I have a public class called dbOPS that has some subs and functions like:
Public Function getSqlReader(ByVal sql As String) As SqlDataReader
Dim cmd As New SqlCommand(sql, getConn)
cmd.CommandTimeout = 360
Dim dr As SqlDataReader = cmd.ExecuteReader(Data.CommandBehavior.CloseConnection)
Return dr
End Function
Public Function getSqlScalar(ByVal sql As String)
Dim cmd As New SqlCommand(sql, getConn)
cmd.CommandTimeout = 360
Dim cnt = cmd.ExecuteScalar
closeCX()
Return cnt
End Function
Public Sub ExecuteSql(ByVal sql As String)
Dim cmd As New SqlCommand(sql, getConn)
cmd.CommandTimeout = 360
cmd.ExecuteNonQuery()
closeCX()
End Sub
I then use the following command once per page and use the db variable many (many) times throughout the page:
Dim db as new dbOPS
Recently, I have started getting many errors
ExecuteScalar requires an open and available connection. the connection's current state is connecting
Is this the cause?
Is there any way around it without rewriting every page and every command to open its own connection?
Thanks
It's probably best to detect the connection and reopen them just in case it is not open prior to executing the command.
If (cmd.Connection.State != ConnectionState.Open) Then
cmd.Connection.Open()
Do this as part of your helper methods.
Related
Trying to update an old VB6 app to VB.Net. I am having trouble with syntax, I think. In any case it is a simple matter of inserting a new record to the autolog table. (code below).
I would like to ask something else that is often not documented too. It seems that I have to use command builders and so on - is there no way I can simply use an SQL statement and execute it against the background table? The tables are in Access while I am developing but will be scaled up on the final release of the software.
I have altered my code to the following by making use of the error suggestions at the foot of mygui.
It now looks like this and the only thing is that it is throwing a logic error at me which is that every end function must have a preceding "function". Perhaps I am being a little bit dim
Function MAutolog(ByVal Action As String) As Boolean
Dim SQL = "Insert Into Autolog (Action) Values (#Action)"
Using con As New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\PC User\Documents\Freightmaster\resources\freightmaster.accdb"),
cmd As New OleDb.OleDbCommand(SQL, con)
cmd.Parameters.Add("#Action", OleDb.OleDbType.VarChar).Value = Action
con.Open()
cmd.ExecuteNonQuery()
End Using
MAutolog = True
End Function
I would like to thank you for your help in advance. I can not tell you how much I will appreciate it.
Code
Module ModFunctions
Function MAutolog(ByVal UserID As Long, ByVal Action As String) As Boolean
Dim dbprovider As String
Dim dbsource As String
Dim mydocumentsfolder As String
Dim fulldatabasepath As String
Dim TheDatabase As String
Dim SQL As String
Dim DS As New DataSet
Dim da As OleDb.OleDbDataAdapter
Dim con As New OleDb.OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\PC User\Documents\Freightmaster\resources\freightmaster.accdb")
con.Open()
'----------------------------
SQL = "Select * from Autolog"
da = New OleDb.OleDbDataAdapter(SQL, con)
da.Fill(DS, "Log")
con.Close()
Dim CB As New OleDb.OleDbCommandBuilder(da)
Dim DSNEWROW As DataRow
DSNEWROW = DS.Tables("Log").NewRow()
DSNEWROW.Item("UserID") = UserID
DSNEWROW.Item("Action") = Action
DS.Tables("log").Rows.Add(DSNEWROW)
da.Update(DS, "log")
MAutolog = True
End function
Database objects like Connection and Command use unmanaged code and need their Dispose methods to release these resources. Either call this method on these objects or use Using...End Using blocks which will do this for you even if there is an error. In this code, both the Connection and Command are included in the Using block by separating them be a comma.
By Val is the default so is not necessary.
Always use parameters to avoid sql injection. Using values directly from user input can allow malicious code to be executed on your database. The value of a parameter is not considered as executable code by the database.
OleDb does not care about parameter names. You could just as easily use ? in the sql statement. I use names for readability. You do need some sort of name to add the parameter. OleDb considers the position of the parameter in the sql statement. The position must match the order that the parameters are added to the parameters collection.
This is the code for the Insert if UserID in an auto-number field. You do not provide a value for auto-number fields. The database will handle that.
