WaitCallback Delegate

Parameters

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

Imports System.Threading

Class Example

    ' This is the UI element that receives the output from the example.
    Private Shared outputBlock As System.Windows.Controls.TextBlock

    ' 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 Shared Sub Demo(ByVal outputBlock As System.Windows.Controls.TextBlock)

        Example.outputBlock = outputBlock
        outputBlock.Text = "Click here to start a background task." & vbCrLf

        ' Set up an event handler to start a task when the TextBlock 
        ' is clicked.
        AddHandler outputBlock.MouseLeftButtonUp, AddressOf HandleMouseUp

    End Sub

    ' Clicking the TextBlock queues a delegate to perform a task on a 
    ' thread pool thread.
    Private Shared Sub HandleMouseUp(ByVal sender As Object, _
                          ByVal e As System.Windows.Input.MouseButtonEventArgs) 

        ThreadPool.QueueUserWorkItem(AddressOf AppendTextTask)

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

    ' 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. In 
    ' this example, the delegate is stored in the field named 'append'.
    Private Shared Sub AppendTextTask(ByVal stateInfo As Object)

        outputBlock.Dispatcher.BeginInvoke(append, "Hello from the thread pool." & vbCrLf)

    End Sub

    ' Invoking this delegate runs the AppendText method.
    Private Shared append As New Action(Of String)(AddressOf AppendText) 

    ' The AppendText method can safely access UI elements because it is run
    ' on the UI thread, by using the Dispatcher.BeginInvoke method.
    Private Shared Sub AppendText(ByVal text As String) 

        outputBlock.Text &= text

    End Sub
End Class 

' 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.

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)
    {
        ThreadPool.QueueUserWorkItem(AppendTextTask);

        // 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.
 */