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ThreadPool.QueueUserWorkItem Method (WaitCallback)

Queues a method for execution. The method executes when a thread pool thread becomes available.

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

public static bool QueueUserWorkItem(
	WaitCallback callBack
)

Parameters

callBack
Type: System.Threading.WaitCallback

A WaitCallback that represents the method to be executed.

Return Value

Type: System.Boolean
true if the method is successfully queued; NotSupportedException is thrown if the work item is not queued.

ExceptionCondition
NotSupportedException

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

ArgumentNullException

callBack is null.

You can place data required by the queued method in the instance fields of the class in which the method is defined, or you can use the QueueUserWorkItem(WaitCallback, Object) overload that accepts an object containing the necessary data.

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.

Version Information

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 uses the QueueUserWorkItem(WaitCallback) method overload to queue a task, which is represented by the ThreadProc method, to execute when a thread becomes available. No task information is supplied with this overload. Therefore, the information that is available to the ThreadProc method is limited to the object the method belongs to.

using System;
using System.Threading;
public class Example {
    public static void Main() {
        // Queue the task.
        ThreadPool.QueueUserWorkItem(new WaitCallback(ThreadProc));

        Console.WriteLine("Main thread does some work, then sleeps.");
        // If you comment out the Sleep, the main thread exits before 
        // the thread pool task runs.  The thread pool 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.");
    }

    // This thread procedure performs the task. 
    static void ThreadProc(Object stateInfo) {
        // No state object was passed to QueueUserWorkItem, so  
        // stateInfo is null.
        Console.WriteLine("Hello from the thread pool.");
    }
}

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

.NET Framework

Supported in: 3.5, 3.0, 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 3.5, 2.0, 1.0

XNA Framework

Supported in: 3.0, 2.0, 1.0

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