using System;
using System.Collections.Generic;
using System.Linq;

namespace ConsoleApplication2
{
    class Program
    {
        public static void Main()
        {
            string[] list = new string[] { "a", "b", "c", "d", "e", "MM" };
            var X = 3;
            var cnt = 0;
            foreach (var s in 你的组合(list, X))
                Console.WriteLine("第{0}个结果:\t{1}", ++cnt, string.Join(string.Empty, s));
            Console.ReadKey();
        }

        private static IEnumerable<string[]> 你的组合(string[] list, int X)
        {
            if (X == 1)
            {
                foreach (var s in list)
                    yield return new string[] { s };
            }
            else if (X > 1)
            {
                for (var i = 0; i < list.Length; i++)
                {
                    var newList = list.Where((s, p) => p != i).ToArray();
                    foreach (var sub in 你的组合(newList, X - 1))
                        for (var j = 0; j < sub.Length; j++)
                        {
                            var ret = sub.ToList();
                            ret.Insert(j, list[i]);
                            yield return ret.ToArray();
                        }
                }
            }
        }
    }
}

------解决方案--------------------
要注意，输出当然有bad这种结果啦，不是一定按照原来排列顺序的。