Function MAutolog(Action As String) As Boolean
Dim SQL = "Insert Into Autolog (Action) Values (#Action)"
Using con As New OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\PC User\Documents\Freightmaster\resources\freightmaster.accdb"),
cmd As New OleDbCommand(SQL, con)
cmd.Parameters.Add("#Action", OleDbType.VarChar).Value = Action
con.Open()
cmd.ExecuteNonQuery()
End Using
MAutolog = True
End Function
If UserID is not auto-number
Function MAutolog(UserID As Long, Action As String) As Boolean
Dim SQL = "Insert Into Autolog (UserID, Action) Values (#UserID, #Action)"
Using con As New OleDb.OleDbConnection("Provider=Microsoft.ACE.OLEDB.12.0;Data Source=C:\Users\PC User\Documents\Freightmaster\resources\freightmaster.accdb"),
cmd As New OleDbCommand(SQL, con)
cmd.Parameters.Add("#UserID", OleDbType.Integer).Value = UserID
cmd.Parameters.Add("#Action", OleDbType.VarChar).Value = Action
con.Open()
cmd.ExecuteNonQuery()
End Using
MAutolog = True
End Function
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
I'm trying to Autocomplete City names from a huge sql table. My code below work but its very slow and sometimes it freezes because I got a lot repeated suggestions. any way to filter all this repeated string to make it faster ? thanks
Public Function GetCompletionList(ByVal prefixText As String, ByVal count As Integer) As String()
Dim strCn As String = "Data Source=sqlserver\sqlexpress;Initial Catalog=zip;User ID=sa;Password=xxx"
cn.ConnectionString = strCn
Dim cmd As New SqlClient.SqlCommand
cmd.Connection = cn
cmd.CommandType = CommandType.Text
cmd.CommandText = "select * from zip_code Where City like #myParameter+'%'"
cmd.Parameters.AddWithValue("#myParameter", prefixText)
Try
cn.Open()
cmd.ExecuteNonQuery()
Dim da As New SqlDataAdapter(cmd)
Dim dt As New DataTable()
da.Fill(ds)
Catch ex As Exception
Finally
cn.Close()
End Try
dt = ds.Tables(0)
Dim txtItems As New List(Of String)()
Dim dbValues As String
For Each row As DataRow In dt.Rows
dbValues = row("City").ToString()
dbValues = dbValues.ToLower()
txtItems.Add(dbValues)
Next
Return txtItems.ToArray
You are executing your SQL twice, once during ExecuteNonQuery, second time during Fill.
You are not using IDisposable objects correctly.
You return all columns.
If you have a control for display, check if it will accept Rows (Dataset?) property directly. Don't copy.
UPDATE
Your connection, command and other objects implement an IDisposable interface. So they should be placed in a using block:
Using { resourcelist | resourceexpression }
[ statements ]
End Using
Then you don't have to worry about closing and disposing a database connection.
UPDATE 2
Your code has thread safety issues, the same connection must not be shared by two different threads. EVER. Two consecutive Fill request and one will close the database connection before the other can finish.
I was asked to do a system that would poll one table in the database every second and if it counters a row that meets a criteria start actions to handle that.
I've done this but every now and then I get a time out exception. I have a WPF application where I have a thread that runs in background. This thread has a loop and sleeps for one second at the end of the loop. The connection to the database is opened inside "using" clause.
Below is my thread sub:
Private Sub PollDatabase()
While m_StopThread = False
Try
Dim listOfRows As List(Of DataObject) = db.GetDataObjects()
... Do something with the rows ...
Catch ex As Exception
m_log.WriteLine(ex.ToString())
End Try
Thread.Sleep(1000)
End While
End Sub
And my SQL function looks like this:
Public Function GetDataObjects() As List(Of DataObject)
Dim result As New List(Of DataObject)
Dim sb As New StringBuilder("... the sql query ...")
Using cnn = New SqlConnection(_connectionString)
cnn.Open()
Using cmd = New SqlCommand(sb.ToString(), cnn)
cmd.CommandTimeout = 0
Using DataReader As SqlDataReader = cmd.ExecuteReader()
Do While DataReader.Read()
... read the columns from table
to the dataobject ...
result.Add(DataObject)
Loop
End Using
End Using
End Using
Return result
End Function
Now what seems randomly my log has a time out exception:
System.Data.SqlClient.SqlException (0x80131904): Timeout expired. The timeout period elapsed prior to completion of the operation or the server is not responding.
...
at System.Data.SqlClient.SqlConnection.Open()
My questions are: is this at all save way of doing this? Or am I doing something fundamentally wrong here? And of course if anyone have a suggestion to fix this issue.
EDIT:
I tried a bit different approach with the SQL function. I'm now opening a connection once when my application starts and dumped the "using" clauses. So my function looks something like this now:
Public Function GetDataObjects() As List(Of DataObject)
Dim result As New List(Of DataObject)
Dim sb As New StringBuilder("... the sql query ...")
_sqlCmd.CommandText = sb.ToString()
Using DataReader As SqlDataReader = _sqlCmd.ExecuteReader()
Do While DataReader.Read()
... fill the list with objects ...
Loop
End Using
Return result
End Function
My log is clean form errors. So is there something wrong opening a connection to the server once in a second as I do with the using?
