I have created a SQLite database file, by migrating the table schema, rows, etc from a SQL server instance on Azure.
I created a util class to populate a DataSet object using a SQLiteDataAdapter object connected the the SQLite database, with a (Select * FROM Users) statement.
For some reason, I can execute a SQLite Reader just fine with the SQLiteCommand, and it returns the queried rows, which I print to the console.
However whenever I try to execute an adapter.Fill(), it appears that nothing happens, and all code that should be executed afterwards(not dependent on anything done by the adapter itself), just ceases to do anything. There is NO exception raised, it simply stops...
Imports System.Data
Imports System.Data.SQLite
Imports System.Threading.Tasks
Public Class SqliteInit
Public Function getUserDataSet()
Dim connectionString As String = "Data Source=users1.db;Version=3;"
Dim mSQL As String = "SELECT * FROM UserProfile;"
Dim dataTable As New DataTable
Dim dataSet As New DataSet
dataSet.Tables.Add("UserProfile")
Try
If My.Computer.Network.IsAvailable Then
MsgBox(" Computer is connected to the internet")
Else
MsgBox(" Computer is NOT connected")
Dim SQLConn As New SQLite.SQLiteConnection(connectionString)
Dim SQLComm As New SQLite.SQLiteCommand(mSQL, SQLConn)
Dim adapter As New SQLiteDataAdapter(SQLComm)
SQLConn.Open()
adapter.Fill(dataSet, "UserProfile")
SQLConn.Close()
Console.WriteLine("Empty")
Return CType(dataSet, AzureDataSet)
End If
Catch ex As Exception
Return Nothing
End Try
Return Nothing
End Function
End Class
Every other execution option seems to work with those defined objects, for example, I tested if I was accessing the database correctly by turning the statement into an insert, and sure enough when I executed the SqlCommand.NonQuery, it added the row. I also used a SQLiteReader with the Select statement, and it successfully printed the rows.
adapter.Fill seems to be the only thing breaking.
Related
I'm using simple DataReader commands very often in my project.
To simplify it, I've created a function:
Public Function DataReaderFromCommand(ByRef uCn As SQLite.SQLiteConnection, ByVal uCommandText As String) As SQLite.SQLiteDataReader
Dim nCmdSel As SQLite.SQLiteCommand = uCn.CreateCommand
With nCmdSel
.CommandText = uCommandText
End With
Dim r As SQLite.SQLiteDataReader = nCmdSel.ExecuteReader
Return r
End Function
In my project I use it like this:
Using r As SQLite.SQLiteDataReader = DataReaderFromCommand(cnUser, "SELECT * FROM settings")
Do While r.Read
'do something
Loop
End Using'this should close the DataReader
In one case, I need to delete my database. However this fails with the error "File is locked by another process".
I tried to isolate the problem, and the locking occurs because of the function "DataReaderFromCommand".
Does anybody see what I'm doing wrong / what keeps the database locked?
I thought that after "End Using" of the datareader, the SQLiteCommand would also be disposed, so there are no further reference to the database.
You should probably be trying to do it this way:
Public Sub UsingDataReader(ByVal connectionString As String, ByVal commandText As String, ByVal action As Action(Of SQLite.SQLiteDataReader))
Using connection As New SQLite.SQLiteConnection(connectionString)
Using command As New SQLite.SQLiteCommand(commandText, connection)
Using reader = command.ExecuteReader()
action(reader)
End Using
End Using
End Using
End Sub
Then you can call the code like this:
UsingDataReader("/* your connection string here */", "SELECT * FROM settings", _
Sub (r)
Do While r.Read
'do something
Loop
End Sub)
This ensures that all of the disposable references are closed when the Sub has completed.
The first problem is that not all the disposables are being disposed of. We are assured that the connection passed to that helper is in a Using block, but the command also needs to be disposed of as it has a reference to the connection:
Dim cmd As New SQLiteCommand(sql, dbcon)
Even if you dont use the overloaded constructor, in order to work, somewhere you set the connection property. This illustrates one of the problems with such "DB helper" methods: the DBConnection, DBCommand and DBReader objects work together very closely, but they are created in different methods with different scopes and you can't normally see if everything is being cleaned up properly.
The code posted will always fail because that DBCommand object - and by extension the DBConnection - are not disposed. But even if you clean up properly, pooling will keep the DBConnection alive for a while as jmcilhinney explains. Here are 2 fixes:
Clear the Pool
Using dbcon As New SQLiteConnection(LiteConnStr),
cmd As New SQLiteCommand(sql, dbcon)
dbcon.Open()
Dim n As Int32 = 0
Using rdr = cmd.ExecuteReader
While rdr.Read
' == DoSomething()
Console.WriteLine("{0} == {1}", n, rdr.GetString(0))
n += 1
End While
End Using
' Clears the connection pool associated with the connection.
' Any other active connections using the same database file will be
' discarded instead of returned to the pool when they are closed.
SQLiteConnection.ClearPool(dbcon)
End Using
File.Delete(sqlFile)
The dbCon and cmd objects are "stacked" into one Using statement to reduce indentation.
