[RESOLVED] [2005] USING statement and multiple resources

[nmadd]

Hi all,

I'd like to use one Using statement with multiple resources. MSDN says that one Using block can be used for multiple resources, however, I don't see any examples and I can't seem to figure out the syntax.

I have already acquired the recources and would like to do something like:

Using conn, da
' Do something.
End Using

Thanks for any help.

[Bulldog]

An example is below, the resources can be of different types;

        Using g As Graphics = e.Graphics, p As New Pen(Color.Black)
            g.DrawLine(p, Me.ClientRectangle.X, Me.ClientRectangle.Top, _
            Me.ClientRectangle.Right, Me.ClientRectangle.Bottom)
        End Using

[nmadd]

Thank you. That works when I am acquiring the resources in the Using statement but what about if I have them already? This doesn't seem to to work:

Dim g as Graphics = e.graphics
        Dim p As New Pen(Color.Black)
 
        Using g, p
            g.DrawLine(p, Me.ClientRectangle.X, Me.ClientRectangle.Top, _
            Me.ClientRectangle.Right, Me.ClientRectangle.Bottom)
        End Using

[Bulldog]

The purpose of the Using/End Using it to create temporary objects etc. that are created, used and then discarded. After the "Using" part, you declare the temporary objects you want to create, you then use them and discard them by virtue of the "End Using" statement.

So putting that new 'pen' into a Using statement is not valid, since it already exists.

[nmadd]

Thanks for the quick reply. I did read up about the purpose of the Using statement and thought I understood it.

MSDN says that you can pass it a variable instead of defining the resource in the Using statement itself.

You're right that it does make sense to do it right in the statement, but I was just curious about using variables instead. I thought it would possibly make it more readable.

Am I misunderstanding this?

[Bulldog]

I understand what you mean, you can do it like this;

    Dim a As Label

    Private Sub Form1_Paint( _
    ByVal sender As System.Object, _
    ByVal e As System.Windows.Forms.PaintEventArgs) _
    Handles MyBase.Paint

        Using g As Graphics = e.Graphics, p As New Pen(Color.Black), a = New Label
            g.DrawLine(p, Me.ClientRectangle.X, Me.ClientRectangle.Top, Me.ClientRectangle.Right, Me.ClientRectangle.Bottom)
        End Using

    End Sub

The difference here is that you declare "a" as a label, but the Dim statement does not create a label. It only creates an identifier "a" which is of type "label". It is in the Using statement, that you actually create a label object.

To be precise, "a 'Using' resource variable must have an explicit initialization".

[nmadd]

Thanks again.

So, for example, something like this is just not going to work?

Dim conn As New OleDb.OleDbConnection(someConnectionString)
Dim da As New OleDb.OleDbDataAdapter(SQLString, conn)
 
Using conn, da
    'Fill a datatable
End Using

However, this does seem to work:

Using conn
    Using da
        'Fill a table
    End Using
End Using

I'm confused as to why so I guess I'll just start defining the resources right in the Using statement since I know that works.

[Bulldog]

Well not really, since in your first example you have gone back to the situation where conn and da exist before the "Using" statement and hence they will still exist afterwards. Compare the two subs below;

    Dim a As Label
    Dim b As Panel

    Private Sub MySub1()
        a = New Label
        b = New Panel
        Using a
            Using b

            End Using
        End Using
        '"a" and "b" will still exist at this point
    End Sub

    Private Sub MySub2()
        Using a = New Label, b = New Panel

        End Using
        '"a" and "b" will not exist at this point
    End Sub

In MySub1 the objects that have already been created are used and not disposed of. In MySub2 the two objects are created and disposed.

[nmadd]

Aha! Understood now. I thought that the Using statement would dispose of the resources regardless of where I acquired them. Thank you very much for the posts.