<?xml version="1.0" encoding="utf-8" ?>
<configuration>
  <connectionStrings>
    <add name="myconn" connectionString="Data Source=.;Initial Catalog=Northwind;
         Persist Security Info=True;User ID=sa;Password=110"/>
  </connectionStrings>
</configuration>

 private void button1_Click(object sender, EventArgs e)
        {
            string connstr = ConfigurationManager.ConnectionStrings["myconn"].ConnectionString;
            using (SqlConnection conn = new SqlConnection(connstr))
            {
                SqlCommand comm = new SqlCommand("select FirstName from Employees", conn);
                conn.Open();
                SqlDataReader sdr = comm.ExecuteReader();
                
                while (sdr.Read())
                {
                    listBox1.Items.Add(sdr[0].ToString());
                }
            }
        }