AsyncCompletedEventArgs.Error 屬性

定義

命名空間:

System.ComponentModel

組件:

System.ComponentModel.EventBasedAsync.dll

來源:

AsyncCompletedEvent.cs

取得值，指出非同步作業期間是否發生錯誤。

public:
 property Exception ^ Error { Exception ^ get(); };

public Exception? Error { get; }

member this.Error : Exception

Public ReadOnly Property Error As Exception

屬性值

Exception

如果非同步作業期間發生錯誤，則為 Exception 執行個體，否則為 null。

範例

下列程式碼範例示範如何使用 AsyncOperation 來追蹤非同步作業的存留期。 此程式碼範例是提供給 類別之較大範例的 System.ComponentModel.AsyncOperationManager 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

備註

如果在非同步作業期間引發例外狀況，類別會將例外狀況指派給 Error 屬性。 用戶端應用程式的事件處理常式委派應該先檢查 Error 屬性，再存取衍生自 AsyncCompletedEventArgs 之類別中的任何屬性;否則，屬性會引發 TargetInvocationException ，其 InnerException 屬性會保存 的 Error 參考。

Error如果作業已取消，則屬性值為 null 。

給繼承者的注意事項

如果您在衍生類別中提供唯讀屬性，請務必在屬性實作中呼叫 RaiseExceptionIfNecessary() 方法。 這可防止用戶端存取因非同步作業失敗而可能不正確屬性。

另請參閱

• AsyncCompletedEventHandler
• AsyncOperationManager
• AsyncOperation
• 實作事件架構非同步模式