GeniuS BraiN GeniuS BraiN - 4 months ago 23
Vb.net Question

DataAdapter is disposed before reaching "End Using"

I know that I should always dispose DataAdapter instances. In most cases I'm disposing it immediately after closing the connection, but in cases like when user will be modifying DataTable items (displayed in ListBox or DataGridView) I create the DataAdapter, use it to fill the DataTable, but don't dispose it until the user clickes

Save
which calls
DataAdapter.Update(DataTable)
... not my main question but is this the right approach?

Back to the main question, I have these two functions:

Public Function LoadCompaniesDT(ByRef dtCompanies As DataTable) As Boolean
Using daCompanies As MySqlDataAdapter = Nothing
Return LoadCompaniesDT(daCompanies, dtCompanies)
End Using
End Function

Public Function LoadCompaniesDT(ByRef daCompanies As MySqlDataAdapter, ByRef dtCompanies As DataTable) As Boolean
Dim sql As String = "SELECT * FROM companies"
Return LoadDT(daCompanies, dtCompanies, sql, Res.CompaniesFailedMsgBody)
End Function


They're used to call
LoadDT
which fills the DataTable so I have the choice to pass a DataAdapter or not.

Now I'm confused about something: When using the first
LoadCompaniesDT
function,
daCompanies
is disposed before reaching
End Using
.. like this:

Public Function LoadCompaniesDT(ByRef dtCompanies As DataTable) As Boolean
Using daCompanies As MySqlDataAdapter = Nothing
Dim tmp As Boolean = LoadCompaniesDT(daCompanies, dtCompanies)
Console.WriteLine(daCompanies Is Nothing) ' ==> True!!
Return tmp
End Using
End Function


Note: if I use
Dim daCompanies
instead of
Using daCompanies
then
daCompanies Is Nothing
will return False.




LoadDT
function code:

Private Function LoadDT(ByRef da As MySqlDataAdapter, ByRef dt As DataTable,
ByVal sqlQuery As String,
ByVal errorText As String) As Boolean
Dim connStr As String = String.Format("server={0}; port={1}; user id={2}; password={3}; database={4}",
DbServer, DbServerPort, DbUserName, DbPassword, DatabaseName)
Dim conn As MySqlConnection = New MySqlConnection(connStr)
Dim cmd As MySqlCommand = New MySqlCommand

Try
conn.Open()

cmd.CommandType = CommandType.Text
cmd.CommandText = sqlQuery
cmd.Connection = conn

da = New MySqlDataAdapter(cmd)
dt = New DataTable
da.Fill(dt)

Return True
Catch ex As Exception
MessageBox.Show(errorText, Res.ServerError, MessageBoxButtons.OK, MessageBoxIcon.Error)
Return False
Finally
cmd.Dispose()
cmd = Nothing
conn.Close()
conn.Dispose()
End Try
End Function

Answer

Update: you're right, you don't get an initialized MySqlDataAdapter back from the methods if the ByRef passed instance is used in a Using-statement. Those variables are readonly. In C# you get this meaningful compiler error:

Error CS1657 Cannot pass 'daCompanies' as a ref or out argument because it is a 'using variable'

It's documented here:

Compiler Error CS1657

Cannot pass 'parameter' as a ref or out argument because 'reason'' This error occurs when a variable is passed as a ref or out argument in a context in which that variable is readonly. Readonly contexts include foreach iteration variables, using variables, and fixed variables.

In VB.NET you can do that(so the compiler ignores it which is almost a bug) but the variable is not initialized afterwards. But as mentioned below, you should not use this approach anyway.


According to the the other question:

If you look at the sample on MSDN you'll see that microsoft also doesn't dispose the dataadapter. So it not really necessary. Having said that, it's always best practise to use the Using statement on anything that implements IDisposable.

A DataAdapter is not an expensive object and it does not hold unmanaged resources(like the connection). So it doesn't hurt to create a new instance from it whereever you need one. And you don't need to dispose it, but that's an implementation detail that might change in future or in a different implementation of DbDataAdapter, so it's still best practise to dispose it, best by using the Using-statement.

I wouldn't use your approach because you are passing the sql-string to the method which often leads to a sql injection vulnerabiliy. Instead use sql parameters.

For example:

Private Function LoadDT() As DataTable
    Dim tbl As New DataTable()
    'Load connection string from app.config or web.config
    Dim sql As String = "SELECT * FROM companies" ' don't use * but list all columns explicitely
    Using conn As New MySqlConnection(My.Settings.MySqlConnection)
        Using da = New MySqlDataAdapter(sql, conn)
            da.Fill(tbl)
        End Using
    End Using
    Return tbl
End Function

Instead of passing an errorText ByRef i'd use a logging framework like log4net.

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