AsyncCompletedEventArgs.Cancelled 属性
重要
一些信息与预发行产品相关，相应产品在发行之前可能会进行重大修改。 对于此处提供的信息，Microsoft 不作任何明示或暗示的担保。
获取一个值，该值指示异步操作是否已被取消。
public:
property bool Cancelled { bool get(); };public bool Cancelled { get; }member this.Cancelled : boolPublic ReadOnly Property Cancelled As Boolean如果后台操作已被取消，则为 true；否则为 false。 默认值为 false。
下面的代码示例演示如何使用 AsyncOperation 来跟踪异步操作的生存期。 此代码示例是为 System.ComponentModel.AsyncOperationManager 类提供的一个更大示例的一部分。
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
当 Cancelled 属性为 true时，异步操作已中断。
客户端应用程序的事件处理程序委托应在访问派生自 AsyncCompletedEventArgs的类中的任何属性之前检查 Cancelled 属性;否则，如果异步操作中断，该属性将引发 InvalidOperationException 。
如果在派生类中提供只读属性，请确保调用 RaiseExceptionIfNecessary() 方法。 这可以防止客户端访问由于异步操作失败而可能无效的属性。
| 产品 | 版本 |
|---|---|
| .NET | Core 1.0, Core 1.1, Core 2.0, Core 2.1, Core 2.2, Core 3.0, Core 3.1, 5, 6, 7, 8, 9 |
| .NET Framework | 2.0, 3.0, 3.5, 4.0, 4.5, 4.5.1, 4.5.2, 4.6, 4.6.1, 4.6.2, 4.7, 4.7.1, 4.7.2, 4.8, 4.8.1 |
| .NET Standard | 2.0, 2.1 |
| UWP | 10.0 |
