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)

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 namespace System;
using namespace System::Threading;

ref class Example

   // This thread procedure performs the task.
   static void ThreadProc(Object^ stateInfo)

      // No state object was passed to QueueUserWorkItem, so stateInfo is 0.
      Console::WriteLine( "Hello from the thread pool." );

int main()
   // Queue the task.
   ThreadPool::QueueUserWorkItem(gcnew WaitCallback(Example::ThreadProc));

   Console::WriteLine("Main thread does some work, then sleeps.");

   Console::WriteLine("Main thread exits.");
   return 0;
// 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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