using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static void Main()

    {

        var words = new []

        {

            "A",

            "B",

            "C",

        }.ToList();

        var combinations = GetCombination(words);

        var permutations = combinations.SelectMany(x => GetPermutations(x, x.Count()));

        permutations.Select(x => string.Join("", x)).ToList().ForEach(Console.WriteLine);

    }

    static IEnumerable<List<string>> GetCombination(List<string> list)

    {

        double count = Math.Pow(2, list.Count);

        for (int i = 1; i <= count - 1; i++)

        {

            string str = Convert.ToString(i, 2).PadLeft(list.Count, '0');

            List<string> strings = new List<string>();

            for (int j = 0; j < str.Length; j++)

            {

                if (str[j] == '1')

                {

                    strings.Add(list[j]);

                }

            }

            yield return strings;

        }

    }

    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 }));

    }

}