BackgroundWorker.CancellationPending Proprietà

Definizione

Spazio dei nomi:

System.ComponentModel

Assembly:

netstandard.dll, System.ComponentModel.EventBasedAsync.dll

Origine:

BackgroundWorker.cs

Ottiene un valore che indica se l'applicazione ha richiesto l'annullamento di un'operazione in background.

public:
 property bool CancellationPending { bool get(); };

public bool CancellationPending { get; }

member this.CancellationPending : bool

Public ReadOnly Property CancellationPending As Boolean

Valore della proprietà

Boolean

true se l'applicazione ha richiesto l'annullamento di un'operazione in background; in caso contrario, false. Il valore predefinito è false.

Esempio

Nell'esempio di codice seguente viene illustrato l'uso della proprietà per eseguire una query sullo stato di CancellationPending annullamento di un BackgroundWorker oggetto . Questo esempio di codice fa parte di un esempio più ampio fornito per la BackgroundWorker classe .

// Abort the operation if the user has cancelled.
// Note that a call to CancelAsync may have set 
// CancellationPending to true just after the
// last invocation of this method exits, so this 
// code will not have the opportunity to set the 
// DoWorkEventArgs.Cancel flag to true. This means
// that RunWorkerCompletedEventArgs.Cancelled will
// not be set to true in your RunWorkerCompleted
// event handler. This is a race condition.
if ( worker->CancellationPending )
{
   e->Cancel = true;
}
else
{
   if ( n < 2 )
   {
      result = 1;
   }
   else
   {
      result = ComputeFibonacci( n - 1, worker, e ) + ComputeFibonacci( n - 2, worker, e );
   }

   // Report progress as a percentage of the total task.
   int percentComplete = (int)((float)n / (float)numberToCompute * 100);
   if ( percentComplete > highestPercentageReached )
   {
      highestPercentageReached = percentComplete;
      worker->ReportProgress( percentComplete );
   }
}

// Abort the operation if the user has canceled.
// Note that a call to CancelAsync may have set 
// CancellationPending to true just after the
// last invocation of this method exits, so this 
// code will not have the opportunity to set the 
// DoWorkEventArgs.Cancel flag to true. This means
// that RunWorkerCompletedEventArgs.Cancelled will
// not be set to true in your RunWorkerCompleted
// event handler. This is a race condition.

if (worker.CancellationPending)
{
    e.Cancel = true;
}
else
{
    result = n < 2
        ? 1
        : ComputeFibonacci(n - 1, worker, e) +
                 ComputeFibonacci(n - 2, worker, e);

    // Report progress as a percentage of the total task.
    int percentComplete =
        (int)(n / (float)numberToCompute * 100);
    if (percentComplete > highestPercentageReached)
    {
        highestPercentageReached = percentComplete;
        worker.ReportProgress(percentComplete);
    }
}

' Abort the operation if the user has canceled.
' Note that a call to CancelAsync may have set 
' CancellationPending to true just after the
' last invocation of this method exits, so this 
' code will not have the opportunity to set the 
' DoWorkEventArgs.Cancel flag to true. This means
' that RunWorkerCompletedEventArgs.Cancelled will
' not be set to true in your RunWorkerCompleted
' event handler. This is a race condition.
If worker.CancellationPending Then
    e.Cancel = True
Else
    If n < 2 Then
        result = 1
    Else
        result = ComputeFibonacci(n - 1, worker, e) +
                 ComputeFibonacci(n - 2, worker, e)
    End If

    ' Report progress as a percentage of the total task.
    Dim percentComplete As Integer =
        CSng(n) / CSng(numberToCompute) * 100
    If percentComplete > highestPercentageReached Then
        highestPercentageReached = percentComplete
        worker.ReportProgress(percentComplete)
    End If

End If

Commenti

Se CancellationPending è true, il CancelAsync metodo è stato chiamato su BackgroundWorker.

Questa proprietà è destinata all'uso da parte del thread di lavoro, che deve controllare CancellationPending periodicamente e interrompere l'operazione in background quando è impostata su true.

Vedi anche

• Come eseguire un'operazione in background
• Procedura: Scaricare un file in background