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ThreadPool.QueueUserWorkItem Method (WaitCallback, Object)

Updated: August 2009

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 static bool QueueUserWorkItem(
	WaitCallback callBack,
	Object state


Type: System.Threading.WaitCallback

A WaitCallback representing the method to execute.

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.


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


callBack is null.

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


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 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. 
using System;
using 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 string Boilerplate;
    public int Value;

    // Public constructor provides an easy way to supply all 
    // the information needed for the task. 
    public TaskInfo(string text, int number) {
        Boilerplate = text;
        Value = number;

public class Example {
    public static void Main()
        // Create an object containing the information needed 
        // for the task.
        TaskInfo ti = new TaskInfo("This report displays the number {0}.", 42);

        // Queue the task and data.
        ThreadPool.QueueUserWorkItem(new WaitCallback(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.)

        Console.WriteLine("Main thread exits.");

    // The thread procedure performs the independent task, in this case 
    // formatting and printing a very simple report. 
    static void ThreadProc(Object stateInfo) 
        TaskInfo ti = (TaskInfo) stateInfo;
        Console.WriteLine(ti.Boilerplate, ti.Value); 

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




August 2009

Removed an extraneous if statement from the example.

Customer feedback.