Are there any implementations of multiset for .Net?

Question

I\'m looking for a .Net implementation of a multiset. Can anyone recommend a good one?

(A multiset, or bag, is a set that can have duplicate values, and on which you

Answer 1

Anything calling itself a C# implementation of a multiset should not be based on a Dictionary internally. Dictionaries are hash tables, unordered collections. C++'s sets, multisets, maps, and multimaps are ordered. Internally each is represented as some flavor of a self-balancing binary search tree.

In C# we should then use a SortedDictionary as the basis of our implementation as according to Microsoft's own documentation a SortedDictionary "is a binary search tree with O(log n) retrieval". A basic multiset can be implemented as follows:

public class SortedMultiSet : IEnumerable
{
    private SortedDictionary _dict; 

    public SortedMultiSet()
    {
        _dict = new SortedDictionary();
    }

    public SortedMultiSet(IEnumerable items) : this()
    {
        Add(items);
    }

    public bool Contains(T item)
    {
        return _dict.ContainsKey(item);
    }

    public void Add(T item)
    {
        if (_dict.ContainsKey(item))
            _dict[item]++;
        else
            _dict[item] = 1;
    }

    public void Add(IEnumerable items)
    {
        foreach (var item in items)
            Add(item);
    }

    public void Remove(T item)
    {
        if (!_dict.ContainsKey(item))
            throw new ArgumentException();
        if (--_dict[item] == 0)
            _dict.Remove(item);
    }

    // Return the last value in the multiset
    public T Peek()
    {
        if (!_dict.Any())
            throw new NullReferenceException();
        return _dict.Last().Key;
    }

    // Return the last value in the multiset and remove it.
    public T Pop()
    {
        T item = Peek();
        Remove(item);
        return item;
    }

    public IEnumerator GetEnumerator()
    {
        foreach(var kvp in _dict)
            for(int i = 0; i < kvp.Value; i++)
                yield return kvp.Key;
    }

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