Использование транзакций для обеспечения безопасности параллелизма в работе с базой данных

Private Sub TryCommand(ByVal cmd As String) 
  Dim connectionString As String = "Data Source=.\SQLExpress;Initial Catalog=AdventureWorks;" + _
                                   "Integrated Security=True;" 
  Using connection As New SqlConnection(connectionString)
  connection.Open()
 
  Dim command As SqlCommand = connection.CreateCommand() 
  Dim transaction As SqlTransaction
  Dim iso As IsolationLevel = IsolationLevel.RepeatableRead
  "Запускаем локальную транзакцию и определяем уровень изоляции 
  transaction = connection.BeginTransaction(iso)
  "Приписываем объекты transaction и connection 
  "к объектам Command 
  command.Connection = connection 
  command.Transaction = transaction
  Try
    command.CommandText = cmd 
    command.ExecuteNonQuery()
    " Пытаемся зафиксировать транзакцию. 
    transaction.Commit() 
    Console.WriteLine("Transaction succeeded.")
  Catch ex As Exception
    Console.WriteLine("Commit Exception Type: {0}", ex.GetType()) 
    Console.WriteLine(" Message: {0}", ex.Message)
    " Пытаемся выполнить откат транзакции. 
    Try
      transaction.Rollback()
    Catch ex2 As Exception
      "Если соединение уже было закрыто, 
      "откат больше не работает
      Console.WriteLine("Rollback Exception Type: {0}", ex2.GetType()) 
      Console.WriteLine(" Message: {0}", ex2.Message) 
    End Try 
  End Try 
  End Using
End Sub

Private Sub btnTryInvalidCmd_Click(ByVal sender As System.Object, _
                                   ByVal e As System.EventArgs) 
  Handles btnSetInvalidCmd.Click 
  TryCommand("This_is_an_invalid_sql_command") 
End Sub

Private Sub btnTryValidCmd_Click(ByVal sender As System.Object, _ 
                                 ByVal e As System.EventArgs) 
  Handles btnSetValidCmd.Click 
  TryCommand("SELECT * FROM dbo.orders")
End Sub