tarik facturation form

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data.SqlClient;
using System.Data;
using System.Drawing;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;
using Microsoft.Office.Interop.Word;

namespace gestion_transport
{
    public partial class facturation : Form
    {
        SqlConnection conn = new SqlConnection("Data Source=ADMIN-PC;Initial Catalog=data_transport;Integrated Security=True");
        SqlCommand cmnd = new SqlCommand();
        SqlDataReader dataReader;
        double tarif;

        double montant;
        double tva;
        double ttc;
        //int fac_num;
        public facturation()
        {
            InitializeComponent();
        }

        private void facturation_Load(object sender, EventArgs e)
        {
            conn.Open();
            cmnd.CommandText = "Select DISTINCT societe from personne ";
            cmnd.Connection = conn;
            dataReader = cmnd.ExecuteReader();
            while (dataReader.Read())
            {
                cbx_societe.Items.Add(dataReader[0]);
            }
            dataReader.Close();
            conn.Close();


        }

        private void cbx_societe_SelectedIndexChanged(object sender, EventArgs e)
        {
            //combobox
            cbx_groupe.Items.Clear();
            conn.Open();
            cmnd.CommandText = "Select DISTINCT numgroupe from personne WHERE societe='" + cbx_societe.Text + "'";
            cmnd.Connection = conn;
            dataReader = cmnd.ExecuteReader();
            while (dataReader.Read())
            {
                if (dataReader[0] != null)
                    cbx_groupe.Items.Add(dataReader[0]);
            }
            dataReader.Close();
            conn.Close();



        }

        private void cbx_groupe_SelectedIndexChanged(object sender, EventArgs e)
        {
            //add grid
            dataPersonne.Rows.Clear();
            conn.Open();
            cmnd.CommandText = "Select * from personne where societe='" + cbx_societe.Text + "'and numgroupe="+int.Parse(cbx_groupe.Text);
            cmnd.Connection = conn;
            dataReader = cmnd.ExecuteReader();
            while (dataReader.Read())
            {
                dataPersonne.Rows.Add(dataReader[0], dataReader[1], dataReader[2], dataReader[3]);
            }
            dataReader.Close();
            conn.Close();


            conn.Open();
            cmnd.CommandText = "Select tarif from tbl_groupe WHERE ngroupe=" + int.Parse(cbx_groupe.Text)+"" ;
            cmnd.Connection = conn;
            dataReader = cmnd.ExecuteReader();
            while (dataReader.Read())
            {
                tarif = int.Parse(dataReader[0].ToString());
            }
            dataReader.Close();
            conn.Close();

             montant = tarif * (dataPersonne.RowCount-1);
             tva = montant / 5;
             ttc= montant + tva;

            label4.Text = montant.ToString() + "DH";
            label6.Text = tva.ToString() + "DH";
            label8.Text=ttc.ToString() + "DH";

        }

        private void button1_Click(object sender, EventArgs e)
        {
            int num = new_fac_num();
            conn.Open();
            cmnd.CommandText = "insert into facturess values("+ num + ",'" + cbx_societe.Text+"',"+montant+ "," +tva+ "," +ttc+")";
            cmnd.Connection = conn;
            cmnd.ExecuteNonQuery();
            conn.Close();
        }


        int new_fac_num()
        {
            List<int> num_fac = new List<int>();
            conn.Open();
            cmnd.CommandText = "Select num_fac from factures";
            cmnd.Connection = conn;
            dataReader = cmnd.ExecuteReader();
            while (dataReader.Read())
            {
                num_fac.Add(int.Parse(dataReader[0].ToString()));
            }
            dataReader.Close();
            conn.Close();

            int x = 0;
            Random rnd = new Random();
            bool exist = false;
            while (!exist)
            {
                x = rnd.Next(1, 10000);
                MessageBox.Show(x.ToString());
                if (!num_fac.Contains(x))
                {
                    exist = true;
                }
            }

            return x;
        }

    }
}