C# repeating click issues

I have a buttonclick that take values from excel and compare it whith a sql data and check if "numnat" is already exist in database, it work fine the first time when i select a file that contanins an already exist value but when i change the file select a file that not contanins an already exist value and click second time it give me the result of the first click.that's my problem. buttonclick code is like this:

private void button2_Click(object sender, EventArgs e)
{
    string excelstringconn = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=" + textselect.Text + ";Extended Properties=\"Excel 8.0;HDR=Yes;\";";
    OleDbConnection conn = new OleDbConnection(excelstringconn);
    bool exist = false;

    if (textselect.Text != "")
    {
        if (textchoice.Text != "")
        {
            try
            {
                Microsoft.Office.Interop.Excel.Application excel = new Microsoft.Office.Interop.Excel.Application();
                Workbook workbook = excel.Workbooks.Open(textselect.Text);
                SqlConnection cnn;
                string connectionString = null;
                string sql = null;

                foreach (Worksheet sheet in workbook.Sheets)
                {
                    if (sheet.Name.Equals(textchoice.Text))
                    {
                        exist = true;
                    }
                }

                if (exist == true)
                {
                    OleDbDataAdapter da = new OleDbDataAdapter("Select * from [" + textchoice.Text + "$]", conn);
                    string numnat = "";
                    string numi = "";
                    da.Fill(dataSet1);

                    connectionString = @"data source=DESKTOP-NN4Q7KD\MSQLSERVER;initial catalog=et;integrated security=true;";
                    cnn = new SqlConnection(connectionString);
                    cnn.Open();
                    sql = "SELECT Num_National FROM Note_et where Num_National=@num";
                    SqlDataAdapter DA = new SqlDataAdapter();
                    DA.SelectCommand = new SqlCommand(sql, cnn);
                    bool exist2 = false;

                    for (int i = 0; i <= dataSet1.Tables[0].Rows.Count - 1; i++)
                    {
                        exist2 = false;
                        numnat = dataSet1.Tables[0].Rows[i].ItemArray[3].ToString();
                        DA.SelectCommand.Parameters.AddWithValue("@num", numnat);
                        SqlDataReader dr = DA.SelectCommand.ExecuteReader();

                        if (dr.HasRows == true)
                        {
                            exist2 = true;
                            numi = numnat;
                            break;
                        }

                        dr.Close();
                    }


                    if (exist2 == true)
                    {
                        MessageBox.Show("User" + numi + "déja exist");
                    }
                    else
                    {
                        dataGridView1.DataSource = dataSet1.Tables[0];
                    }
                }
                else
                    MessageBox.Show("entrez une sheet valide");
            }
            catch
            {
                MessageBox.Show("Entrez un fichier valide");
            }
        }
        else
            MessageBox.Show("champs vide");
    }
    else
        MessageBox.Show("Selectioner un fichier");
}

btw sorry for my bad english, hope you give asolution for my problem tnx.