BackgroundWorker.RunWorkerCompleted 事件

定义

命名空间:

System.ComponentModel

程序集:

System.ComponentModel.EventBasedAsync.dll

Source:

BackgroundWorker.cs

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;

member this.RunWorkerCompleted : System.ComponentModel.RunWorkerCompletedEventHandler 

Public Custom Event RunWorkerCompleted As RunWorkerCompletedEventHandler 

事件类型

RunWorkerCompletedEventHandler

示例

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

注解

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。

另请参阅

• 如何：在后台运行操作
• 如何：在后台下载文件