using System;
using System.Collections.Generic;
using System.Text;

namespace Combine {
    class Program {

        public static bool EnumerableHasAtLeast<T>(IEnumerable<T> Enumerable, int xElements) {
            int x = 0;
            foreach (T entry in Enumerable) {
                x++;
                if (x >= xElements) {
                    return (true);
                }
            }
            return (false);
        }

        public static IEnumerable<IEnumerable<T>> Combine<T>(IEnumerable<T> sequence, int elementsPerEntry) {
            if (elementsPerEntry <= 0) {
                yield break;
            }

            if (elementsPerEntry == 1) {
                foreach (T element in sequence) {
                    List<T> tmp = new List<T>();
                    tmp.Add(element);
                    yield return tmp;
                }
            } else {
                foreach (T element in sequence) {
                    
                    // manually select items after element from the original sequence
                    List<T> subset = new List<T>(sequence);
                    while (!subset[0].Equals(element)) {
                        subset.RemoveAt(0);
                    }
                    subset.RemoveAt(0);

                    if (!EnumerableHasAtLeast(subset, elementsPerEntry - 1)) {
                        yield break;
                    } else {
                        foreach (IEnumerable<T> leftovers in Combine(subset, elementsPerEntry - 1)) {
                            List<T> tmp = new List<T>(leftovers);
                            tmp.Insert(0, element);
                            yield return (tmp);
                        }
                    }
                }
            }
        }

        static void Main(string[] args) {
            List<string> seq = new List<string>();
            seq.Add("A");
            seq.Add("B");
            seq.Add("C");
            seq.Add("D");
            seq.Add("E");
            seq.Add("F");

            foreach (IEnumerable<string> combo in Combine(seq, 3)) {
                foreach (string element in combo) {
                    Console.Write(element);
                }
                Console.WriteLine();
            }
        }
    }
}

static void Main(string[] args) {
            int elements = 3;
            List<string> seq = PopulateSeq25;

            int x = 0;
            foreach (IEnumerable<string> combo in Combine(seq, elements)) {
                foreach (string element in combo) {
                    //Console.Write("{0} ",element);
                }
                x++;
                //Console.WriteLine();
            }
            Console.WriteLine("Tel:");
            Console.WriteLine("Sequence Size: 25");
            Console.WriteLine("Elements per result: {0}", elements);
            Console.WriteLine("{0} combos.", x);
            Console.WriteLine("----------");
            Console.WriteLine("Processor Time: {0}", System.Diagnostics.Process.GetCurrentProcess().TotalProcessorTime);
            Console.WriteLine("GC memory usage: {0}", System.GC.GetTotalMemory(false));
        }

Tel:
Sequence Size: 25
Elements per result: 3
2300 combos.
----------
Processor Time: 00:00:00.0468750
GC memory usage: 367220

Mike:
Sequence Size: 25
Elements per result: 3
2300 combos.
----------
Processor Time: 00:00:00.0625000
GC memory usage: 878176


Tel:
Sequence Size: 25
Elements per result: 10
3268760 combos.
----------
Processor Time: 00:00:23.1093750
GC memory usage: 572120

Mike:
Sequence Size: 25
Elements per result: 10
3268760 combos.
----------
Processor Time: 00:00:40.2968750
GC memory usage: 1180774852


Tel:
Sequence Size: 100
Elements per result: 3
161700 combos.
----------
Processor Time: 00:00:00.3281250
GC memory usage: 536524

Mike:
Sequence Size: 100
Elements per result: 3
161700 combos.
----------
Processor Time: 00:00:02.4375000
GC memory usage: 38762176


Tel:
Sequence Size: 100
Elements per result: 5
75287520 combos.
----------
Processor Time: 00:03:39.9843750
GC memory usage: 356340

Mike:
Sequence Size: 100
Elements per result: 5
DNF - OutOfMemory