What's the proper use of WaitOne() function - vb.net

Question

I was experimented some Thread pool examples. I've started from Fibonacci example on MSDN web site, but this wasn't suitable for more than 64 calculations, so i've resolved with this code:
Imports System.Threading
Module Module1
Public Class Fibonacci
Private _n As Integer
Private _fibOfN
Private _doneEvent As ManualResetEvent
Public ReadOnly Property N() As Integer
Get
Return _n
End Get
End Property
Public ReadOnly Property FibOfN() As Integer
Get
Return _fibOfN
End Get
End Property
Sub New(ByVal n As Integer, ByVal doneEvent As ManualResetEvent)
_n = n
_doneEvent = doneEvent
End Sub
' Wrapper method for use with the thread pool.
Public Sub ThreadPoolCallBackMar(ByVal threadContext As Object)
Dim threadIndex As Integer = CType(threadContext, Integer)
Console.WriteLine("thread {0} started...", threadIndex)
_fibOfN = Calculate(_n)
Console.WriteLine("thread {0} result calculated...", threadIndex)
_doneEvent.Set()
End Sub
Public Function Calculate(ByVal n As Integer) As Integer
If n <= 1 Then
Return n
End If
Return Calculate(n - 1) + Calculate(n - 2)
End Function
End Class
<MTAThread()>
Sub Main()
Const FibonacciCalculations As Integer = 65
' One event is used for each Fibonacci object
Dim doneEvents(FibonacciCalculations) As ManualResetEvent
Dim fibArray(FibonacciCalculations) As Fibonacci
Dim r As New Random()
' Configure and start threads using ThreadPool.
Console.WriteLine("launching {0} tasks...", FibonacciCalculations)
For i As Integer = 0 To FibonacciCalculations
doneEvents(i) = New ManualResetEvent(False)
Dim f = New Fibonacci(r.Next(20, 40), doneEvents(i))
fibArray(i) = f
ThreadPool.QueueUserWorkItem(AddressOf f.ThreadPoolCallBackMar, i)
Next
Console.WriteLine("All calculations are complete.")
For i As Integer = 0 To FibonacciCalculations
doneEvents(i).WaitOne()
Dim f As Fibonacci = fibArray(i)
Console.WriteLine("Fibonacci({0}) = {1}", f.N, f.FibOfN)
Next
Console.Read()
End Sub
End Module
The use of WaitOne() instead of WaitAll() resolve the problem but the question is: If I don't need to display the results then I don't need neither the second loop, but... without the second loop where I've to put the waitOne() function?

Answer 1

Your code does essentially this:
// start a bunch of threads to do calculations
Console.WriteLine("All calculations are complete."); // This is a lie!
// Wait for the threads to exit
The primary problem here is that the calculations are not complete when you make that call to Console.WriteLine. Well, they might be complete, but you don't know unless you've waited on the event to see that it's signaled.
The purpose of WaitOne is to tell you if the calculation has completed. Your code should be written like this:
For i As Integer = 0 To FibonacciCalculations
doneEvents(i) = New ManualResetEvent(False)
Dim f = New Fibonacci(r.Next(20, 40), doneEvents(i))
fibArray(i) = f
ThreadPool.QueueUserWorkItem(AddressOf f.ThreadPoolCallBackMar, i)
Next
Console.WriteLine("All calculations are started. Waiting for them to complete.")
For i As Integer = 0 To FibonacciCalculations
doneEvents(i).WaitOne()
Dim f As Fibonacci = fibArray(i)
Console.WriteLine("Fibonacci({0}) = {1}", f.N, f.FibOfN)
Next
Console.WriteLine("All calculations are complete.")
You must check the event to know that the calculation is complete.
Now, if you don't need to know if the calculation is complete, then there's no need for the WaitOne at all. And if you're not going to wait on the event, then there's no real need to have the event, is there? Although one wonders why you're going to do a calculation and then not use the result.