public partial class Form1 : Form
    {
        private string _connectionString =
        "Data Source=localhost;Initial Catalog=Test;" +
        "Integrated Security=SSPI;";

        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            SqlDependency.Start(_connectionString);
            SubscribeForChanges(_connectionString);
        }

        private void SubscribeForChanges(string connectionString)
        {
            using (SqlConnection conn = new SqlConnection(connectionString))
            {
                conn.Open();
                SqlCommand cmd = new SqlCommand("SELECT ProductName FROM Products", conn);
                SqlDependency dependency = new SqlDependency(cmd);
                dependency.OnChange += new OnChangeEventHandler(dependency_OnChange);
                cmd.ExecuteNonQuery();
            }
        }

        private void dependency_OnChange(object sender, SqlNotificationEventArgs e)
        {
            MessageBox.Show("Aaa!");
            //SubscribeForChanges(_connectionString);
        }
    }