using System;

using System.Collections.Generic;

using System.Linq;

​

public class Program

{

    public static IEnumerable<IEnumerable<T>> GetPowerSetOfLength<T>(IList<T> list, int length)

    {

        return from m in Enumerable.Range(0, 1 << list.Count)

                  let setResult = ( from i in Enumerable.Range(0, list.Count)

                                    where (m & (1 << i)) != 0

                                    select list[i]).ToList()

                  where setResult.Count==length

                  select setResult;

    }

    public static void Main()

    {

        Console.WriteLine(string.Join(", " , GetPowerSetOfLength<int>(new[]{1,2,3,4,5,6,7,8,9},3).Select(b=>"{ " + string.Join(" ,",b) +" }")));

    }

}