theodorzoulias

// Related: https://github.com/dotnet/runtime/issues/101003 -- [API Proposal]: New method HashSet<T>.Add that replaces an existing element if found
abstract class JsonKeyedCollectionBase<TKey, TItem> : ICollection<TItem>, IEqualityComparer<TItem>
{
    private readonly HashSet<TItem> _set;

    protected abstract TKey GetKeyForItem(TItem item);
    protected abstract TItem GetItemForKey(TKey key);

    protected virtual IEqualityComparer<TKey> KeyComparer => EqualityComparer<TKey>.Default;
    protected virtual IComparer<TItem> EnumerationComparer => null;

theodorzoulias

// https://stackoverflow.com/questions/60695167/concurrentdictionary-with-multiple-values-per-key-removing-empty-entries
// Alternative implementation
using System.Collections;
using System.Collections.Generic;
using System.Collections.Concurrent;
using System.Collections.Immutable;
using System.Linq;

public class ConcurrentMultiDictionary<TKey, TValue>
    : IEnumerable<KeyValuePair<TKey, TValue[]>>