GroupedDictionary

GroupedDictionary.cs

public class GroupedDictionary<TKey, TValue> : Dictionary<TKey, ICollection<TValue>>
{
	private readonly Func<TValue, TKey> _groupingKeyExtractor;

	public GroupedDictionary(IEnumerable<TValue> values, Expression<Func<TValue, TKey>> groupingKey)
	{
		_groupingKeyExtractor = groupingKey.Compile();

		PopulateDictionary(values);
	}

	public GroupedDictionary<TResultKey, TValue> GetGroupedDictionaryAt<TResultKey>(TKey key, Expression<Func<TValue, TResultKey>> groupingKey)
	{
		return new GroupedDictionary<TResultKey, TValue>(this[key], groupingKey);
	}

	private void PopulateDictionary(TValue valueToAdd)
	{
		var key = _groupingKeyExtractor(valueToAdd);

		if (ContainsKey(key))
		{
			this[key].Add(valueToAdd);
		}
		else
		{
			this[key] = new Collection<TValue> { valueToAdd };
		}
	}

	private void PopulateDictionary(IEnumerable<TValue> valuesToAdd)
	{
		foreach (var valueToAdd in valuesToAdd)
		{
			PopulateDictionary(valueToAdd);
		}
	}
}