using System;

using System.Collections.Generic;

using System.IO;

using System.Linq;

using System.Text;

using System.Threading.Tasks;

​

namespace Stream

{

    public class Program

    {

        static readonly string file = "file.txt";

​

        public static void Main(string[] args)

        {

            Generate();

            Console.WriteLine("Original");

            Display();

​

            Console.WriteLine();

​

            Modify();

            Console.WriteLine("Modified");

            Display();

        }

​

        static void Display()

        {

            using (var sr = new StreamReader(file))

            {

                string line;

                while ((line = sr.ReadLine()) != null)

                    Console.WriteLine(line);

            }

        }

​

        static void Generate()

        {

            using (var sw = new StreamWriter(file))

            {

                for (var i = 10; i < 20; i++)

                {

                    var line = i % 2 == 0 ? "line" : "long line";

                    sw.WriteLine($"{line},{i}");

                }

            }

        }

​

        static void Modify()

        {

            using (var fs = new FileStream(file, FileMode.Open, FileAccess.ReadWrite))

            {

                string line;

                long position;

​

                while ((line = fs.ReadLine(out position)) != null)

                {

                    var tmp = line.Split(',');

                    tmp[1] = "00"; // new value

                    var newLine = string.Join(",", tmp);

                    fs.WriteLine(position, newLine);

                }

            }

        }

    }

​

    static class FileStreamExtensions

    {

        private static readonly char[] newLine = Environment.NewLine.ToCharArray();

        private static readonly int length = Environment.NewLine.Length;

        private static readonly char eof = '\uFFFF';

​

        public static string ReadLine(this FileStream fs, out long position)

        {

            position = fs.Position;

            var chars = new List<char>();

            char c;

            while ((c = (char)fs.ReadByte()) != eof && (chars.Count < length || !chars.Skip(chars.Count - 2).SequenceEqual(newLine)))

            {

                chars.Add(c);

            }

            fs.Position--;

​

            if (chars.Count == 0)

                return null;

​

            return new string(chars.ToArray());

        }

​

        public static void WriteLine(this FileStream fs, long position, string line)

        {

            var bytes = line.ToCharArray().Concat(newLine).Select(c => (byte)c).ToArray();

            fs.Position = position;

            fs.Write(bytes, 0, bytes.Length);

        }

    }

}

​