BackgroundWorker.CancellationPending Свойство

Определение

Пространство имен:

System.ComponentModel

Сборка:

System.ComponentModel.EventBasedAsync.dll

Исходный код:

BackgroundWorker.cs

Возвращает значение, показывающее, запросило ли приложение отмену фоновой операции.

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

public bool CancellationPending { get; }

member this.CancellationPending : bool

Public ReadOnly Property CancellationPending As Boolean

Значение свойства

Boolean

Значение true, если приложение запросило отмену фоновой операции; в противном случае — значение false. Значение по умолчанию — false.

Примеры

В следующем примере кода показано использование свойства для CancellationPending запроса BackgroundWorker о состоянии отмены. Этот пример входит в состав более крупного примера использования класса BackgroundWorker.

// 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
{   
    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 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

Комментарии

Если CancellationPending имеет значение true, метод CancelAsync был вызван для BackgroundWorker.

Это свойство предназначено для использования рабочим потоком, который должен периодически проверка CancellationPending и прерывать фоновую операцию, если для нее задано значение true.

См. также раздел

• Практическое руководство. Фоновое выполнение операции
• Практическое руководство. Фоновая загрузка файла