Problème C# & MySQL

    public partial class Fm_Saisie_Absence : Form
    {
        string server = "xxxx";
        string database = "gestion_absences";
        string user = "xxxx";
        string pwd = "xxxx";

        string pseudo_prof;

        public Fm_Saisie_Absence(string pseudo)
        {
            pseudo_prof = pseudo;
            InitializeComponent();
            RecupPromotions(server, database, user, pwd);


        }


        private void bt_valider_Click(object sender, EventArgs e)
        {

        }

        private void Visualiser_Absence_Click(object sender, EventArgs e)
        {
            Form Visualiser_Absence = new Visualiser_Absences(pseudo_prof);
            Visualiser_Absence.Show();
            this.Hide();
        }

        private void palmares_absences_Click(object sender, EventArgs e)
        {
            Form Palmarès_Absence = new Palmarès_Absence(pseudo_prof);
            Palmarès_Absence.Show();
            this.Hide();
        }

        private void RecupPromotions(string server, string database, string user,string pwd)
        {
            string connectionString;
            connectionString = "SERVER=" + server + ";" + "DATABASE=" +
            database + ";" + "UID=" + user + ";" + "PASSWORD=" + pwd + ";";
            MySqlConnection Cnx = new MySqlConnection(connectionString);

            //Execution de la requête
            string requete = "SELECT designation FROM promotions";
            MySqlCommand Cmd = new MySqlCommand(requete, Cnx);
            Cnx.Open();

            MySqlDataReader rdr = Cmd.ExecuteReader();

            while (rdr.Read())
            {
                CB_Promotion.Items.Add(rdr["designation"]);
            };

            Cnx.Close();
        }

        private void CB_Promotion_SelectedIndexChanged(object sender, EventArgs e)
        {
            CLB_Eleves.Items.Clear();
            string id_promotion = (CB_Promotion.SelectedIndex + 1).ToString();

            string connectionString;
            connectionString = "SERVER=" + server + ";" + "DATABASE=" +
            database + ";" + "UID=" + user + ";" + "PASSWORD=" + pwd + ";";
            MySqlConnection Cnx = new MySqlConnection(connectionString);

            //Execution de la requête
            string requete = "SELECT nom_etudiant, prenom_etudiant FROM etudiants WHERE id_promotion='" + id_promotion +"';" ;
            MySqlCommand Cmd = new MySqlCommand(requete, Cnx);
            Cnx.Open();

            MySqlDataReader rdr = Cmd.ExecuteReader();

            while (rdr.Read())
            {
                CLB_Eleves.Items.Add(rdr["nom_etudiant"].ToString() + " " + rdr["prenom_etudiant"].ToString());
            };
            Cnx.Close();

            RecupMatieres(server, database, user, pwd, pseudo_prof, id_promotion);

        }

        private void RecupMatieres(string server, string database, string user, string pwd, string pseudo, string id_promotion)
        {
            string connectionString;
            connectionString = "SERVER=" + server + ";" + "DATABASE=" +
            database + ";" + "UID=" + user + ";" + "PASSWORD=" + pwd + ";";
            MySqlConnection Cnx = new MySqlConnection(connectionString);

            //Execution de la requête
            string requete = "SELECT designation FROM matieres, enseignants, lien_matiere_enseignant WHERE enseignants.pseudo='" + pseudo +
                "' AND lien_matiere_enseignant.id_enseignant = enseignants.id_enseignant " +
                "AND lien_matiere_enseignant.id_matiere = matieres.id_matiere" +
                "AND lien_matiere_promotion.id_promotion='" + id_promotion + "';";
            MySqlCommand Cmd = new MySqlCommand(requete, Cnx);
            Cnx.Open();

            MySqlDataReader rdr = Cmd.ExecuteReader();

            while (rdr.Read())
            {
                CB_Matieres.Items.Add(rdr["designation"]);
            };

            Cnx.Close();
        }
    }