EDIT:
I've done a lot of testing now to identify the problem. What I discovered is that just connecting multiple times to the server doesn't cause any problems. Neither does adding a select statement after the connection. But when I actually implement a function where is the complete reader part and return my results I run into the time out problems. Here is two examples.
This isn't causing issues:
Private Sub Window_Loaded(sender As System.Object, e As System.Windows.RoutedEventArgs)
Me.DataContext = Me
m_Thread = New Thread(AddressOf ConnectionTestFunction)
m_Thread.IsBackground = True
m_Thread.Start()
End Sub
Private Sub ConnectionTestFunction()
While m_stopThread = False
Try
m_log.WriteLine("GetData (" & m_ThreadCounter & ")")
Using cnn As SqlConnection = New SqlConnection("Data Source=server;Initial Catalog=db;Integrated Security=True;MultipleActiveResultSets=True")
cnn.Open()
Using cmd As SqlCommand = New SqlCommand("SELECT * FROM Data", cnn)
Using DataReader As SqlDataReader = cmd.ExecuteReader()
Do While DataReader.Read()
Loop
End Using
End Using
End Using
Catch ex As Exception
m_log.WriteLine(ex.ToString())
End Try
m_ThreadCounter += 1
Thread.Sleep(1000)
End While
End Sub
This is causing timeout errors:
Private Sub Window_Loaded(sender As System.Object, e As System.Windows.RoutedEventArgs)
Me.DataContext = Me
m_Thread = New Thread(AddressOf ConnectionTestFunction)
m_Thread.IsBackground = True
m_Thread.Start()
End Sub
Private Sub ConnectionTestFunction()
While m_stopThread = False
Try
m_log.WriteLine("GetData (" & m_ThreadCounter & ")")
Dim datarows As List(Of Data) = Me.GetData()
Catch ex As Exception
m_log.WriteLine(ex.ToString())
End Try
m_ThreadCounter += 1
Thread.Sleep(1000)
End While
End Sub
Private Function GetData() As List(Of Data)
Dim result As New List(Of Data)
Using cnn As SqlConnection = New SqlConnection("Data Source=server;Initial Catalog=db;Integrated Security=True;MultipleActiveResultSets=True")
cnn.Open()
Using cmd As SqlCommand = New SqlCommand("SELECT * FROM Data", cnn)
Using DataReader As SqlDataReader = cmd.ExecuteReader()
Do While DataReader.Read()
Dim d As New Data()
d.DataId = DataReader("DataId")
... etc fields about 10 of them ...
result.Add(d)
Loop
End Using
End Using
End Using
Return result
End Function
I'm really happy if anyone have any thoughts about this... I have to admit I'm really confused now.
Probably, your code is taking longer to complete than the default time-out value for the connection is. Try specifying the time-out when you create your Sql Connection. Make sure that it's longer than the time your code needs to complete.
This approach doesn't seem very good.. Instead of pooling, why not react when new data comes? You can use a trigger or SqlDependency?
http://dotnet.dzone.com/articles/c-sqldependency-monitoring
Should I use using?
Private Sub btntest_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btntest.Click
If sqlConnection.State = ConnectionState.Closed Then
sqlConnection.Open()
End If
Dim query = "Select * from tablebusiness"
Dim cmd = New MySqlCommand(query, sqlConnection)
Dim data = cmd.ExecuteReader()
Do While data.Read
Loop
Dim cmd1 = New MySqlCommand(query, sqlConnection)
Dim data1 = cmd1.ExecuteReader //Error. Already have data reader
//Error There is already an open DataReader associated with this Connection which must be closed first.
Dim check = 1
'sqlConnection.Close()
End Sub
Although you have not let us know what the error is (which makes solving any problem much harder), I expect the issue is arising because you are trying to re-use the SqlConnection object for 2 different commands. Especially since you are not disposing your first command before initialising the second.
Firstly, use 2 different SqlConnection objects to manage the connection to the database. You are not putting any more overhead on the database or the code if you do this. Let the .NET framework connection pooling do its job - don't try to do it yourself. You don't need to do anything specific to enable connection pooling (although you can disable it by setting Pooling=false in your connection string).
Secondly use the using statement to correctly dispose your SqlConnection, SqlCommand, and SqlDataReader objects. e.g.
Using connection As New SqlConnection(connectionString)
connection.Open()
Using Command As New SqlCommand(query, connection)
Using reader As SqlDataReader = Command.ExecuteReader()
While reader.Read()
'Do Stuff'
End While
End Using
End Using
connection.Close()
End Using
You have missed the parenthesis after cmd1.ExecuteReader. It should be cmd1.ExecuteReader().
You Need another Conncetion if you want both the datareaders to work simultaneously, else close/ dispose the previous command before using cmd1.ExecuteReader()