perm.cs

using System;
using System.Collections.Generic;

namespace PermutationsMany
{
    public class Program
    {
        public static void Main()
        {
            var collections = new[] { new[] { 1, 2, }, new[] { 4, 5, 100, 234 }, new[] { 6, 7 } };
            foreach (var permutation in PermutationsMany(collections))
                Console.WriteLine(string.Join(", ", permutation));
            Console.ReadKey();
        }

        public static IEnumerable<T[]> PermutationsMany<T>(IList<T[]> collections)
        {
            var indexes = new int[collections.Count];
            while (indexes[0] < collections[0].Length)
            {
                var current = collections.Count - 1;
                var item = new T[collections.Count];
                for (var i = 0; i < collections.Count; i++)
                    item[i] = collections[i][indexes[i]];
                yield return item;
                indexes[current]++;
                for (; current > 0 && indexes[current] >= collections[current].Length; current--)
                    indexes[current - 1]++;
                for (var i = current + 1; i < indexes.Length; i++)
                    indexes[i] = 0;
            }
        }
    }
}