c# - Write to text file - appending new text ot the top of the file

try
      {
      FileStream fStream = File.Open("C:\\test.txt" , FileMode.Open  );
        if(fStream.CanWrite)
        {
        string s = textBox1.Text;
           fStream.Seek(0 , SeekOrigin.Begin);
          byte[] bytes = new byte[100];
          bytes = System.Text.ASCIIEncoding.ASCII.GetBytes(s);
             
           fStream .Write (bytes,0,bytes .Length );
 
          
        }
        fStream.Flush();
        fStream.Close();
        
      }
      catch(IOException ex)
      {
        Console.WriteLine(ex.Message);
      }

                                              

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    string oldPath = 'old.txt';
    string tmpPath = 'temp.txt';
    string newText = 'This is the new text.';
 
    // Write the new text in a temporary file:
    File.WriteAllText(tmpPath, newText);
 
    // Open the temporary file to append and the
    // old file for reading.
    StreamWriter tmpFile = File.AppendText(tmpPath);
    StreamReader oldFile = File.OpenText(oldPath);
    try
        {
            // Write all lines of oldFile to tmpFile.
            string line = "";
            while ((line = oldFile.ReadLine()) != null)
            {
                tmpFile.WriteLine(line);
            }
        }
    finally
        {
        oldFile.Dispose();
        tmpFile.Dispose();
        }
 
    // Rename the temporary file to overwrite the old one
    File.Move(tmpFile, oldFile);
                                              

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    // Obviously, these will come from other parts of your program:
    string oldFile = 'old.txt';
    string tmpFile = 'temp.txt';
    string newText = 'This is the new text.';
 
    // Open the tmpFile for appending and the oldFile for reading.
    // If tmpFile does not exist (and it shouldn't!), it is
    // created.  Alternatively, you may want to call File.Delete()
    // first to make sure the tmpFile does not exist.
    StreamWriter tmpFile = File.AppendText(tmpFile);
    StreamReader oldFile = File.OpenText(oldFile);
    try
        {
            // Write the new text to tmpFile.
            File.WriteLine(newText);
 
            // Write all lines of oldFile to tmpFile.
            string line = "";
            while ((line = oldFile.ReadLine()) != null)
            {
                tmpFile.WriteLine(line);
            }
        }
    finally
        {
        oldFile.Dispose();
        tmpFile.Dispose();
        }
 
    // Rename the temporary file to overwrite the old one
    File.Move(tmpFile, oldFile);
                                              

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