I have an application which does quite a few database queries. In order to speed up the application I do some queries which are independent of each other (it doesn't matter which order they get updated in as they aren't referencing one another) on a separate thread.
However, it appears that if two queries are executed at the same time on the different threads, but one finishes earlier, the DataAdapter attempts to close the connection which is still being used on another thread resulting in a RaceOnRCWCleanup warning:
An attempt has been made to free an RCW that is in use. The RCW is in use on the active thread or another thread. Attempting to free an in-use RCW can cause corruption or data loss.
I may be incorrect about this assumption but if I don't call any queries on the secondary thread, or delay them, I don't seem to get this issue. If I force the queries to happen at the same time, I get the warning.
My query functions are, with a few exceptions, all structured as such:
Dim strSQL As String = "SELECT..."
Try
da = New OleDb.OleDbDataAdapter(strSQL, conn)
da.Fill(dsData, "Progress")
Catch ex As Exception
MessageBox.Show(ex.Message)
End Try
Return dsData.Tables("Progress")
And da is declared in a separate module as:
Public da As OleDbDataAdapter
I thought by declaring da = New I force it to be a new object and therefore, closing the connection on one should not effect the other?
Where am I going wrong here?
I believe I have solved this by using a separate OleDbConnection object for each thread (I never call the same functions across different threads).
This way a single connection object is only ever used in serial.
I don't know if this is the correct way to handle this but it seems to work.
Related
I need to run a number of queries as part of a transaction.
There's a method in my project, creatively named RunQuery(), that concentrates the database queries, so I'll be using it. It has four parameters:
a string indicating which database to run the query on,
a string containing the query,
a table name for when the result needs to be a data set,
an Enum that indicates what kind of query/results is wanted: Select query returning a data reader, Select query returning a data set, Insert/Update/Delete query.
(If you need more detail, I'll supply it gladly. I'll keep this short-ish for now.)
The point is that when you want to run a query, you don't have to worry about the connection or command objects, you just write your query and call RunQuery(). Fine.
Now, I've added a new ByRef parameter to RunQuery(), an IDbTransaction to track the transaction. It needs to be handled at least partially outside RunQuery(), if only to commit after the very last call, but on the first call it's set at Nothing.
I've also added an item to the Enum so RunQuery() knows it has to deal with a transaction.
My code looks like this:
Dim z_lisQuery As New List(Of String)
' [...] Filling z_lisQuery with queries
Dim z_dtrTransaction As IDbTransaction = Nothing
Dim z_blnExecutionOk As Boolean = True
For Each z_strQuery As String In z_lisQuery
z_blnExecutionOk = z_blnExecutionOk And RunQuery(p_strDbId,
z_strQuery,
"",
z_dtrTransaction,
QueryAction.ExecutionTransaction)
If Not z_blnExecutionOk Then
Exit For
End If
Next
If z_blnExecutionOk Then
z_dtrTransaction.Commit()
End If
z_dtrTransaction?.Connection.Dispose()
z_dtrTransaction?.Dispose()
(A Rollback() is executed within RunQuery() if something goes wrong.)
When I run that and no errors happen, everything goes fine until the penultimate line:
z_dtrTransaction?.Connection.Dispose() throws an exception because z_dtrTransaction.Connection is Nothing.
The obvious workaround is to use z_dtrTransaction?.Connection?.Dispose(), so that's not what I'm puzzled about.
What I'm puzzled about is that z_dtrTransaction.Connection is still a functional OleDbConnection object when execution reaches z_dtrTransaction.Commit(). But after z_dtrTransaction.Commit() is executed, z_dtrTransaction.Connection is Nothing.
I can see the point of that, but the language reference doesn't seem to indicate that it should happen that way and I haven't found a reference to that way of doing things.
Is it officially stated somewhere that this behavior is the normal one? Or is it just with OleDbConnection and not with SqlConnection, for instance?
(Even if it is, obviously I'm keeping z_dtrTransaction?.Connection?.Dispose() because I'm not entirely sure what happens in case of a rollback.)
Hi friends of stackoverflow,
I'm writing a question here because i'm having problems to detect why sometimes a read to a field of a datareader returns a invalid cast exception.
I will give all information possible to understand my situation. I'm working with ASP.NET 3.5
I have a Module that have a function that returns a IDataReader and receives a sql query. something like this
function get_dr(query as String) as IDataReader
dim connection = new SqlClient.SqlConnection("connection string")
connection.open()
dim command = connection.createCommand
command.commandText = query
dim reader = command.executeReader(CommandBehavior.CloseConnection)
return reader
end function
I have a Class with a Shared function that recovers a new dataReader and returns a date. something like this:
public shared function getDate() as Date
using dr = get_dr("SELECT dbo.getDate()")
if dr.read() and dr(0) isnot dbnull.value then
return dr.GetDateTime(0)
end if
end using
end function
when in another code i call the getDate() function, it gives me a call stack like this.
