c^n_k.cs

private static IEnumerable<List<T>> GenerateAllCombinations<T>(IList<T> set, int subsetPower)
{
    var subsetIndexes = Enumerable.Range(0, subsetPower).ToArray();
    while (true)
    {
        yield return subsetIndexes.Select(x => set[x]).ToList();
        var index = subsetPower - 1;
        while (index >= 0 && subsetIndexes[index] == set.Count + index - subsetPower) index--;
        if (index < 0) break;
        subsetIndexes[index]++;
        while (index++ < subsetPower - 1) subsetIndexes[index] = subsetIndexes[index - 1] + 1;
    }
}