AsyncCompletedEventArgs.Cancelled AsyncCompletedEventArgs.Cancelled AsyncCompletedEventArgs.Cancelled AsyncCompletedEventArgs.Cancelled Property

定义

获取一个值,该值指示异步操作是否已被取消。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

属性值

如果后台操作已被取消,则为 true;否则为 falsetrue if the background operation has been canceled; otherwise false. 默认值为 falseThe default is false.

示例

下面的代码示例演示AsyncOperation如何使用跟踪异步操作的生存期。The following code example demonstrates the using an AsyncOperation to track the lifetime of asynchronous operations. 此代码示例是为System.ComponentModel.AsyncOperationManager类提供的更大示例的一部分。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

注解

当属性为true时, 异步操作中断。 CancelledWhen the Cancelled property is true, the asynchronous operation was interrupted.

客户端应用程序的事件处理程序委托应该在Cancelled访问派生自AsyncCompletedEventArgs的类中的任何属性之前检查属性; 否则, InvalidOperationException如果异步操作为, 则属性将引发。中断.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.

继承者说明

如果在派生类中提供只读属性, 请确保调用RaiseExceptionIfNecessary()方法。If you provide read-only properties in a derived class, be sure to call the RaiseExceptionIfNecessary() method. 这会阻止客户端访问由于异步操作中的错误而可能无效的属性。This prevents clients from accessing properties that are potentially not valid due to a failure in the asynchronous operation.

适用于

另请参阅