WaitHandle.WaitAll Runs before all Threads completed - VB.Net 4.0

Question

I'm working on a program that scans a list of servers for different information.

Everything is working fine except sometimes I get errors when the threads are done. Either by the scan completing or the cancel button stopping the loop but letting the current threads continue.

I see the UI say the scan is complete but the progressUpdate tries to run again. I can solve the problem by making a longer thread.sleep after the waitall.

For the most part it waits to the last thread but I get an error sometimes saying it can't access the progressUpdate function because OperationsComplete has already been run.

I get the error at BackgroundWorker1.ReportProgress(_completedCount, ScanResult) below but it shouldn't be calling that because WaitAll should wait until the threads are complete.

Private Sub ScanIsDone(ByVal ar As IAsyncResult)
    Dim d As PingDelegate = DirectCast(ar.AsyncState, PingDelegate)
    Dim ScanResult As ServerInfo = d.EndInvoke(ar)

    SyncLock (_lockObject)
        _completedCount = _completedCount + 1
        BackgroundWorker1.ReportProgress(_completedCount, ScanResult)
    End SyncLock
End Sub



Private Sub BackgroundWorker1_DoWork (ByVal sender As System.Object, ByVal e As System.ComponentModel.DoWorkEventArgs) Handles BackgroundWorker1.DoWork

    Dim servers As List(Of ServerInfo) = DirectCast(e.Argument, List(Of ServerInfo))
    Dim waitHandles As New List(Of WaitHandle)
    Dim waitHandles2 As New List(Of WaitHandle)
    Dim waitHandles3 As New List(Of WaitHandle)
    Dim waitHandles4 As New List(Of WaitHandle)

    For Each server As ServerInfo In servers
        _ThreadsOpen += 1
        _WaitCountAll += 1
        Dim d As New PingDelegate(AddressOf ScanServer)
        Dim ar As IAsyncResult = d.BeginInvoke(server, AddressOf ScanIsDone, d)
        Select Case _WaitCountAll
            Case 1 To 64
                waitHandles.Add(ar.AsyncWaitHandle)
            Case 65 To 128
                waitHandles2.Add(ar.AsyncWaitHandle)
            Case 129 To 192
                waitHandles3.Add(ar.AsyncWaitHandle)
            Case 193 To 256
                waitHandles4.Add(ar.AsyncWaitHandle)
        End Select

        While _ThreadsOpen > _ThreadMax - 1
            Thread.Sleep(200)
        End While
        If Cancel_Scan = True Then Exit For

    Next

    If waitHandles.Count <> 0 Then WaitHandle.WaitAll(waitHandles.ToArray())
    If waitHandles2.Count <> 0 Then WaitHandle.WaitAll(waitHandles2.ToArray())
    If waitHandles3.Count <> 0 Then WaitHandle.WaitAll(waitHandles3.ToArray())
    If waitHandles4.Count <> 0 Then WaitHandle.WaitAll(waitHandles4.ToArray())

    Thread.Sleep(1000)

End Sub

Edit The array is empty when i break it. But somehow something runs still. Maybe I am missing catching one.

Answer 1

Nearly all of the complexity in your code can be avoided by using the new Parallel.ForEach method in .Net 4. This will eliminate the need to maintain WaitHandles, as well as offering an easy mechanism to limit the maximum number of threads if you so choose.