public static IEnumerable<IEnumerable<T>> Permute<T>(this IList<T> v)
{
    ICollection<IList<T>> result = new List<IList<T>>();

    Permute(v, v.Count, result);

    return result;
}

private static void Permute<T>(IList<T> v, int n, ICollection<IList<T>> result)
{
    if (n == 1)
    {
        result.Add(new List<T>(v));
    }
    else
    {
        for (var i = 0; i < n; i++)
        {
            Permute(v, n - 1, result);
            Swap(v, n % 2 == 1 ? 0 : i, n - 1);
        }
    }
}

private static void Swap<T>(IList<T> v, int i, int j)
{
    var t = v[i];
    v[i] = v[j];
    v[j] = t;
}