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BackgroundWorker.CancelAsync Method

Requests cancellation of a pending background operation.

Namespace: System.ComponentModel
Assembly: System (in system.dll)

'Declaration
Public Sub CancelAsync
'Usage
Dim instance As BackgroundWorker

instance.CancelAsync
public void CancelAsync ()
public function CancelAsync ()
Not applicable.

CancelAsync submits a request to terminate the pending background operation and sets the CancellationPending property to true.

When you call CancelAsync, your worker method has an opportunity to stop its execution and exit. The worker code should periodically check the CancellationPending property to see if it has been set to true.

Caution noteCaution:

Be aware that your code in the DoWork event handler may finish its work as a cancellation request is being made, and your polling loop may miss CancellationPending being set to true. In this case, the Cancelled flag of System.ComponentModel.RunWorkerCompletedEventArgs in your RunWorkerCompleted event handler will not be set to true, even though a cancellation request was made. This situation is called a race condition and is a common concern in multithreaded programming. For more information about multithreading design issues, see Managed Threading Best Practices.

The following code example demonstrates the use of the CancelAsync method to cancel an asynchronous ("background") operation. This code example is part of a larger example provided for the BackgroundWorker class.

Private Sub cancelAsyncButton_Click( _
ByVal sender As System.Object, _
ByVal e As System.EventArgs) _
Handles cancelAsyncButton.Click
    
    ' Cancel the asynchronous operation.
    Me.backgroundWorker1.CancelAsync()

    ' Disable the Cancel button.
    cancelAsyncButton.Enabled = False
    
End Sub 'cancelAsyncButton_Click

private void cancelAsyncButton_Click(Object sender, System.EventArgs e)
{   
    // Cancel the asynchronous operation.
    this.backgroundWorker1.CancelAsync();

    // Disable the Cancel button.
    cancelAsyncButton.set_Enabled(false);
}

Windows 98, Windows Server 2000 SP4, Windows Millennium Edition, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0

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