QueueUserWorkItem Method (WaitCallback)

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


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 could not be queued.

Exception Condition

callBack is null.


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

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.


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 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.)

        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.");
// The example displays output like the following:
//       Main thread does some work, then sleeps.
//       Hello from the thread pool.
//       Main thread exits.

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