Permutations by Ehrlich swaps

Permutation.cs

static IEnumerable<IEnumerable<T>> Permute<T>(IEnumerable<T> collection)
{
    // Initialize
    int length = collection.Count();
    int[] b = Enumerable.Range(0, length).ToArray();
    // Set all in c to zero.
    int[] c = Enumerable.Range(0, length + 1).Select(x => 0).ToArray();
    // Create a temporary copy.
    List<T> set = collection.Select(x => x).ToList();
    while (true)
    {
        // Return the permutation.
        yield return set;
        int k = 1;
        // Find k.
        while (c[k] == k)
        {
            c[k] = 0;
            k++;
        }
        // Termination if k == n.
        if (k == length) 
            yield break;
        c[k]++;
        // Swap a[0] <-> a[b[k]].
        T tmp = set[0];
        set[0] = set[b[k]];
        set[b[k]] = tmp;
        // Flip contents.
        set.Reverse(1, k - 1);
    }
}