public class DbCommandExample
{
    public void Execute(string connectionString)
    {
        using (var conn = new SqlConnection(connectionString))
        {
            var rnd = new Random();
            var trx = $"transaction#{rnd.Next()}";

            conn.Open();

            using (var trans = conn.BeginTransaction(IsolationLevel.Serializable, trx))
            {
                using (var cmd = new SqlCommand("Select * from Sales.Invoices", conn, trans))
                {
                    cmd.ExecuteNonQuery();
                }

                trans.Commit();
            }
        }
    }
}