Want to call the same BackgroundWorker multiple times without using Application.DoEvents

Question

I ran into a problem that I was able to fix with Application.DoEvents, but I do not want to leave it because it can cause all kinds of unpleasant problems.

Background: Our application is basically a desktop application that makes many web service calls. We control everything, but changes in the overall system design will not be seriously considered. One of these Calculate calls is used very often, and sometimes it may take several minutes to process all the data in order to return valid results.

Previously, this Calculate call was executed synchronously and thus would block the user interface, leaving the user wondering whether the application was frozen or not, etc. I successfully moved all the long wait calls to BackgroundWorker, and then made a simple Wait Screen, which will cycle through the animated message “Calculation ...”.

Now the problem occurs when our user interface code tries to call the calculation procedure again before the first completion. I would get the message "This BackgroundWorker is currently busy and cannot start multiple instances ...". It seems that I should control the calls to resetEvent.WaitOne (). This is not so, I thought, maybe another event controlling access to the whole routine would help, so I added calcDoneEvent. This has not fixed the problem yet, but will cause it to block indefinitely on the second call to Calculate calcDoneEvent.WaitOne (). Then, on a whim, I added Application.DoEvents at the bottom of Calculate and viola, the problem is resolved.

I do not want to leave this .DoEvents there, because I read it, it can cause problems that are very difficult to track in the future. Is there a better way to handle this situation?

.

Private WithEvents CalculateBGW As New System.ComponentModel.BackgroundWorker
Dim resetEvent As New Threading.AutoResetEvent(False)
Dim calcDoneEvent As New Threading.AutoResetEvent(True)

Public Sub Calculate()

    calcDoneEvent.WaitOne() ' will wait if there is already a calculate running.'
    calcDoneEvent.Reset()

    ' setup variables for the background worker'

    CalculateBGW.RunWorkerAsync() ' Start the call to calculate'

    Dim nMsgState As Integer = 0
    ' will block until the backgorundWorker is done'
    Do While Not resetEvent.WaitOne(200) ' sleep for 200 miliseconds, then update the status window'
        Select Case nMsgState
            Case 1
                PleaseWait(True, vbNull, "Calculating.   ")
            Case 2
                PleaseWait(True, vbNull, "Calculating..  ")
            Case 3
                PleaseWait(True, vbNull, "Calculating... ")
            Case 4
                PleaseWait(True, vbNull, "Calculating....")
            Case Else
                PleaseWait(True, vbNull, "Calculating    ")
        End Select
        nMsgState = (nMsgState + 1) Mod 5
    Loop

    PleaseWait(False, vbNull) 'make sure the wait screen goes away'

    calcDoneEvent.Set() ' allow another calculate to proceed'
    Application.DoEvents() ' I hate using this here'
End Sub

Private Sub CalculateBGW_DoWork(ByVal sender As System.Object, _
    ByVal e As System.ComponentModel.DoWorkEventArgs) Handles CalculateBGW.DoWork
    Try
        'make WS Call, do data processing on it, can take a long time..'
        'No Catch inside the DoWork for BGW, or exception handling wont work right...'
        'Catch'
    Finally
        resetEvent.Set() 'unblock the main thread'
    End Try
End Sub

Private Sub CalculateBGW_RunWorkerCompleted(ByVal sender As Object, _
    ByVal e As System.ComponentModel.RunWorkerCompletedEventArgs) Handles CalculateBGW.RunWorkerCompleted

    'If an error occurs we must check e.Error prior to touching e.Result, or the BGW' 
    'will possibly "eat" the exception for breakfast (I hear theyre tasty w/ jam)'
    If Not (e.Error Is Nothing) Then

        'If a Web Exception timeout, retry the call'
        If TypeOf e.Error Is System.Net.WebException And _
            e.Error.Message = "The operation has timed out" And _
            intRetryCount < intRetryMax Then

            ' Code for checking retry times, increasing timeout, then possibly recalling the BGW'

            resetEvent.Reset()
            CalculateBGW.RunWorkerAsync() 'restart the call to the WS'
        Else
            Throw e.Error ' after intRetryMax times, go ahead and throw the error up higher'
        End If
    Else
        Try

            'normal completion stuff'

        Catch ex As Exception
            Throw
        End Try
    End If

End Sub

Answer 1

:

Private WithEvents CalculateBGW As New System.ComponentModel.BackgroundWorker
Dim resetEvent As New Threading.AutoResetEvent(False)
Dim calcDoneEvent As New Threading.AutoResetEvent(True)

. , RunWorkerAsync() BackgroundWorker ( ). "". BackgroundWorker .

, UI Calculate() , , CalculateBGW Calculate(), BackgroundWorker ( ). , Calculate(), AddHandler RemoveHandler.

, BackgroundWorker.ProgressChanged BackgroundWorker.ReportProgress.

BackgroundWorker.RunWorkerCompleted , , , . , DoEvents(). , , , .