System.InvalidCastException: Specified cast is not valid.
at System.Data.SqlClient.SqlBuffer.get_DateTime()
at System.Data.SqlClient.SqlDataReader.GetDateTime(Int32 i)
Why sometimes i'm getting this error? i was thinking this is because that a lot of users is calling this function in conjunction with another functions of my application (those functions eventually uses get_dr too), mixing the data of the dataReader on another executions, but i need to know if im doing something wrong or maybe to do something better.
Notes:
dbo.getDate is a sql function that ALWAYS returns a date.
don't worry about bad writing code, those are only examples but they have the necessary to understand the scenario.
sorry for my bad english
Really thanks in advance.
One possible reason - you declare connection inside of the function that returns DataReader. When you're out of the function that connection goes out of scope. That means that at some unpredictable point (depends on memory usage etc.) Garbage Collector will collect it. If you try to use the DataReader at that point - all bets are off.
One way to solve it is to declare connection outside of function get_dr and pass it there as a parameter. But also seeing that you're returning a single value and if you don't plan to use the reader for multiple values I suggest using ExecuteScalar instead - it will save you a lot of headaches.
In my Winforms app which uses a remote database, I have the function below. (I also have two other functions which work similarly: One for scalar queries which returns zero upon failure, the other for updates and inserts which returns false upon failure.)
Currently, ALL data manipulation is pumped through these three functions.
It works fine, but overall please advise if I'd be better off establishing the connection upon launching my app, then closing it as the app is killed? Or at another time? (Again, it's a windows forms app, so it has the potential to be sitting stagnant while a user takes a long lunch break.)
So far, I'm not seeing any ill effects as everything seems to happen "in the blink of an eye"... but am I getting data slower, or are there any other potential hazards, such as memory leaks? Please notice I am closing the connection no matter how the function terminates.
Public Function GetData(ByVal Query As String) As DataTable
Dim Con As New SqlConnection(GlobalConnectionString)
Dim da As New SqlDataAdapter
Dim dt As New DataTable
Try
Con.Open()
da = New SqlDataAdapter(Query, Con)
Con.Close()
da.Fill(dt)
Catch ex As Exception
Debug.Print(Query)
MsgBox("UNABLE TO RETRIEVE DATA" & vbCrLf & vbCrLf & ex.Message, MsgBoxStyle.Critical, "Unable to retrieve data.")
End Try
da.Dispose()
Con.Close()
Return dt
End Function
There are exceptions to this, but best practices in .Net do indeed call for creating a brand new connection object for most queries. Really.
To understand why is, first understand actually connecting to a database involves lots of work in terms of protocol negotiations, authentication, and more. It's not cheap. To help with this, ADO.Net provides a built-in connection pooling feature. Most platforms take advantage of this to help keep connections efficient. The actual SqlConnection, MySqlConnection, or similar object used in your code is comparatively light weight. When you try to re-use that object, you're optimizing for the small thing (the wrapper) at the expense of the much larger thing (the actual underlying connection resources).
Aside from the benefits created from connection pooling, using a new connection object makes it easier for your app to scale to multiple threads. Imagine writing an app which tries to rely on a single global connection object. Later you build a process which wants to spawn separate threads to work on a long-running task in the background, only to find your connection is blocked, or is itself blocking other normal access to the database. Worse, imagine trying to do this for a web app, and getting it wrong such that the single connection is shared for your entire Application Domain (all users to the site). This is a real thing I've seen happen.
So this is something that your existing code does right.
However, there are two serious problems with the existing method.
The first is that the author seems not to understand when to open and when to close a connection. Using the .Fill() method complicates this, because this method will open and close your connection all on its own.1 When using this method, there is no good reason to see a single call to .Open(), .Close(), or .Dispose() anywhere in that method. When not using the .Fill() method, connections should always be closed as part of a Finally block: and the easiest way to do that is with Using blocks.
The second is SQL Injection. The method as written allows no way to include parameter data in the query. It only allows a completed SQL command string. This practically forces you to write code that will be horribly vulnerable to SQL Injection attacks. If you don't already know what SQL Injection attacks are, stop whatever else you're doing and go spend some time Googling that phrase.
Let me suggest an alternative method for you to address these problems:
Public Function GetData(ByVal Query As String, ParamArray parameters() As SqlParameter) As DataTable
Dim result As New DataTable()
Using Con As New SqlConnection(GlobalConnectionString), _
Cmd As New SqlCommand(Query, Con),
da As New SqlDataAdpapter(Cmd)
If parameters IsNot Nothing Then Cmd.Parameters.AddRange(parameters)
Try
da.Fill(result)
Catch ex As Exception
Debug.Print(Query)
'Better to allow higher-level method to handle presenting the error to the user
'Just log it here and Rethrow so presentation tier can catch
Throw
End Try
End Using 'guarantees connection is not left hanging open
Return result
End Function
1See the first paragraph of the "Remarks" section.
