BackgroundWorker.CancelAsync Method (System.ComponentModel)

CancelAsync Method

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Microsoft Visual Studio 2008/.NET Framework 3.5

Other versions are also available for the following:

.NET Framework Class Library

BackgroundWorker..::.CancelAsync Method

Updated: November 2007

Requests cancellation of a pending background operation.

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

Syntax

Visual Basic (Declaration)

Public Sub CancelAsync

Visual Basic (Usage)

Dim instance As BackgroundWorker

instance.CancelAsync()

public void CancelAsync()

Visual C++

public:
void CancelAsync()

public void CancelAsync()

JScript

public function CancelAsync()

Exceptions

Exception	Condition
InvalidOperationException	WorkerSupportsCancellation is false.

Remarks

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:
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.

Caution:

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.

Examples

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.

Visual Basic

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

    // Disable the Cancel button.
    cancelAsyncButton.Enabled = false;
}

Visual C++

void cancelAsyncButton_Click( System::Object^ /*sender*/, System::EventArgs^ /*e*/ )
{  
   // Cancel the asynchronous operation.
   this->backgroundWorker1->CancelAsync();

   // Disable the Cancel button.
   cancelAsyncButton->Enabled = false;
}

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

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

Platforms

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

The .NET Framework and .NET Compact Framework do not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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