Threading Exception: The number of WaitHandles must be less than or equal to 64

Question

The title is to make this easy to find for others having this error. I'm new to Threading, so this is really giving me heck. I'm getting this runtime error that crashed Cassini. This is code that I'm maintaining originally developed as a website project in VS 2003 and converted to a VS 2008 website project.

Important Info:

• The number of objects in the manualEvents array is 128 in this case.
• products is an array of Strings
• Need to support .NET 2.0

For Each product As String In products
    If Not product.Trim().ToUpper().EndsWith("OBSOLETE") Then
        calls += 1
    End If
Next
Dim results(calls - 1) As DownloadResults
'Dim manualEvents(calls - 1) As Threading.ManualResetEvent '128 objects in this case.
Dim manualEvents(0) As Threading.ManualResetEvent
manualEvents(0) = New Threading.ManualResetEvent(False)
'NOTE: I don't think this will work because what is not seen here, is that
'    this code is being used to populate and cache a long list of products,
'    each with their own category, etc. Am I misunderstanding something?

'initialize results structures
'spawn background workers
calls = 0
For Each product As String In products
    If Not product.Trim().ToUpper().EndsWith("OBSOLETE") Then
        Dim result As New DownloadResults

        'manualEvents(calls) = New Threading.ManualResetEvent(False)
        'Moved above For Each after declaration of variable

        result.params.product = product
        result.params.category = docType
        'result.ManualEvent = manualEvents(calls)
        result.ManualEvent = manualEvents(0)
        result.Context = Me._context

        results(calls) = result

        Threading.ThreadPool.QueueUserWorkItem(AddressOf ProcessSingleCategoryProduct, results(calls))
        Threading.Interlocked.Increment(calls) 'Replaces below incrementation

        'calls += 1
    End If
Next

Threading.WaitHandle.WaitAll(manualEvents) 'CRASHES HERE

Thread Helper Function (for the sake of completion)

Public Shared Sub ProcessSingleCategoryProduct(ByVal state As Object)
    Dim drs As DownloadResults = CType(state, DownloadResults)
    Dim adc As New cADCWebService(drs.Context)

    drs.docs = adc.DownloadADC(drs.params.category, drs.params.product)
    drs.ManualEvent.Set()
End Sub

Answer 1

You don't need an array of 128 manual events to check for completion of all 128 threads.

Create only one manual reset event and a plain integer starting at 128. Decrement that integer using Interlocked.Decrement at the end of ProcessSingleCategoryProduct, and only signal the event when the count reaches zero:

if (Interlocked.Decrement(ByRef myCounter) = 0) myEvent.Set();

Then declare only one Threading.ManualResetEvent as opposed to an array of them, and you can call WaitOne instead of WaitAll on it, and you are done.

See also usr's comment for an easier alternative in case you have .NET 4.