using System;

using System.Collections.Generic;

using System.Linq;

public class Program

{

    public static void Main()

    {

        string[] sentence = new string[] { "The quick brown", "fox jumps over", "the lazy dog." };

​

            string[] result = sentence.Select(x => String.Join("#", x.Split(' ')))

                                     .ToArray();

​

            foreach (var item in result)

            {

                Console.WriteLine(item);

            }

    }

}