Imports System
Imports System.Threading
' TaskInfo holds state information for a task that will be
' executed by a ThreadPool thread.Public Class TaskInfo
    ' State information for the task.These members
    ' can be implemented as read-only properties, read/write
    ' properties with validation, and so on, as required.Public Boilerplate As String
    Public Value As Integer

    ' Public constructor provides an easy way to supply all
    ' the information needed for the task.Public Sub New(text As String, number As Integer)
        Boilerplate = text
        Value = number
    End Sub
End Class

Public Class Example
    Public Shared Sub Main()
        ' Create an object containing the information needed
        ' for the task.Dim ti As New TaskInfo("This report displays the number {0}.", 42)

        ' Queue the task and data.If ThreadPool.QueueUserWorkItem( _
            New WaitCallback(AddressOf ThreadProc), ti) Then
        
            Console.WriteLine("Main thread does some work, then sleeps.")

            ' If you comment out the Sleep, the main thread exits before
            ' the ThreadPool task has a chance to run.ThreadPool uses 
            ' background threads, which do not keep the application 
            ' running.(This is a simple example of a race condition.)
            Thread.Sleep(1000)

            Console.WriteLine("Main thread exits.")
        Else
            Console.WriteLine("Unable to queue ThreadPool request.")
        End If
    End Sub

    ' The thread procedure performs the independent task, in this case
    ' formatting and printing a very simple report.
    'Shared Sub ThreadProc(stateInfo As Object)
        Dim ti As TaskInfo = CType(stateInfo, TaskInfo)
        Console.WriteLine(ti.Boilerplate, ti.Value)
    End Sub
End Class

Imports System
Imports System.Threading

' TaskInfo contains data that will be passed to the callback
' method.Public Class TaskInfo
    public Handle As RegisteredWaitHandle = Nothing
    public OtherInfo As String = "default"
End Class

Public Class Example
    Public Shared Sub Main()
        ' The main thread uses AutoResetEvent to signal the
        ' registered wait handle, which executes the callback
        ' method.Dim ev As New AutoResetEvent(false)

        Dim ti As New TaskInfo()
        ti.OtherInfo = "First task"
        ' The TaskInfo for the task includes the registered wait
        ' handle returned by RegisterWaitForSingleObject.This
        ' allows the wait to be terminated when the object has
        ' been signaled once (see WaitProc).ti.Handle = ThreadPool.RegisterWaitForSingleObject( _
            ev, _
            New WaitOrTimerCallback(AddressOf WaitProc), _
            ti, _
            1000, _
            false _
        )

        ' The main thread waits about three seconds, to demonstrate 
        ' the time-outs on the queued task, and then signals.Thread.Sleep(3100)
        Console.WriteLine("Main thread signals.")
        ev.Set()

        ' The main thread sleeps, which should give the callback
        ' method time to execute.If you comment out this line, the
        ' program usually ends before the ThreadPool thread can execute.Thread.Sleep(1000)
        ' If you start a thread yourself, you can wait for it to end
        ' by calling Thread.Join.This option is not available with 
        ' thread pool threads.End Sub
   
    ' The callback method executes when the registered wait times out,
    ' or when the WaitHandle (in this case AutoResetEvent) is signaled.
    'WaitProc unregisters the WaitHandle the first time the event is 
    ' signaled.Public Shared Sub WaitProc(state As Object, timedOut As Boolean)
        ' The state object must be cast to the correct type, because the
        ' signature of the WaitOrTimerCallback delegate specifies type
        ' Object.Dim ti As TaskInfo = CType(state, TaskInfo)

        Dim cause As String = "TIMED OUT"
        If Not timedOut Then
            cause = "SIGNALED"
            ' If the callback method executes because the WaitHandle is
            ' signaled, stop future execution of the callback method
            ' by unregistering the WaitHandle.If Not ti.Handle Is Nothing Then
                ti.Handle.Unregister(Nothing)
            End If
        End If 

        Console.WriteLine("WaitProc( {0} ) executes on thread {1}; cause = {2}.", _
            ti.OtherInfo, _
            Thread.CurrentThread.GetHashCode().ToString(), _
            cause _
        )
    End Sub
End Class