C#

[Serializable]
public class ParticipantList : List<Participant>
{
  private Guid _ID = Guid.NewGuid();
  public Guid ID
  {
    get { return _ID; }
    set { _ID = value; }
  }

  public static void ToFile(ParticipantList list, string filename)
  {
    using (FileStream fs = new FileStream(filename, FileMode.Create, FileAccess.Write))
    {
      XmlSerializer xs = new XmlSerializer(typeof(ParticipantList));
      xs.Serialize(fs, list);
    }
  }

  public static ParticipantList FromFile(string filename)
  {
    using (FileStream fs = new FileStream(filename, FileMode.Open, FileAccess.Read, FileShare.ReadWrite))
    {
      XmlSerializer xs = new XmlSerializer(typeof(ParticipantList));
      return (ParticipantList)xs.Deserialize(fs);
    }
  }
}

void Cmd_Login_Click(object sender, EventArgs e)
        {

            string host = "localhost";
            string database = "database";
            string user = "username";
            string password = "password";

            string strSQL = "SELECT * FROM users";

            string strProvider = "Data Source=" + host + 
                                ";Database=" + database + 
                                ";User ID=" + user + 
                                ";Password=" + password;
            try
            {
                MySqlConnection mysqlCon = new MySqlConnection(strProvider);
                mysqlCon.Open();

                if (mysqlCon.State.ToString() == "Open")
                {
                    MySqlCommand mysqlCmd = new MySqlCommand(strSQL, mysqlCon);
                    MySqlDataReader mysqlReader = mysqlCmd.ExecuteReader();

                    while (mysqlReader.Read())
                    {
                        if (Txt_Name.Text == mysqlReader.GetString(1) && Txt_Code.Text == mysqlReader.GetString(2))
                        {

                            <big></big>//POINT TO INSERT QUERY TO EXECUTE

                            if (checkBox1.Checked )
                            {
                                Frm_Main Frm_Main = new Frm_Main();
                                Frm_Main.Close();

                                Form1 form1 = new Form1();
                                form1.ShowDialog();
                            }
                            else
                            {
                                Pan_Step1.Visible = false;
                                Closer.Start();
                            }
                        }
                    }
                }
                    else 
                    {
                        System.Media.SystemSounds.Hand.Play();
                    }

                mysqlCon.Close();

            }
            catch (Exception er)
            {
                Console.WriteLine("Errore " + er.Message);
            }
        }

strSQL = "UPDATE users SET access='" + DateTime.Now.ToString() + "', admin='" + checkBox1.CheckState + "' WHERE username='" + mysqlReader.GetString(1) + "'";

MySqlConnection mysqlCon = new MySqlConnection(strProvider);
mysqlCon.Open();
MySqlCommand mysqlCon = new MySqlCommand(strSQL); 
mysqlCon.ExecuteNonQuery();