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BackgroundWorker.CancelAsync Method

  • Reference

Definition

Namespace:
System.ComponentModel
Assemblies:
System.dll, System.ComponentModel.EventBasedAsync.dll
Assembly:
System.ComponentModel.EventBasedAsync.dll
Assembly:
System.dll
Assembly:
netstandard.dll
Source:
BackgroundWorker.cs
Source:
BackgroundWorker.cs
Source:
BackgroundWorker.cs

Requests cancellation of a pending background operation.

public:
 void CancelAsync();
public void CancelAsync ();
member this.CancelAsync : unit -> unit
Public Sub CancelAsync ()

Exceptions

InvalidOperationException

WorkerSupportsCancellation is false.

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.

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

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

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

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.

Applies to

See also