C# export large table data to multiple excel sheets

<pre>public void DumpTableToFile(SqlConnection connection, string tableName, string destinationFile)
        {
            using (var command = new SqlCommand("select * from tabl", connection))
            {
                using (var reader = command.ExecuteReader())
                {
                    using (StreamWriter sw = new StreamWriter(File.Open(destinationFile, FileMode.Create), Encoding.UTF8))
                    {
                        string[] columnNames = GetColumnNames(reader).ToArray();
                        int numFields = columnNames.Length;
                        sw.WriteLine(string.Join(",", columnNames)); // writing headers  

                        if (reader.HasRows)
                        {
                            while (reader.Read())
                            {
                                string[] columnValues = Enumerable.Range(0, numFields)
                                      .Select(i => reader.GetValue(i).ToString())
                                      .Select(field => string.Concat("\"", field.Replace("\"", "\"\""), "\""))
                                      .ToArray();
                                sw.WriteLine(string.Join(",", columnValues));
                            }
                        }
                    }
                }
            }
        }

        private IEnumerable<string> GetColumnNames(IDataReader reader)
        {
            foreach (DataRow row in reader.GetSchemaTable().Rows)
            {
                yield return (string)row["ColumnName"];
            }
        }

        private void Button5_Click(object sender, EventArgs e)
        {
            SqlConnection _connection = new SqlConnection();
            SqlDataAdapter _dataAdapter = new SqlDataAdapter();
            SqlCommand _command = new SqlCommand();
            DataTable _dataTable = new DataTable();

            _connection = new SqlConnection();
            _dataAdapter = new SqlDataAdapter();
            _command = new SqlCommand();
            _dataTable = new DataTable();

            //dbk is my database name that you can change it to your database name
            _connection.ConnectionString = "connaction";
            _connection.Open();

            SaveFileDialog saveFileDialogCSV = new SaveFileDialog();
            saveFileDialogCSV.InitialDirectory = Application.ExecutablePath.ToString();

            saveFileDialogCSV.Filter = "CSV files (*.csv)|*.csv|All files (*.*)|*.*";
            saveFileDialogCSV.FilterIndex = 1;
            saveFileDialogCSV.RestoreDirectory = true;

            string path_csv = "";
            if (saveFileDialogCSV.ShowDialog() == DialogResult.OK)
            {
               
                // Runs the export operation if the given filenam is valid.
                path_csv = saveFileDialogCSV.FileName.ToString();
            }

           
            DumpTableToFile(_connection, "tbl_trmc", path_csv);

        }
    }