MainForm.cs

//Подключить namespace
using System.Data.OleDb;

//Добавить поля в класс
public static string connectString = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=../../../Exam.mdb;";
private OleDbConnection myConnection;

//Конструктор формы
public MainForm()
{
  InitializeComponent();
  //Новое соединение
  myConnection = new OleDbConnection(connectString);
  myConnection.Open();
}


//SQL-запросы и Access
//Один элемент
private void OneElementButton_Click(object sender, EventArgs e)
        {
            string query = "SELECT user FROM data WHERE Код=1";
            OleDbCommand command = new OleDbCommand(query, myConnection);
            textBox1.Text = command.ExecuteScalar().ToString();
        }
        
//Несколько элементов
 private void MultiElementsButton_Click(object sender, EventArgs e)
        {
            string query = "SELECT * FROM data WHERE user=2";
            OleDbCommand command = new OleDbCommand(query, myConnection);
            OleDbDataReader reader = command.ExecuteReader();
            listBox1.Items.Clear();
            while (reader.Read())
                listBox1.Items.Add(reader[0].ToString() + " " + reader[1].ToString() + " " + reader[2].ToString() + " ");
            reader.Close();
        }
        
//Для команд без возврата значения (INSERT/DELETE)
private void InsertElementButton_Click(object sender, EventArgs e)
        {
            string query = "INSERT INTO data (user, active, passive) VALUES ("+UserTextBox.Text+", '"+ActiveTextBox.Text+"', '"+PassiveTextBox.Text+"')";
            OleDbCommand command = new OleDbCommand(query, myConnection);
            command.ExecuteNonQuery();
        }