BackgroundWorker.RunWorkerCompleted 事件
重要
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当后台操作已完成、被取消或引发异常时发生。
public:
event System::ComponentModel::RunWorkerCompletedEventHandler ^ RunWorkerCompleted;public event System.ComponentModel.RunWorkerCompletedEventHandler RunWorkerCompleted;public event System.ComponentModel.RunWorkerCompletedEventHandler? RunWorkerCompleted;member this.RunWorkerCompleted : System.ComponentModel.RunWorkerCompletedEventHandler Public Custom Event RunWorkerCompleted As RunWorkerCompletedEventHandler 下面的代码示例演示如何使用 RunWorkerCompleted 事件来处理异步操作的结果。 此代码示例是为 BackgroundWorker 类提供的一个更大示例的一部分。
// This event handler deals with the results of the
// background operation.
void backgroundWorker1_RunWorkerCompleted( Object^ /*sender*/, RunWorkerCompletedEventArgs^ e )
{
// First, handle the case where an exception was thrown.
if ( e->Error != nullptr )
{
MessageBox::Show( e->Error->Message );
}
else
if ( e->Cancelled )
{
// Next, handle the case where the user cancelled
// the operation.
// Note that due to a race condition in
// the DoWork event handler, the Cancelled
// flag may not have been set, even though
// CancelAsync was called.
resultLabel->Text = "Cancelled";
}
else
{
// Finally, handle the case where the operation
// succeeded.
resultLabel->Text = e->Result->ToString();
}
// Enable the UpDown control.
this->numericUpDown1->Enabled = true;
// Enable the Start button.
startAsyncButton->Enabled = true;
// Disable the Cancel button.
cancelAsyncButton->Enabled = false;
}
// This event handler deals with the results of the
// background operation.
private void backgroundWorker1_RunWorkerCompleted(
object sender, RunWorkerCompletedEventArgs e)
{
// First, handle the case where an exception was thrown.
if (e.Error != null)
{
MessageBox.Show(e.Error.Message);
}
else if (e.Cancelled)
{
// Next, handle the case where the user canceled
// the operation.
// Note that due to a race condition in
// the DoWork event handler, the Cancelled
// flag may not have been set, even though
// CancelAsync was called.
resultLabel.Text = "Canceled";
}
else
{
// Finally, handle the case where the operation
// succeeded.
resultLabel.Text = e.Result.ToString();
}
// Enable the UpDown control.
this.numericUpDown1.Enabled = true;
// Enable the Start button.
startAsyncButton.Enabled = true;
// Disable the Cancel button.
cancelAsyncButton.Enabled = false;
}
' This event handler deals with the results of the
' background operation.
Private Sub backgroundWorker1_RunWorkerCompleted( _
ByVal sender As Object, ByVal e As RunWorkerCompletedEventArgs) _
Handles backgroundWorker1.RunWorkerCompleted
' First, handle the case where an exception was thrown.
If (e.Error IsNot Nothing) Then
MessageBox.Show(e.Error.Message)
ElseIf e.Cancelled Then
' Next, handle the case where the user canceled the
' operation.
' Note that due to a race condition in
' the DoWork event handler, the Cancelled
' flag may not have been set, even though
' CancelAsync was called.
resultLabel.Text = "Canceled"
Else
' Finally, handle the case where the operation succeeded.
resultLabel.Text = e.Result.ToString()
End If
' Enable the UpDown control.
Me.numericUpDown1.Enabled = True
' Enable the Start button.
startAsyncButton.Enabled = True
' Disable the Cancel button.
cancelAsyncButton.Enabled = False
End Sub
当事件处理程序返回时， DoWork 将引发此事件。
如果操作成功完成并在事件处理程序中 DoWork 分配了其结果，则可以通过 属性访问结果 RunWorkerCompletedEventArgs.Result 。
Error的 System.ComponentModel.RunWorkerCompletedEventArgs 属性指示操作引发了异常。
Cancelled的 System.ComponentModel.RunWorkerCompletedEventArgs 属性指示取消请求是否由后台操作处理。 如果事件处理程序中的DoWork代码通过检查 CancellationPending 标志并将 的 System.ComponentModel.DoWorkEventArgs 标志设置为 Canceltrue来检测到取消请求，则 Cancelled 的 System.ComponentModel.RunWorkerCompletedEventArgs 标志也将设置为 true。
注意
请注意，事件处理程序中的 DoWork 代码可能会在发出取消请求时完成其工作，并且轮询循环可能会错过 CancellationPending 设置为 true。 在这种情况下，即使发出了取消请求，Cancelled事件处理程序中的 RunWorkerCompleted 标志System.ComponentModel.RunWorkerCompletedEventArgs也不会设置为 true。 这种情况称为 争用条件 ，是多线程编程中常见的问题。 有关多线程设计问题的详细信息，请参阅 托管线程处理最佳做法。
在RunWorkerCompleted访问 RunWorkerCompletedEventArgs.Result 属性之前，AsyncCompletedEventArgs.Error事件处理程序应始终检查 和 AsyncCompletedEventArgs.Cancelled 属性。 如果引发异常或取消操作，则 RunWorkerCompletedEventArgs.Result 访问 属性将引发异常。
| 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 |
