ThreadPoolUse the ThreadPool type from System.Threading. Pass a WaitCallback argument.
This page was last reviewed on Sep 29, 2022.
ThreadPool handles many threads. This VB.NET type allows us to easily manage these threads. We optimize when they run for the best performance.
Type notes. With ThreadPool, we account for multiple cores and processors. ThreadPool eliminates our need to implement complicated logic that allocates threads.
Example. We use QueueUserWorkItem. This function receives 2 parameters: a WaitCallback instance, which is assigned to a subroutine address, and an argument of type Object.
AddressOf Please use the AddressOf operator and specify the method name as the first argument.
Tip You can pass anything as the second argument. It will be cast to an Object, which you can then recast in the specified method.
Tip 2 Pay attention to how the WaitCallback is passed as an argument. This syntax is a bit confusing.
Info This program creates a thread in the ThreadPool and executes the code. It passes an argument to the threaded method.
Imports System.Threading Module Module1 Sub Main() ' Use this argument to the thread. Dim t As String = "test" ThreadPool.QueueUserWorkItem(New WaitCallback(AddressOf Process), t) ' Wait a while. Thread.Sleep(50) End Sub Sub Process(ByVal arg As Object) ' Cast the argument and display its length. Dim t As String = arg Console.WriteLine(t.Length) End Sub End Module
Example 2. This ThreadPool example builds on our understanding of the thread pool. Two fields are used—an Integer is accessed by the threaded methods.
And The second is an Object that is used by the Monitor.Enter and Monitor.Exit subroutines.
First A loop creates 10 threaded calls to the subroutine Process. In Process we recast the argument. We call Monitor.Enter and Monitor.Exit.
Tip The monitor calls ensure that no two threads will mutate the Integer at once.
Important This example demonstrates the use of QueueUserWorkItem and it further shows the Monitor.Enter and Monitor.Exit methods.
Imports System.Threading Module Module1 Private _int As Integer Private _obj As Object = New Object Sub Main() ' WaitCallback instance. Dim w As WaitCallback = New WaitCallback(AddressOf Process) ' Loop over these values. For var As Integer = 0 To 10 ThreadPool.QueueUserWorkItem(w, var) Next ' Wait a while. Thread.Sleep(50) ' Write integer. Console.WriteLine(_int) End Sub Sub Process(ByVal arg As Object) ' Cast the argument and display it. Dim t As Integer = arg ' Lock the integer. Monitor.Enter(_obj) ' Change value. _int += t ' Unlock. Monitor.Exit(_obj) End Sub End Module
Program output. The program prints 55, the sum of all numbers in the range of 0 to 10. It did this by using 10 threaded method calls.
Note On the ThreadPool, these could be run on any number of processors. For a more complex calculation, this could be useful.
BackgroundWorker. Often we prefer the BackgroundWorker type over the ThreadPool type. The BackgroundWorker seems easier to use and overall more intuitive.
Note For threaded programs, I almost always reach for BackgroundWorker because it is very reliable and less confusing for novices.
Summary. As part of the System.Threading namespace, the ThreadPool type provides multiple threads support in the VB.NET language. ThreadPool simplifies thread management logic.
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This page was last updated on Sep 29, 2022 (rewrite).
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