[ADO.NET] DataSet & SqlDataAdapter

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

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
        private string connectionString = "Server=VAKHO-PC; Database=TestDB; Integrated Security=true;";
        private SqlConnection con;
        private SqlDataAdapter adapter;
        private DataSet dataSet = null;

        public Form1()
        {
            InitializeComponent();
        }

        private void refreshTable()
        {
            this.dataSet = new DataSet("users");

            adapter.Fill(dataSet, "users");
            dataGridView1.DataSource = dataSet.Tables[0];
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            this.con = new SqlConnection(this.connectionString);
            this.adapter = new SqlDataAdapter("SELECT * FROM dbo.Users", con);

            this.refreshTable();

            try
            {
                con.Open();
            }
            catch (SqlException ex)
            {
                MessageBox.Show(ex.Message, "Sql Exception!", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
            }
            finally
            {
                MessageBox.Show(con.State.ToString(), "Sql Exception!", MessageBoxButtons.OK, MessageBoxIcon.Asterisk);
                con.Close();
            }
        }

        private void button1_Click(object sender, EventArgs e)
        {
            this.refreshTable();
        }
    }
}