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.
Related
I have 3 text boxes and based on Customer Name suggestion other 2 boxes should be auto filled
I am new to database so I am finding codes on internet and trying to implement it but it
is not working. So please help me with this.
I am working in for Windows Form Application in visual basic.
Private Sub AutoComplete()
'sql = "Select * FROM Table1 where CustomerName='" & custnm.Text & "'"
com = New OleDbCommand(sql, con)
reader = com.ExecuteReader()
Dim autoComp As New AutoCompleteStringCollection()
While reader.Read()
autoComp.Add(reader("CustomerName"))
autoComp.Add(reader("ContactNO"))
End While
reader.Close()
custnm.AutoCompleteMode = AutoCompleteMode.Suggest
custnm.AutoCompleteSource = AutoCompleteSource.CustomSource
custnm.AutoCompleteCustomSource = autoComp
contactno.AutoCompleteMode = AutoCompleteMode.Suggest
contactno.AutoCompleteSource = AutoCompleteSource.CustomSource
contactno.AutoCompleteCustomSource = autoComp
wcontno.AutoCompleteMode = AutoCompleteMode.Suggest
wcontno.AutoCompleteSource = AutoCompleteSource.CustomSource
wcontno.AutoCompleteCustomSource = autoComp
End Sub
This is only a partial solution to straighten out your database code. It is too much code to put in a comment.
Keep your database objects local so you can control when they are closed and disposed. Objects that expose a .Dispose method may have unmanaged resources that need to be released. A Using...End Using block will handle this for you even if there is and error.
Always use Parameters to avoid sql injection which can damage your database. Also it makes the sql string easier to write. You will need to check your database for the proper datatype because I had to guess.
A DataReader uses an open connection until it is done. You don't want to hold your connection open while you use the data so I chose a DataTable. Just load it and pass it off to where you want to use it. The connection and command are closed and disposed.
I wasn't sure why you combined the data in a single AutoCompleteStringCollection. It seems that one for each combo box would be better. They are form level variables so you can used them in another method to attach to a combo box.
I think you need to look into BindingSource and BindingNavigator to sync the combo boxes.
Private Function GetData() As DataTable
Dim dt As New DataTable
Using con As New OleDbConnection("Your connection string"),
com As New OleDbCommand("Select * FROM Table1 where CustomerName= #Name;", con)
com.Parameters.Add("#Name", OleDbType.VarChar).Value = custnm.Text
con.Open()
dt.Load(com.ExecuteReader)
End Using
Return dt
End Function
Private autoCompName As New AutoCompleteStringCollection()
Private autoCompNO As New AutoCompleteStringCollection()
Private Sub FillAutoCompleteCollection()
Dim dt = GetData()
For Each row As DataRow In dt.Rows
autoCompName.Add(row("CustomerName").ToString)
autoCompNO.Add(row("ContractNO").ToString)
Next
End Sub
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 having a problem in project of mine in VB.NET. The problem is whenever I want to save, delete or update data with an Access database I get an error with a message saying that "not allowed to change the connection string property. connection's current state is open".
I have used If con.State = ConnectionState.Open Then con.Close() End If
command using finally in every section where I have called the database.
But still I'm having the same problem. What am I doing wrong?
Use the "USING"-Keyword. Exiting a using block calls .Dispose() on the object which for a SqlConnection will close the connection and any open resources.
Using connection As New SqlConnection(connection)
Dim command As New SqlCommand("Select * From dbo.table1",connection)
command.ExecuteNonQuery()
End Using
EDIT:
Module Module1
Public Sub DbConnection()
Dim connectionString as String = "Provider=Microsoft.Jet.OLEDB.4.0;Data Source=yourServerAddress;Initial Catalog=university.mdb;
Integrated Security=SSPI;"
Using connection as New Sqlconnection(connectionstring)
Dim command As New SqlCommand("Select * From dbo.table1",connection)
command.ExecuteNonQuery()
End Using
End Sub
End Module
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.
I have an ASP.Net 2.0 Web Forms Application using SQL Server 2008. The application has a UI layer and Data Access Layer. I use Enterprise Libray 5.0 to persist the data.
Recently my site has been running very slow, especially on pages where there are maybe 15-20 separate reads from the database. I am very concerned that my SqlDataReader database connections are not being closed properly. Below is an example of how code works. Please take a look and let me know if you see any issues with it in terms of leaking connections.
Data Access Class
Public Class DataAccess
Private db As Database = DatabaseFactory.CreateDatabase()
Public Function ExecuteDataReader(ByVal params() As SqlParameter, ByVal SProc As String) As SqlDataReader
Dim i As Integer
Dim dr As SqlDataReader = Nothing
Dim cmd As DbCommand
cmd = db.GetStoredProcCommand(SProc)
cmd.CommandTimeout = 120
For i = 0 To params.Length - 1
db.AddInParameter(cmd, params(i).ParameterName.ToString, params(i).DbType, params(i).Value)
Next
dr = TryCast(DirectCast(db.ExecuteReader(cmd), RefCountingDataReader).InnerReader, SqlDataReader)
Return dr
End Function
UI Code Behind Page
Dim drSource As SqlDataReader = Nothing
Try
Dim params(0) As SqlParameter
params(0) = New SqlParameter("#applicant_id", Session("ApplicantID"))
drSource = DataAccess.ExecuteDataReader(params, "sp_get_date_last_login")
If drSource.HasRows Then
drSource.Read()
'Do code
End If
Finally
If Not (drSource Is Nothing) Then
drSource.Close()
End If
End Try
I tried to put the code below into my ExecuteDataReader method, but this then closes the SqlDataReader before it gets a chance to do a Read
if (cmd.Connection.State == ConnectionState.Open)
cmd.Connection.Close();
Can someone please look at the code above and let me know how to properly close my database connections, or maybe I am already doing it?
Thanks for your help folks.
Have you tried getting the underlying call to ExecuteReader to take the CommandBehavior.CloseConnection parameter? At least this will ensure that the connection is closed when the DataReader is also closed. This will rely on consumers of the DataReader passed back from ExecuteDataReader() to close it explicitly or dispose via a Using block.
Alternatively, try adding the following to the code behind after the drSource.Close() line:
drSource.Connection.Close()