This isn't a real "answer" to my own question, but I have something to add and I wanted to add some code.
To Joe: Thank you, my code is well on the way to using parameterized queries almost exclusively. Although I knew what SQL injection attacks were, and that they're a pretty big deal, here's my exuse: In the past I had used stored procedures for parameterized queries, and I kinda hate writing those and for the first year my code will be used only within by my small company of 5 employees who are family members... I had planned to switch everything to stored procedures later if I sold the software. This approach is better and I will probably not need stored procedures at all.
I especially like how elegantly parameterized queries handle dates, as I don't have to convert dates to appropriate text. Much easier.
Anopther advantage I'm seeing: Sometimes a "Save button" must execute either Insert or Update, depending on whether the record displayed is new. Using parameters allows me to write two alternate short basic queries, but to use the same parameters for either with less code.
Overall, this means a whole lot less code-intensive construction of the query string.
The part I didn't have, and I learned to do it elsewhere, was assigning the parameter array, calling the procedure, so I'm including an example here hoping others find it useful:
Dim query As String = "Select Phone from Employees where EmpNo = #EmployeeNumber and Age = #Age"
Dim params As SqlParameter() = {
New SqlParameter("#EmployeeNumber", txtEmployeeNumber.Value),
New SqlParameter("#Age", txtAge.Value)
}
Dim Phone as String = GetData(query, params).Rows.Item(0)
Take code like this:
Dim dt As New DataTable("Table")
Dim sda As New SqlDataAdapter(pQuery, pCon)
sda.SelectCommand.CommandTimeout = pCommandTimeout
sda.Fill(dt)
sda.Dispose()
Return dt
and:
Dim myconnection As SqlConnection = New SqlConnection(DBValues.m_sDBString)
myconnection.Open()
Try
Dim com As New SqlCommand(cmd, myconnection)
com.ExecuteNonQuery()
com.Dispose()
Catch ex As Exception
Throw ex
Finally
If myconnection.State = ConnectionState.Open Then
myconnection.Close()
myconnection.Dispose()
End If
End Try
When is it getting excessive to explicity call things like Close() and Dispose()? Because apparently the GC handles this internally most of the time, but not all the time (?). So far I haven't really seen somebody put down a very clear and unambiguous explanation on how to distinguish between when this is excessive and when it isn't, or what the difference is between doing this explicitly and just letting the GC handle it. Could somebody explain this? Thanks!
If you aren't doing anything more complicated than what's in the example, then it is probably excessive.
You could use the using keyword to make things simpler. It automates the calling of the Dispose method of objects from classes that implement IDisposable.
Relevant documentation on the using keyword, for VB.NET
Also note that the Dispose method of SqlConnection calls its Close method too (running the same state validation you do in your code).
Notice that there are situations where you might have to call Dispose manually instead of going with using. For example, you might want to use a data reader in more than one context, so using using in either of them may not be desired. You'll know when you're in one of these situations, and then calling Dispose yourself won't be overkill. But only then.
I'm wondering if this is a good way of doing data access, in terms of all the database objects being properly closed and disposed? For example:
Using conn As New SqlConnection(MyConnectionString)
Using cmd As New SqlCommand("some SQL here", conn)
... add parameters ...
conn.Open()
Using dr As SqlDataReader = cmd.ExecuteReader()
While dr.Read()
... do stuff ...
Wend
End Using
End Using
End Using
Is nesting Using like acceptable practice? If I exit the method at some point within the Read() loop, will the use of Using like this ensure all objects are cleaned up properly regardless?
Using guarantees orderly disposal in an implicit try/finally block.
' THE FOLLOWING TRY CONSTRUCTION IS EQUIVALENT TO THE USING BLOCK
Dim resource As New resourceType
Try
' Insert code to work with resource.
Catch ex As Exception
' Insert code to process exception.
Finally
' Insert code to do additional processing before disposing of resource.
resource.Dispose()
End Try
Nested usings work in a similar fashion. If you exit a block of code, it will execute the finally block, and properly dispose your objects.
http://msdn.microsoft.com/en-US/library/htd05whh(v=VS.80).aspx
To add, the Using block will "behind the scenes" add a Try Finally statement. In the finally statement it will call IDisposable.Dispose on the object. In other words, no matter what you do or what happens, the object will get disposed.
Yes, this is ok. The Dispose method of IDisposable objects is always called.
PS: In this case, the Dispose method also contains the Close method.
You are writing VB.Net, so this only partially applies, but for the C# folks out there using StyleCop, multiple using statements like this will cause exception 2202 in StyleCop.
There is a lengthy section at the msdn link of dissenting comments as to the usefulness of this rule in stylecop.
I will not make a judgment call as to whether you should heed StyleCops warnings for C# in your VB.Net code.