WaitCallback Delegate

Represents a callback method to be executed by a thread pool thread.

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

public delegate void WaitCallback(
	Object state


Type: System.Object
An object that contains information to be used by the callback method.

WaitCallback represents a callback method that you want to execute on a ThreadPool thread. Create the delegate by passing your callback method to the WaitCallback constructor. Your method must have the signature shown in the Syntax section.

Queue your task for execution by passing the WaitCallback delegate to the ThreadPool.QueueUserWorkItem method. Your callback method executes when a thread pool thread becomes available.


Visual Basic and C# users can omit the WaitCallback constructor, because the compilers infer the delegate type automatically and supply the correct constructor. In Visual Basic, use the AddressOf operator when passing the callback method to QueueUserWorkItem.

If you want to pass information to your callback method, create an object that contains the necessary information and pass it to QueueUserWorkItem when you queue your task for execution. Every time your callback method executes, the state parameter contains this object.

For more information about how to use callbacks to synchronize thread pool threads, see The Managed Thread Pool.

The following example shows how to use the WaitCallback delegate to queue a task for execution by the thread pool. The code example uses the ThreadPool.QueueUserWorkItem(WaitCallback) method overload to queue a task, which is represented by a WaitCallback that wraps 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.

The example displays its output in a TextBlock on the UI thread. To access the TextBlock from the callback thread, the example uses the Dispatcher property to obtain a Dispatcher object for the TextBlock, and then uses the Dispatcher.BeginInvoke method to make the cross-thread call.

using System;
using System.Threading;

class Example
    // This is the UI element that receives the output from the example.
    private static System.Windows.Controls.TextBlock outputBlock;

    // The Demo method runs the example. It saves the TextBlock that is 
    // used for output, and sets up an event handler to start tasks.
    public static void Demo(System.Windows.Controls.TextBlock outputBlock)
        Example.outputBlock = outputBlock;
        outputBlock.Text = "Click here to start a background task.\r\n";

        // Set up an event handler to start a task when the TextBlock 
        // is clicked.
        outputBlock.MouseLeftButtonUp += HandleMouseUp;

    // Clicking the TextBlock queues a delegate to perform a task on a 
    // thread pool thread.
    private static void HandleMouseUp(object sender, 
                                      System.Windows.Input.MouseButtonEventArgs e)

        // Note: The stateInfo object for the task in this example is
        // Nothing, because this overload of QueueUserWorkItem does not
        // take a stateInfo parameter. 

    // This method performs the task, which is to append text to the
    // TextBlock. To communicate with objects on the UI thread, get the 
    // Dispatcher for one of the UI objects. Use the Dispatcher object's 
    // BeginInvoke method to queue a delegate that will run on the UI thread,
    // and therefore can safely access UI elements like the TextBlock.
    private static void AppendTextTask(object stateInfo)
        outputBlock.Dispatcher.BeginInvoke(delegate () {
            outputBlock.Text += "Hello from the thread pool.\r\n";

/* This code produces output similar to the following:

Click here to start a background task.
Hello from the thread pool.
Hello from the thread pool.
Hello from the thread pool.


Supported in: 5, 4, 3

Silverlight for Windows Phone

Supported in: Windows Phone OS 7.1, Windows Phone OS 7.0

XNA Framework

Supported in: Xbox 360, Windows Phone OS 7.0

For a list of the operating systems and browsers that are supported by Silverlight, see Supported Operating Systems and Browsers.