static IEnumerable<IEnumerable<T>>
 GetKCombs<T>(IEnumerable<T> list, int length) where T : IComparable
 {
        if (length == 1) return list.Select(t => new T[] { t });
        return GetKCombs(list, length - 1)
            .SelectMany(t => list.Where(o => o.CompareTo(t.Last()) > 0),
                (t1, t2) => t1.Concat(new T[] { t2 }));
 }

string[] files = Directory.GetFiles(@"C:UsersDownloadsSamples", "*.txt");
IEnumerable<IEnumerable<string>> filescombination = GetKCombs(files, 2);

foreach(var x in filescombination)
        {
            Console.WriteLine(x);
        }

?x
          {System.Linq.Enumerable.ConcatIterator<string>}
            first: null
            second: null

class Program
{
    static void Main(string[] args)
    {
        List<int> list = new List<int> { 1, 2, 3, 4 };

        IEnumerable<IEnumerable<int>> result = GetKCombs(list, 2);

        foreach (var line in result)
        {
            foreach (var item in line)
            {
                Console.Write("{0}, ", item);
            }
            Console.WriteLine();
        }

        Console.ReadKey();
    }

    static IEnumerable<IEnumerable<T>> GetKCombs<T>(IEnumerable<T> list, int length) where T : IComparable
    {
        if (length == 1) return list.Select(t => new T[] { t });
        return GetKCombs(list, length - 1)
            .SelectMany(t => list.Where(o => o.CompareTo(t.Last()) > 0),
                (t1, t2) => t1.Concat(new T[] { t2 }));
    }
}

foreach(IEnumerable<string> x in filescombination)
    {
        Console.WriteLine(x);
    }

foreach(var x in filescombination)
        {
            var y = x.ToList()[0] + ',' + x.ToList()[1];
        }

IEnumerable<string>

IEnumerable<IEnumerable<string>> filescombination = GetKCombs(files,2);
        List<string> flist = new List<string>();
        foreach(var x in filescombination)
        {
            var y = x.ToList()[0] + ',' + x.ToList()[1];
            flist.Add(y);
        }