Umbraco-CMS/src/Umbraco.Core/Collections/OrderedHashSet.cs

using System.Collections.ObjectModel;

namespace Umbraco.Core.Collections
{
    /// <summary>
    /// A custom collection similar to HashSet{T} which only contains unique items, however this collection keeps items in order
    /// and is customizable to keep the newest or oldest equatable item
    /// </summary>
    /// <typeparam name="T"></typeparam>
    internal class OrderedHashSet<T> : KeyedCollection<T, T>
    {
        private readonly bool _keepOldest;

        public OrderedHashSet(bool keepOldest = true)
        {
            _keepOldest = keepOldest;
        }

        protected override void InsertItem(int index, T item)
        {
            if (Dictionary == null)
            {
                base.InsertItem(index, item);
            }
            else
            {
                var exists = Dictionary.ContainsKey(item);

                //if we want to keep the newest, then we need to remove the old item and add the new one
                if (exists == false)
                {
                    base.InsertItem(index, item);
                }
                else if(_keepOldest == false)
                {
                    if (Remove(item))
                    {
                        index--;
                    }
                    base.InsertItem(index, item);
                }
            }
        }

        protected override T GetKeyForItem(T item)
        {
            return item;
        }
    }
}