asynchronous programming question

[Chris]

Hello,

With asynchronous programming :

Why does the callback-function (running in a worker thread) may not update
the state of a control on the main-form ?

The docs say that you must use a MethodInvoker instead as in following
example :

Private Sub CallBtn_Click(B yVal sender ...) Handles CallBtn.Click

webServiceObj = New MyDatabaseWSCla ss()|
Dim delCB As New AsyncCallback(A ddressOf MyCallBack)
aResult = webServiceObj.B eginConnectToDB (CInt(txtSecond s.Text), delCB,
Now)

End Sub

Private Sub UpdateUI()
OutputLabel.Tex t = "Async: Web service call complete."
End If

Private Sub MyCallBack(ByVa l result As IAsyncResult)
If webServiceObj.E ndConnectToDB(r esult) = True Then
DisplayMessage( result)
End If

' following is NOT allowed although it works fine ?????
OutputLabel.Tex t = "Async: Web service call complete."
Dim mi As New MethodInvoker(A ddressOf Me.UpdateUI)
Me.BeginInvoke( mi)
End Sub

Any ideas ?

Thank you

Chris

[AlexS]

Because most if not all of controls on Windows forms are not thread-safe.
It's component / controls design issue.

HTH
Alex
"Chris" <ch********@pan dora.be> wrote in message
news:xp******** ***********@pho bos.telenet-ops.be...

Hello,

With asynchronous programming :

Why does the callback-function (running in a worker thread) may not update
the state of a control on the main-form ?

The docs say that you must use a MethodInvoker instead as in following
example :

Private Sub CallBtn_Click(B yVal sender ...) Handles CallBtn.Click

webServiceObj = New MyDatabaseWSCla ss()|
Dim delCB As New AsyncCallback(A ddressOf MyCallBack)
aResult = webServiceObj.B eginConnectToDB (CInt(txtSecond s.Text), delCB,
Now)

End Sub

Private Sub UpdateUI()
OutputLabel.Tex t = "Async: Web service call complete."
End If

Private Sub MyCallBack(ByVa l result As IAsyncResult)
If webServiceObj.E ndConnectToDB(r esult) = True Then
DisplayMessage( result)
End If

' following is NOT allowed although it works fine ?????
OutputLabel.Tex t = "Async: Web service call complete."
Dim mi As New MethodInvoker(A ddressOf Me.UpdateUI)
Me.BeginInvoke( mi)
End Sub

Any ideas ?

Thank you

Chris

[Shawn D Shelton]

"Controls in Windows Forms are bound to a specific thread and are not
thread safe. Therefore, if you are calling a control's method from a
different thread, you must use one of the control's invoke methods to
marshal the call to the proper thread. This property can be used to
determine if you must call an invoke method, which can be useful if you
do not know what thread owns a control. There are four methods on a
control that are safe to call from any thread: Invoke, BeginInvoke,
EndInvoke and CreateGraphics. For all other method calls, you should use
one of these invoke methods when calling from a different thread."
******
instead of
OutputLabel.Tex t = "Async: Web service call complete."

use

OutputLabel.Inv oke(New UpdateLabelDele gate(AddressOf LabelUpdate), New
Object() {OutputLabel})

******

Declare the following:

Delegate Sub UpdateLabelDele gate(ByVal lbl As Label)

******

and add the following sub:

Private Sub UpdateLabel(ByV al lbl As Label)
OutputLabel.Tex t = "Async: Web service call complete."
End Sub

*************
Chris wrote:

Hello,

With asynchronous programming :

Why does the callback-function (running in a worker thread) may not update
the state of a control on the main-form ?

The docs say that you must use a MethodInvoker instead as in following
example :

Private Sub CallBtn_Click(B yVal sender ...) Handles CallBtn.Click

webServiceObj = New MyDatabaseWSCla ss()|
Dim delCB As New AsyncCallback(A ddressOf MyCallBack)
aResult = webServiceObj.B eginConnectToDB (CInt(txtSecond s.Text), delCB,
Now)

End Sub

Private Sub UpdateUI()
OutputLabel.Tex t = "Async: Web service call complete."
End If

Private Sub MyCallBack(ByVa l result As IAsyncResult)
If webServiceObj.E ndConnectToDB(r esult) = True Then
DisplayMessage( result)
End If

' following is NOT allowed although it works fine ?????
OutputLabel.Tex t = "Async: Web service call complete."
Dim mi As New MethodInvoker(A ddressOf Me.UpdateUI)
Me.BeginInvoke( mi)
End Sub

Any ideas ?

Thank you

Chris