This will close and discard any and all connections in the pool, provided they have been Disposed - as well as any objects which reference them. If you use Dim cmd ... you will need to explicitly dispose of it.
Force Garbage Collection
I think this is much more ham-fisted, but it is included for completeness.
Using dbcon As New SQLiteConnection(LiteConnStr),
cmd As New SQLiteCommand(sql, dbcon)
...
Using rdr = cmd.ExecuteReader
...
End Using
End Using
GC.WaitForPendingFinalizers()
File.Delete(sqlFile)
This also works as long as everything has been properly disposed of. I prefer not to mess with GC unless absolutely necessary. The issue here is that clean up will not be limited to DBProvider objects but anything which has been disposed and is awaiting GC.
Yet a third workaround would be to turn off pooling, but you would still have to dispose of everything.
You are going to need to also close your cnUser connection to the database.
Closing/disposing the reader does not necessarily close/dispose the open connection.
I'm attempting to overload the "Delete" method of a TableAdapter (approach). How can I execute an SQL statement from 'here' to handle the delete?
I've got:
Namespace AFL_BackendDataSetTableAdapters
Partial Class Log_entry_unitTableAdapter
Public Overloads Function Delete(ByVal LogEntryID As Integer) As Integer
Dim SQL As String
SQL = "DELETE FROM log_entry_unit WHERE log_entry_unit_id=" & LogEntryID
'?????.Execute SQL
Return 0
End Function
End Class
End Namespace
The overload is working fine, but I don't know how to do the hard part and actually manipulate the data from here. Previously, I've just gone into the Dataset Designer and manually updated the generated methods to work like I want them, but whenever I use the wizard to regenerate the dataset, that (as expected) gets overwritten.
I've previously only ever manipulated Data using the generated methods, and now I'm stuck.
EDIT w/ Final Answer
Based on William's help below here's the final working solution (Note I just had to use OleDb instead of SQL since my Dataset is Access:
Imports System.Data.OleDb
Namespace AFL_BackendDataSetTableAdapters
Partial Class Log_entry_unitTableAdapter
Public Overloads Function Delete(ByVal LogEntryID As Integer) As Integer
Dim queryString As String = "DELETE FROM log_entry_unit WHERE log_entry_unit_id=" & LogEntryID
Dim command As New OleDbCommand(queryString, Connection)
Dim r As Integer
Try
Connection.Open()
r = command.ExecuteNonQuery()
Catch ex As Exception
MsgBox(ex.Message)
r = 0
Finally
Connection.Close()
End Try
Return r
End Function
End Class
End Namespace
I hardcoded a connection string for reference only. This should be in a config file. As an example:
Dim connectionString As String = _
"Data Source=(local);Initial Catalog=YourDatabase;" _
& "Integrated Security=true"
Dim queryString As String = "DELETE FROM log_entry_unit WHERE log_entry_unit_id=" & LogEntryID
' Create and open the connection in a using block. This
' ensures that all resources will be closed and disposed
' when the code exits.
Using connection As New SqlConnection(connectionString)
' Create the Command
Dim command As New SqlCommand(queryString, connection)
' Open the connection in a try/catch block.
Try
connection.Open()
command.ExecuteNonQuery()
Catch ex As Exception
' handle exception here
End Try
End Using
EDIT
I probably should of mentioned you will probably want to fill your adapter again after the delete.
In my application I create a function that allow the user to change the settings of the app. This settings are stored into a table 'cause there's a lot of records. Anyway, the problem's that if the settings isn't valorized yet, when the application start and load the settings from the table take of course a null GUID field and the message:
GUID format not recognized
appear. A code explaination:
Sub LoadSettings()
Using dbCon As MySqlConnection = establishConnection()
Try
dbCon.Open()
Dim MysqlCommand = New MySqlCommand("SELECT * FROM settings", dbCon)
Dim reader = MysqlCommand.ExecuteReader
For Each row In reader
Select Case row(2)
Case "company_name"
Setting.name.Text = row(3)
Case "company_email"
Setting.email.Text = row(3)
...
End Select
Next
End Sub
This function is called when the settings form is opened. If the settings aren't inserted yet, I get a message of bad format. I want to know how I can avoid this message.
You are not using the DataReader correctly. Consider this code:
Dim reader = MysqlCommand.ExecuteReader
For Each row In reader
... something
Next
MysqlCommand.ExecuteReader returns a DataReader object, but it is not - nor does it contain - a row collection you can iterate. If you hold the mouse over row you should see that it is a Data.Common.DataRecordInternal object which does have an Item property but a reference like row(2) will only compile with Option Strict Off.
Used correctly, when you Read a row the data in that internal object is available via the indexer (Item) and the various Getxxxxx() methods. This just prints the Id and Name from a table in a loop. I cant quite tell what you are trying to do with your results...it sort of looks like a Name/Value pair type thing maybe.
