AsyncCompletedEventArgs.Cancelled 屬性

定義

取得值,指出非同步作業是否已取消。Gets a value indicating whether an asynchronous operation has been canceled.

public:
 property bool Cancelled { bool get(); };
public bool Cancelled { get; }
member this.Cancelled : bool
Public ReadOnly Property Cancelled As Boolean

屬性值

如果背景作業已取消,則為 true,否則為 falsetrue if the background operation has been canceled; otherwise false. 預設為 falseThe default is false.

範例

下列程式碼範例將示範AsyncOperation如何使用來追蹤非同步作業的存留期。The following code example demonstrates the using an AsyncOperation to track the lifetime of asynchronous operations. 這個程式碼範例是針對System.ComponentModel.AsyncOperationManager類別提供之較大範例的一部分。This code example is part of a larger example provided for the System.ComponentModel.AsyncOperationManager class.

using System;
using System.Collections;
using System.Collections.Specialized;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Globalization;
using System.Threading;
using System.Windows.Forms;
Imports System.Collections
Imports System.Collections.Specialized
Imports System.ComponentModel
Imports System.Drawing
Imports System.Globalization
Imports System.Threading
Imports System.Windows.Forms
// This event handler updates the ListView control when the
// PrimeNumberCalculator raises the CalculatePrimeCompleted
// event. The ListView item is updated with the appropriate
// outcome of the calculation: Canceled, Error, or result.
private void primeNumberCalculator1_CalculatePrimeCompleted(
    object sender, 
    CalculatePrimeCompletedEventArgs e)
{
    Guid taskId = (Guid)e.UserState;

    if (e.Cancelled)
    {   
        string result = "Canceled";

        ListViewItem lvi = UpdateListViewItem(taskId, result);

        if (lvi != null)
        {
            lvi.BackColor = Color.Pink;
            lvi.Tag = null;
        }
    }
    else if (e.Error != null)
    {
        string result = "Error";

        ListViewItem lvi = UpdateListViewItem(taskId, result);

        if (lvi != null)
        {
            lvi.BackColor = Color.Red;
            lvi.ForeColor = Color.White;
            lvi.Tag = null;
        }
    }
    else
    {   
        bool result = e.IsPrime;

        ListViewItem lvi = UpdateListViewItem(
            taskId, 
            result, 
            e.FirstDivisor);

        if (lvi != null)
        {
            lvi.BackColor = Color.LightGray;
            lvi.Tag = null;
        }
    }
}
' This event handler updates the ListView control when the
' PrimeNumberCalculator raises the CalculatePrimeCompleted
' event. The ListView item is updated with the appropriate
' outcome of the calculation: Canceled, Error, or result.
Private Sub primeNumberCalculator1_CalculatePrimeCompleted( _
    ByVal sender As Object, _
    ByVal e As CalculatePrimeCompletedEventArgs) _
    Handles primeNumberCalculator1.CalculatePrimeCompleted

    Dim taskId As Guid = CType(e.UserState, Guid)

    If e.Cancelled Then
        Dim result As String = "Canceled"

        Dim lvi As ListViewItem = UpdateListViewItem( _
            taskId, _
            result)

        If (lvi IsNot Nothing) Then
            lvi.BackColor = Color.Pink
            lvi.Tag = Nothing
        End If

    ElseIf e.Error IsNot Nothing Then

        Dim result As String = "Error"

        Dim lvi As ListViewItem = UpdateListViewItem( _
            taskId, result)

        If (lvi IsNot Nothing) Then
            lvi.BackColor = Color.Red
            lvi.ForeColor = Color.White
            lvi.Tag = Nothing
        End If
    Else
        Dim result As Boolean = e.IsPrime

        Dim lvi As ListViewItem = UpdateListViewItem( _
            taskId, _
            result, _
            e.FirstDivisor)

        If (lvi IsNot Nothing) Then
            lvi.BackColor = Color.LightGray
            lvi.Tag = Nothing
        End If
    End If

End Sub

備註

當屬性為時true, 非同步作業就會中斷。 CancelledWhen the Cancelled property is true, the asynchronous operation was interrupted.

用戶端應用程式的事件處理常式委派應檢查Cancelled屬性, 然後才存取衍生自之AsyncCompletedEventArgs類別中的任何屬性; 否則, InvalidOperationException如果非同步作業為, 則屬性會引發。會.The client application's event-handler delegate should check the Cancelled property before accessing any properties in a class derived from AsyncCompletedEventArgs; otherwise, the property will raise an InvalidOperationException if the asynchronous operation was interrupted.

給繼承者的注意事項

如果您在衍生類別中提供唯讀屬性, 請務必呼叫RaiseExceptionIfNecessary()方法。If you provide read-only properties in a derived class, be sure to call the RaiseExceptionIfNecessary() method. 這可防止用戶端因為非同步作業失敗而存取可能不正確屬性。This prevents clients from accessing properties that are potentially not valid due to a failure in the asynchronous operation.

適用於

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