c#中byte[]和string的轉換

Using System.Text;
byte[ ] 轉換為string
byte[ ] image;
string ll = Encoding.Default.GetString(image);
string 轉換為byte[ ]
string ss;
byte[] b = Encoding.Default.GetBytes(ss);
資料庫中image類型的欄位的處理。
首先我想從資料庫中讀出圖片（以image類型儲存的), 並且寫入txt檔案中：
private void GetImage()
  {
    string conn = "Server=192.168.0.11; User id=user; Pwd=pwd; Database=database";
    SqlConnection sqlCon = new SqlConnection(conn);
    string sql = "SELECT ImageFile, PersonID FROM Persons where PersonID = 1";
    SqlCommand myCommand = new SqlCommand();
    myCommand.Connection = sqlCon;
    myCommand.CommandType = CommandType.Text;
    myCommand.CommandText = sql;
    DataTable myTable = new DataTable();
    SqlDataAdapter myDataAda = new SqlDataAdapter();
    myDataAda.SelectCommand = myCommand;
    try
    {
      sqlCon.Open();
      myDataAda.Fill(myTable);
      sqlCon.Close();
      if(myTable.Rows.Count>0)
      {
        byte[] image = (byte[])myTable.Rows[0]["ImageFile"];
        string ll = Encoding.Default.GetString(image);
        WriteStr(@"F:\test.txt",ll);
      }
    }
    catch(Exception ex)
    {
      string err = ex.Message;
    }
    finally
    {
      sqlCon.Close();
      myCommand.Dispose();
      myDataAda.Dispose();
    }
  }
  private void WriteStr(string strLogFileName, string strLogContent)
  {
    try
    {
      FileInfo objFileInfo = new FileInfo(strLogFileName);
      using (FileStream objFileStream = objFileInfo.OpenWrite())
      {
        using (StreamWriter objStreamWriter = new StreamWriter(objFileStream))
        {
          objStreamWriter.BaseStream.Seek(0, SeekOrigin.End);
          objStreamWriter.Write("{0}", strLogContent);
        }
      }
    }
    catch
    {
    }
  }
其次，需要把圖片從txt檔案中讀出然後存入資料庫中。
string ss = ReadStr(@"F:\test.txt");
    byte[] b = Encoding.Default.GetBytes(ss);
    string conn = "Server=server; User id=user; Pwd=pwd; Database=database";
    SqlConnection sqlCon = new SqlConnection(conn);
    string sql = "update Persons set ImageFile=@img where PersonID = 1";
    SqlCommand myCommand = new SqlCommand();
    SqlParameter sp = new SqlParameter("@img",SqlDbType.Image);
    myCommand.Connection = sqlCon;
    myCommand.CommandType = CommandType.Text;
    myCommand.CommandText = sql;
    sp.Value = b;
    myCommand.Parameters.Add(sp);
    try
    {
      sqlCon.Open();
      myCommand.ExecuteNonQuery();
      sqlCon.Close();
    }
    catch(Exception eS)
    {
      string ee = eS.Message;
    }
    finally
    {
      sqlCon.Close();
      myCommand.Dispose();
    }