AsyncCompletedEventArgs.Error Property
Definition
Important
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Gets a value indicating which error occurred during an asynchronous operation.
public:
property Exception ^ Error { Exception ^ get(); };public Exception Error { get; }public Exception? Error { get; }member this.Error : ExceptionPublic ReadOnly Property Error As ExceptionProperty Value
An Exception instance, if an error occurred during an asynchronous operation; otherwise null.
Examples
The following code example demonstrates using an AsyncOperation to track the lifetime of asynchronous operations. 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
Remarks
If an exception is raised during an asynchronous operation, the class will assign the exception to the Error property. The client application's event-handler delegate should check the Error property before accessing any properties in a class derived from AsyncCompletedEventArgs; otherwise, the property will raise a TargetInvocationException with its InnerException property holding a reference to Error.
The value of the Error property is null if the operation was canceled.
Notes to Inheritors
If you provide read-only properties in a derived class, be sure to call the RaiseExceptionIfNecessary() method in your property implementation. This prevents clients from accessing properties that are potentially not valid due to a failure in the asynchronous operation.
