`ThreadPool`

The VB.NET ThreadPool type allows us to create and handle many threads. We optimize when they run for the best performance.

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 two parameters: a WaitCallback instance, which is assigned to a subroutine address, and an argument of type Object.

Detail 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
4

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.

Detail 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
55

Output

The example 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.

As part of the System.Threading namespace, the ThreadPool type provides multiple threads support in the VB.NET language. ThreadPool simplifies thread management logic.