append-lines-txt

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;

namespace AppendLinesToTxt
{
    class Program
    {
        
        static void Main(string[] args)
        {
            loadToArray();
          
        }
        static void loadToArray()
        {
            String txt = @"C:\Users\marek\Downloads\stackoverflow\append-lines\file1.txt";
            string[] lines = new string[]{};
            lines = File.ReadAllLines(txt);

            lines[Array.IndexOf(lines, "write1")] = "newline1\nnewline2" ;
            File.WriteAllText(txt, String.Empty);
            foreach (string str in lines)
            {
                File.AppendAllText(txt, str + Environment.NewLine);
            }

        }
        static void loadToList()
        {
            List<string> lines = new List<string>();

            String txt = @"C:\Users\marek\Downloads\stackoverflow\append-lines\file1.txt";
            using (StreamReader sr = File.OpenText(txt))
            {
                lines.Clear();

                string s = String.Empty;
                while ((s = sr.ReadLine()) != null)
                {
                    lines.Add(s);
                    if (s == "write1")
                    {
                        lines.Add("added item 1");
                    }

                }
            }
            File.WriteAllText(txt, String.Empty);
            foreach (string str in lines)
            {
                File.AppendAllText(txt, str + Environment.NewLine);
            }
            Console.Read();
        }
    }
}