C#

public partial class Form1 : Form
    { 

        public Form1()
        {
            InitializeComponent();

        }

 ConnectionDB d = new ConnectionDB();

    using (OleDbCommand cmbox = d.sql_con.CreateCommand())
                {
                    cmbox.CommandText = "SELECT Code_article FROM Catalogue";
                    d.CONNECTER();// i have an error here
                    d.DR = cmbox.ExecuteReader();

                    try
                    {
                        while (d.DR.Read())
                        {
                            CmbRef_Produit.Items.Add(d.DR["Code_article"]).ToString();
                        }

                        //si le textbox est vide
                        if (string.IsNullOrWhiteSpace(CmbRef_Produit.Text ))
                        {
                            // Message box

                            Lbl_affich_TxtQteStock.Text = "";
                            Lbl_Affich_Designation.Text = "";
                            Lbl_Affich_PrixUnitaire.Text = "";
                        }

                    }
                    catch (Exception ex)
                    {
                        MessageBox.Show(ex.ToString());

                        d.DECONNECTER();

                    }
                }

            }

}

using (ConnectionDB d = new ConnectionDB())
{
    d.CONNECTER();
    
    using (OleDbCommand cmbox = d.sql_con.CreateCommand())
    {
        cmbox.CommandText = "SELECT Code_article FROM Catalogue";

        // remaining code
    }
    d.DECONNECTER();
}

Thanks i will try it and you will get back

I try and the error comes from me thank you very much mmister it works.

using (SqlConnection con = new SqlConnection(strConnect))
    {
    con.Open();
    using (SqlCommand cmd = new SqlCommand("SELECT Age, Description FROM myTable WHERE ID = @ID", con))
        {
        cmd.Parameters.AddWithValue("@ID", myTextBox.Text);
        using (SqlDataReader reader = cmd.ExecuteReader())
            {
            while (reader.Read())
                {
                int age = (int) reader["Age"];
                string desc = (string) reader["Description"];
                Console.WriteLine($"{age}\n{desc}");
                }
            }
        }
    }

thank you sir message well received. I haven't tried the configuration file side for the connection to the database yet, but I will study it and see if I can manage it.

public class ConnectionDB
    {

        // Declaration des objets sql
        public OleDbConnection sql_con;
        public OleDbCommand sql_cmd;
        public OleDbDataAdapter DB;
        public DataSet DS = new DataSet();
        public DataTable DT = new DataTable();
        public OleDbDataReader DR;
        public string connetionString = null;

        // declaration de la methode connecter
        public void CONNECTER()
        {                           
                //connection à la base de donnée avec mot de passe
                connetionString = @"Provider=Microsoft.Jet.OLEDB.4.0;Data source=" + Application.StartupPath + @"\DB_CaisseEnregistreuse.mdb;Persist Security Info=True;Jet OLEDB:Database Password=B@sta08091987";
                sql_con = new OleDbConnection(connetionString);
        }

        // declaration de la methode deconnecter
        public void DECONNECTER()
        {
            

                if (sql_con.State == ConnectionState.Open)
                {

                    sql_con.Close();
                }
            
           
        }

 public partial class Form1 : Form
   {

       public Form1()
       {
           InitializeComponent();



       }
       ConnectionDB d = new ConnectionDB();

private void LoadDB()
       {
           using (OleDbConnection sql_con = new OleDbConnection(d.connetionString))
           {
               sql_con.Open();
               using (OleDbCommand sql_cmd = d.sql_con.CreateCommand())
               {
                   string CommandText = "SELECT * FROM Detail_temp";
                   d.DB = new OleDbDataAdapter(CommandText, d.sql_con);
                   d.DS.Reset();
                   d.DB.Fill(d.DS);
                   d.DT = d.DS.Tables[0];
                   dataGridView1.DataSource = d.DT;
                   d.DECONNECTER();
               }
           }
       }


private void Form1_Load(object sender, EventArgs e)
       {

           try
           {

               LoadDB();
               d.sql_con.Open();
               TxtQteCmd.Text = "1";
               Lbl_Affich_TotalCmd.Text = "0";
               LblAfficDate.Text = DateTime.Now.ToString();

               using (OleDbCommand cmbox = d.sql_con.CreateCommand())
               {
                   cmbox.CommandText = "SELECT Code_article FROM Catalogue";
                   //d.sql_con.Open();
                   using (d.DR = cmbox.ExecuteReader())
                   {

                       try
                       {
                           while (d.DR.Read())
                           {
                               CmbRef_Produit.Items.Add(d.DR["Code_article"]).ToString();
                           }

                           //si le textbox est vide
                           if (string.IsNullOrWhiteSpace(CmbRef_Produit.Text))
                           {
                               // Message box

                               Lbl_affich_TxtQteStock.Text = "";
                               Lbl_Affich_Designation.Text = "";
                               Lbl_Affich_PrixUnitaire.Text = "";
                           }

                       }
                       catch (Exception ex)
                       {
                           MessageBox.Show(ex.ToString());

                           d.DECONNECTER();

                       }
                       d.DECONNECTER();
                   }
               }

           }
           catch (Exception ex)
           {
               MessageBox.Show(ex.ToString());
           }

       }

Sir I tried to do as you suggested and I would like you to take a look at my code, just see if it's good. And I would also like to know if the DISCONNECT method is worth it