Modifier un fichier

FileInfo fl=new FileInfo(@"D:\lama.txt");
if(!(fl.exist))
{
fl.create();
}
StreamWrite wr=new SreamWrite(@"D:\lama.txt");
wr.WriteLine(TextBox1.Text);
wr.close();

using System.IO;

if(!File.Exists(@"D:\lama.txt")) {
   File.Create(@"D:\lama.txt");
}
else
{
   StreamReader rd = new StreamReader(@"D:\lama.txt");
   string tx1 = rd.RealAllText();
   rd.Close();
   StreamWriter wr = new StreamWriter(@"D:\lama.txt");
   wr.WriteLine(tx1);
wr.Close();
}

using(FileStream fs = new FileStream(@"D:\lama.txt", FileMode.Append))
{
  // TO DO: écrire...

  fs.Close();
}

using(FileStream fs = new FileStream(@"D:\lama.txt", FileMode.Append))
{
  using (StreamWriter sw = new StreamWriter(fs))
  {
    // TO DO: écrire...
    sw.Close();
  }

  fs.Close();
}