ThreadPool.QueueUserWorkItem Method (WaitCallback, Object)

 

Queues a method for execution, and specifies an object containing data to be used by the method. The method executes when a thread pool thread becomes available.

Namespace:   System.Threading
Assembly:  mscorlib (in mscorlib.dll)

Public Shared Function QueueUserWorkItem (
	callBack As WaitCallback,
	state As Object
) As Boolean

Parameters

callBack
Type: System.Threading.WaitCallback

A WaitCallback representing the method to execute.

state
Type: System.Object

An object containing data to be used by the method.

Return Value

Type: System.Boolean

true if the method is successfully queued; NotSupportedException is thrown if the work item could not be queued.

Exception Condition
NotSupportedException

The common language runtime (CLR) is hosted, and the host does not support this action.

ArgumentNullException

callBack is null.

If the callback method requires complex data, you can define a class to contain the data.

System_CAPS_noteNote

Visual Basic users can omit the WaitCallback constructor, and simply use the AddressOf operator when passing the callback method to QueueUserWorkItem. Visual Basic automatically calls the correct delegate constructor.

In the .NET Framework version 2.0, the Thread.CurrentPrincipal property value is propagated to worker threads queued using the QueueUserWorkItem method. In earlier versions, the principal information is not propagated.

The following example shows how to create an object that contains task information. It also demonstrates how to pass that object to a task that is queued for execution by the thread pool.

' This example shows how to create an object containing task
' information, and pass that object to a task queued for
' execution by the thread pool.
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

    <MTAThread> _
    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.
        ThreadPool.QueueUserWorkItem( _
                New WaitCallback(AddressOf ThreadProc), ti)

        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.")
    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
' The example displays output like the following:
'        Main thread does some work, then sleeps.
'        This report displays the number 42.
'        Main thread exits.

.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Silverlight
Available since 2.0
Windows Phone Silverlight
Available since 7.0
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