

https://stackoverflow.com/questions/491375/readonlycollection-or-ienumerable-for-exposing-member-collections

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