Access to MySQL after closing connection - mysql

Question

I have a problem with access to my datebase MySQL after closing it. I use Windows Form and MySql Connector from MySql official website to connect to datebase. I use MySQL in XAMPP.
If I open my connection I can do a simple query. But when I close a connection I can still do this query. Why?
Here is my code:
Imports MySql.Data.MySqlClient
Public Class Form1
Dim polaczenieMySql As New MySqlConnection
Dim mySQL As PolaczenieMySQL
Public Sub New()
InitializeComponent()
mySQL = New PolaczenieMySQL(Me)
End Sub
Private Sub btnPolaczMySQL_Click(sender As Object, e As EventArgs) Handles btnPolaczMySQL.Click
mySQL.Polacz()
End Sub
Private Sub RozlaczMySQL_Click(sender As Object, e As EventArgs) Handles btnRozlaczMySQL.Click
mySQL.Rozlacz()
End Sub
Private Sub btnZapytanie_Click(sender As Object, e As EventArgs) Handles btnZapytanieMySQL.Click
Dim zapytanie As String = "SELECT title, price, book_id from books"
mySQL.WykonajZapytanie(zapytanie)
End Sub
End Class
And my connection class:
Imports MySql.Data.MySqlClient
Public Class PolaczenieMySQL
Dim polaczenie As MySqlConnection = New MySqlConnection()
Dim glowneOkno As Form1
Public Sub New(form As Form1)
glowneOkno = form
End Sub
Public Sub Polacz()
polaczenie.ConnectionString = ("server=localhost;user id=root;password=;database=test")
Try
polaczenie.Open()
MessageBox.Show("Połączono z bazą danych MySQL.")
glowneOkno.labelPolaczenieMySQL.Text = "Połączono"
glowneOkno.labelPolaczenieMySQL.ForeColor = Color.Green
Catch myerror As MySqlException
MessageBox.Show("Nie udało się połączyć z bazą danych MySQL: " & myerror.Message)
End Try
End Sub
Public Sub Rozlacz()
Try
polaczenie.Close()
MessageBox.Show("Rozłączono z bazy danych MySQL.")
glowneOkno.labelPolaczenieMySQL.Text = "Nie połączono"
glowneOkno.labelPolaczenieMySQL.ForeColor = Color.Red
Catch myerror As MySqlException
MessageBox.Show("Błąd rozłączenia z bazy danych MySQL: " & myerror.Message)
Finally
polaczenie.Dispose()
End Try
End Sub
Public Sub WykonajZapytanie(ByVal zapytanie As String)
Dim SDA As New MySqlDataAdapter
Dim dbDataSet As New DataTable
Dim bSource As New BindingSource
Try
Dim komenda As MySqlCommand = New MySqlCommand(zapytanie, polaczenie)
SDA.SelectCommand = komenda
SDA.Fill(dbDataSet)
bSource.DataSource = dbDataSet
glowneOkno.DataGridViewMySQL.DataSource = bSource
SDA.Update(dbDataSet)
Catch ex As MySqlException
MessageBox.Show("Nie jesteś połączony z bazą")
End Try
End Sub
End Class
As I said before, after closing connection (mySQL.Rozlacz()) I can still do a query (mySQL.WykonajZapytanie(zapytanie)). What is wrong with this?
PS. Sorry for my polish names

Answer 1

If you look at the docs about the DbDataAdapter you could read this remark
The Fill method retrieves the data from the data source using a SELECT
statement. The IDbConnection object associated with the select command
must be valid, but it does not need to be open. If the IDbConnection
is closed before Fill is called, it is opened to retrieve data and
then closed. If the connection is open before Fill is called, it
remains open.
The MySqlDataAdapter inherits from the DbDataAdapter and there is no override of the Fill method, so it is the inherited class (DbDataAdapter) that works with the associated command, reopening the closed connection and then closing it.
EDIT
You asked also how to avoid this behavior, well you could add a boolean property to your class PolaczenieMySQL that maintains the state of your MySqlConnection
Private _isDead As Boolean
Public Property IsDead() As Boolean
Get
Return _isDead
End Get
Private Set(ByVal value As String)
_isDead = value
End Set
End Property
Now you could set this property to False when you open the connection and True when you close it.
Then check its value before executing the WykonajZapytanie sub
Public Sub WykonajZapytanie(ByVal zapytanie As String)
Dim SDA As New MySqlDataAdapter
Dim dbDataSet As New DataTable
Dim bSource As New BindingSource
if Me.IsDead Then
Throw new Exception("Invalid usage of this class")
End If
......
End Sub
Notice that there is a simpler check using the MySqlConnection property State
if polaczenie.State <> ConnectionState.Open Then
......
but, given the fact that in your Rozlacz method you dispose the connection I suggest to implement the first workaround.
Finally, I really think that you should change the whole pattern of usage. The connection should be opened, used and closed in the same method where it is required. Your actual infrastructure render this impossible. It is better to get rid of the Polacz and Rozlacz and use the mentioned pattern without the need of a global variable for the connection.