HashSet that preserves ordering

I need a HashSet that preserves insertion ordering, are there any implementations of this in the framework?

Standard .NET HashSet do not preserve the insertion order. For simple tests the insertion order may be preserved due to an accident, but it's not guaranteed and would not always work that way. To prove that it is enough to do some removals in between.

See this question for more information on that: https://stackoverflow.com/questions/657263/does-hashset-preserve-insertion-order?rq=1

I have briefly implemented a HashSet which guarantees insertion order. It uses the Dictionary to look up items and the LinkedList to preserve order. All three insertion, removal and lookup work still in O(1).

public class OrderedSet<T> : ICollection<T>
{
    private readonly IDictionary<T, LinkedListNode<T>> m_Dictionary;
    private readonly LinkedList<T> m_LinkedList;

    public OrderedSet()
        : this(EqualityComparer<T>.Default)
    {
    }

    public OrderedSet(IEqualityComparer<T> comparer)
    {
        m_Dictionary = new Dictionary<T, LinkedListNode<T>>(comparer);
        m_LinkedList = new LinkedList<T>();
    }

    public int Count => m_Dictionary.Count;

    public virtual bool IsReadOnly => m_Dictionary.IsReadOnly;

    void ICollection<T>.Add(T item)
    {
        Add(item);
    }

    public bool Add(T item)
    {
        if (m_Dictionary.ContainsKey(item)) return false;
        var node = m_LinkedList.AddLast(item);
        m_Dictionary.Add(item, node);
        return true;
    }

    public void Clear()
    {
        m_LinkedList.Clear();
        m_Dictionary.Clear();
    }

    public bool Remove(T item)
    {
        if (item == null) return false;
        var found = m_Dictionary.TryGetValue(item, out var node);
        if (!found) return false;
        m_Dictionary.Remove(item);
        m_LinkedList.Remove(node);
        return true;
    }

    public IEnumerator<T> GetEnumerator()
    {
        return m_LinkedList.GetEnumerator();
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    public bool Contains(T item)
    {
        return item != null && m_Dictionary.ContainsKey(item);
    }

    public void CopyTo(T[] array, int arrayIndex)
    {
        m_LinkedList.CopyTo(array, arrayIndex);
    }
}

You can get this functionality easily using KeyedCollection<TKey,TItem> specifying the same type argument for TKey and TItem:

public class OrderedHashSet<T> : KeyedCollection<T, T>
{
    protected override T GetKeyForItem(T item)
    {
        return item;
    }
}

If you need constant complexity of Add, Remove, Contains and order preservation, then there's no such collection in .NET Framework 4.5.

If you're okay with 3rd party code, take a look at my repository (permissive MIT license): https://github.com/OndrejPetrzilka/Rock.Collections

There's OrderedHashSet<T> collection:

• based on classic HashSet<T> source code (from .NET Core)
• preserves order of insertions and allows manual reordering
• features reversed enumeration
• has same operation complexities as HashSet<T>
• Add and Remove operations are 20% slower compared to HashSet<T>
• consumes 8 more bytes of memory per item

You can use OrderedDictionary to preserve the order of insertion. But beware of the cost of Removing items (O(n)).