using System;

using System.Linq;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        List<string> listArray = new List<string>();

        listArray.Add("This is TEXT");

        listArray.Add("Convert it to words");

        var result = listArray.SelectMany(el => el.Split(" ".ToCharArray())).ToArray();

        Console.WriteLine(string.Join(",", result));

        Console.ReadLine();

    }

}