DataAdapter is disposed before reaching "End Using" - vb.net

Question

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 1

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.