Dim SQL = "SELECT * FROM Demo"
Using dbcon = GetMySQLConnection(),
cmd As MySqlCommand = New MySqlCommand(SQL, dbcon)
dbcon.Open()
Using rdr As MySqlDataReader = cmd.ExecuteReader
If rdr.HasRows Then
Do While rdr.Read()
Console.WriteLine("{0} - {1}", rdr("Id").ToString, rdr("Name").ToString)
Loop
End If
End Using ' dispose of reader
End Using ' dispose of Connection AND command object
Alternatively, you could fill a DataTable and iterate the rows in that. Seems 6:5 and pick-em whether that would gain anything.
Note also that the Connection, Command and DataReader objects are properly disposed of when we are done using them.
Can you please tell me what's wrong with this code?
Do I need to use DataAdapter to insert into a table?
I know the connectionString is ok, because I tested it on the Server Explorer.
Dim mydao As New Connection
Dim connectionString As String = mydao.GetConnectionString()
Dim connection As New SqlConnection(connectionString)
Dim cmd As New SqlCommand
Public Function add(ByVal area As String, ByVal user As String) As Integer
cmd.CommandText = "INSERT into Area (Area, user) VALUES ('" + area + "','" + user + "')"
Try
connection.Open()
Dim cant As Integer = cmd.ExecuteNonQuery()'it throws exception here
connection.Close()
Return cant
Catch ex As Exception
Console.WriteLine(ex.Message)
Return 0
End Try
End Function
The above code fails just after ExecuteNonQuery() and can´t figure why.
TARGET FIELDS (SQL Server 2008):
AREA varchar(100) NOT NULL ,
USER varchar(100) NOT NULL
The exception I receive is: Connection property has not initialized
There's a few issues with this code.
The most significant is that you aren't setting the Command's Connection property, so the command has no way of knowing how to connect to the database.
I would also strongly recommend utilizing using, and also parameterizing your query:
Finally, don't declare the connection and command outside of the function unless you need to. You should only keep the connection and command around for as long as you need them.
So your function would end up looking like:
Public Function add(ByVal area As String, ByVal user As String) As Integer
Dim mydao As New Connection
Using connection As New SqlConnection(mydao.ConnectionString())
Using command As New SqlCommand()
' Set the connection
command.Connection = connection
' Not necessary, but good practice
command.CommandType = CommandType.Text
' Example query using parameters
command.CommandText = "INSERT into Area (Area, user) VALUES (#area, #user)"
' Adding the parameters to the command
command.Parameters.AddWithValue("#area", area)
command.Parameters.AddWithValue("#user", user)
connection.Open()
Return command.ExecuteNonQuery()
End Using ' Dispose Command
End Using ' Dispose (and hence Close) Connection
End Function
Note that currently, you will be returning 0 all the time. Rather than having to check the value returned from the function, the above example will simply throw an exception. This makes for slightly cleaner code (as the caller would have to understand that 0 is an error condition), and, if you needed to handle the exception, simply wrap the call to this function in a Try-Catch block
I'm starting to put in a database into my application, however I'm drawing a blank on how to share a database connection among the dozen or so different forms in my MDI application. I'm assuming this has to do with interfaces or something but I can't find any relevant examples anywhere. Can someone help me out? Ideally what I'd like is when the app is loaded up there is a call to a function in the forms loading area which establishes a single connection to the mdb, that I can then call via any form so I don't always have to open/close connections everytime I need to update the db (assuming what I'm suggesting is better for overhead), unless that is a better option?
Here's a basic example of the mdb database access code I've got working:
Dim dt As DataTable = New DataTable()
Dim OleDbTran As OleDbTransaction = Nothing
Using connJET As OleDbConnection = New OleDbConnection("connection string here...")
Try
connJET.Open()
Dim sqlCount As OleDbCommand = New OleDbCommand("select * from mytable", connJET)
Using aReader As OleDbDataReader = sqlCount.ExecuteReader()
dt.Load(aReader)
End Using
If (dt.Rows.Count > 0) Then
MsgBox(dt.Rows.Count)
End If
OleDbTran = connJET.BeginTransaction()
Dim aCommand As OleDbCommand = connJET.CreateCommand()
aCommand.CommandText = "INSERT INTO Programs (title) VALUES (#title)"
aCommand.Transaction = OleDbTran
aCommand.Parameters.Add("#title", OleDbType.VarChar)
aCommand.Parameters("#title").Value = "Test"
aCommand.ExecuteNonQuery()
OleDbTran.Commit()
Catch ex As Exception
MessageBox.Show(ex.Message)
End Try
End Using
Assuming that you create the connection in your startup form, then you could just add constructors to the other forms that accept a SqlConnection and send that in whenever you create an instance of that form.
Or if you prefer, you create something like this:
Public Class Connection
Private Shared connection As OleDb.OleDbConnection
Public Shared ReadOnly Property Instance As OleDb.OleDbConnection
Get
If connection Is Nothing Then
connection = New OleDb.OleDbConnection("connstring")
End If
Return connection
End Get
End Property
End Class
And then you could access it by just calling Connection.Instance whenever you need it.