static IEnumerable<IEnumerable<T>> GetPermutations<T>(IEnumerable<T> list, int length)
        {
            if (length == 1) return list.Select(t => new T[] { t });

            return GetPermutations(list, length - 1)
                .SelectMany(t => list.Where(e => !t.Contains(e)),
                    (t1, t2) => t1.Concat(new T[] { t2 }));
        }

        static void Main(string[] args)
        {
            IEnumerable<IEnumerable<int>> result = GetPermutations(Enumerable.Range(1, 20), 20);

            foreach (var permutation in result )
            {
                Console.WriteLine(string.Join(" ", permutation));
            }
}