string[][] test_array = new string[4][] { 
                    new string[]{"aaa","ddd","fff"},
                    new string[]{"bbb",null,"ggg"},
                    new string[]{null,null,"hhh"},
                    new string[]{"ccc","eee","iii"}
                };
            for (int k = 0; k < test_array.Count() - 1; k++)
            {
                for (int i = 0; i < test_array.FirstOrDefault().Count(); i++)
                {
                    for (int j = test_array.Count() - 1; j > 0; j--)
                    {
                        if (string.IsNullOrEmpty(test_array[j][i]) && !string.IsNullOrEmpty(test_array[j - 1][i]))
                        {
                            string temp = test_array[j][i];
                            test_array[j][i] = test_array[j - 1][i];
                            test_array[j - 1][i] = temp;
                        }
                    }
                }
            }
            for (int i = 0; i < test_array.Count(); i++)
            {
                for (int j = 0; j < test_array[i].Count(); j++)
                {
                    Console.Write(test_array[i][j]+"\t");
                }
                Console.Write("\n");
            }