using System;
using System.Windows.Forms;
using System.IO;
using System.Text;
using System.Text.RegularExpressions;

namespace ConcurrentReadWrite
{
  public partial class Form1 : Form
  {
    public Form1()
    {
      InitializeComponent();
    }

    string path = "Datei.csv";

    private void btnStart_Click(object sender,EventArgs e)
    {
      Cursor = Cursors.WaitCursor;
      FillFileWithTestdata(path);
      FileStream fs = File.Open(path,FileMode.Open,FileAccess.ReadWrite,FileShare.ReadWrite);
      StreamWriter sw = new StreamWriter(fs,Encoding.Default);
      StreamReader sr = new StreamReader(fs,Encoding.Default);
      sw.AutoFlush = true; long aktPos = 0; bool firstTime = true;
      long bufferLen = 0; int lenReturn = "\r\n".Length;

      while (aktPos < fs.Length)
      {
        string line = sr.ReadLine(); int lineLen = line.Length;
        if (firstTime)
        {
          bufferLen = sw.BaseStream.Position;
          sw.BaseStream.Position = 0;  firstTime = false;
        }
        int diff = (int)(bufferLen - aktPos);
        if (lineLen >= diff)
        {
          if (diff > 0) line = line.Substring(diff);
          bufferLen = sr.BaseStream.Position;
          sr.BaseStream.Position = aktPos;
        }
        line = Regex.Replace(line, "^.", "*");  
        sw.WriteLine(line);
        aktPos += (line.Length + lenReturn);
      }
      sw.Close();
      Cursor = Cursors.Default;
      grpNachher.Text = "Nachher (ein * wurde in allen Zeilen in Spalte1 'on the fly' ersetzt)";
      txbNachher.Text = File.ReadAllText(path);
    }

    /// <summary>Füllen von Beispiel-Daten in die Datei.</summary>
    private void FillFileWithTestdata(string path)
    {
      StreamWriter sw = new StreamWriter(path,false,Encoding.Default);
      sw.AutoFlush = true;
      int step = 5;
      int maxNumber = 3000;
      Random rnd = new Random(DateTime.Now.Millisecond);
      int minRandom = 1; int maxRandom = 10;
      int padding = rnd.Next(minRandom,maxRandom);

      for (int i = 1; i <= maxNumber; i += step)
      {
        for (int j = 0; j < step; j++)
        {
          sw.Write((i + j).ToString().PadLeft(rnd.Next(minRandom,maxRandom),'0'));
          if (j != step - 1) sw.Write(", ");
        }
        sw.WriteLine();
      }
      sw.Close();
      txbVorher.Text = File.ReadAllText(path);
    }
  }
}