Discord-History-Tracker/app/Utils/Collections/MultiDictionary.cs

using System.Collections.Generic;

namespace DHT.Utils.Collections {
	public sealed class MultiDictionary<TKey, TValue> where TKey : notnull {
		private readonly Dictionary<TKey, List<TValue>> dict = new();

		public void Add(TKey key, TValue value) {
			if (!dict.TryGetValue(key, out var list)) {
				dict[key] = list = new List<TValue>();
			}

			list.Add(value);
		}

		public List<TValue>? GetListOrNull(TKey key) {
			return dict.TryGetValue(key, out var list) ? list : null;
		}
	}
}