Shuffles a list in C# (Unity compatible)

shuffler.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Collections.ObjectModel;

namespace Utils
{
    /// <summary>
    /// Implements "iTunes shuffle" that is to say, it's a random where you can't get twice the same value unless you
    /// already got all the values.
    /// 
    /// To avoid having a chance to get twice the same value when refilling the pool, we store the last picked value in
    ///  `_last` and do not put it back when refilling the pool
    /// 
    /// author: [email protected]
    /// </summary>
    public class Shuffler<T> : IEnumerable
    {
        private readonly List<T> _allItems = new List<T>(); // All items used in the shuffler
        private readonly List<T> _pool = new List<T>(); // Remaining items for current series
        private Random _random = new Random();
        private T _last; //Last object extracted from the pool

        private int PoolCount
        {
            get { return _pool.Count; }
        }

        public Shuffler(Random random = null)
        {
            SetRandom(random);
        }

        public Shuffler(IEnumerable<T> allItems, Random random = null)
        {
            _allItems.AddRange(allItems);
            SetRandom(random);
        }

        /// <summary>
        /// Always returns next element. If the pool is empty, GetNext() refills it before returning the next element
        /// </summary>
        /// <returns>The next element</returns>
        public T GetNext()
        {
            if (IsPoolEmpty()) PopulatePool();
            return PopRandomElement();
        }

        /// <summary>
        /// Enumerates the REMAINING elements in the pool. Stops when the pool is empty
        /// </summary>
        /// <returns>enumerator on elements left</returns>
        public IEnumerator GetEnumerator()
        {
            if (IsPoolEmpty()) PopulatePool();
            while (PoolCount > 0)
            {
                yield return PopRandomElement();
            }
        }

        /// <summary>
        /// A readonly list of all items in the Shuffler. Some of them may already have been picked and may not be 
        /// chosen until the pool is refiled
        /// </summary>
        /// <returns>readonly list of all possible values</returns>
        public ReadOnlyCollection<T> GetAllPossibleValues()
        {
            return _allItems.AsReadOnly();
        }

        public void Add(T element)
        {
            _allItems.Add(element);
        }

        private T PopRandomElement()
        {
            if (PoolCount == 0) throw new ArgumentException("List of elements is empty !");
            int randomIndex = _random.Next(PoolCount);
            T item = _pool[randomIndex];
            _pool.RemoveAt(randomIndex);
            if (IsPoolEmpty()) _last = item;
            return item;
        }

        private void PopulatePool()
        {
            _pool.Clear();
            _pool.AddRange(_allItems);
            if (PoolCount > 1) // If _allItems is size 1 we don't remove from the pool : it would be empty
            {
                RemoveLastFromPool();
            }
        }

        private void RemoveLastFromPool()
        {
            _pool.Remove(_last);
            _last = default(T);
        }

        private bool IsPoolEmpty()
        {
            return PoolCount == 0;
        }


        private void SetRandom(Random random)
        {
            if (random != null) _random = random;
        }
    }
}

Rev 2 can now be used as follows :

Shuffler<int> shuffler = new Shuffler<int> {1, 2, 3, 4, 5, 6, 7, 8, 9, 0};
foreach (int number in shuffler)
{
	Debug.Log("Random element : " + number);
}

bool gameIsPlaying = true;
while (gameIsPlaying)
{
	Debug.Log("Random element : " + shuffler.GetNext());
}