BackgroundWorker.RunWorkerCompleted 事件

定義

命名空間:

System.ComponentModel

組件:

netstandard.dll, System.ComponentModel.EventBasedAsync.dll

來源:

BackgroundWorker.cs

當背景操作已完成、已被取消或提出例外時，會發生這種情況。

public:
 event System::ComponentModel::RunWorkerCompletedEventHandler ^ RunWorkerCompleted;

public event System.ComponentModel.RunWorkerCompletedEventHandler? RunWorkerCompleted;

member this.RunWorkerCompleted : System.ComponentModel.RunWorkerCompletedEventHandler 

Public Custom Event RunWorkerCompleted As RunWorkerCompletedEventHandler 

事件類型

RunWorkerCompletedEventHandler

範例

以下程式碼範例展示了如何利用 RunWorkerCompleted 事件處理非同步操作的結果。 此程式碼範例是本類別更大範例 BackgroundWorker 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.
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.
    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.
    numericUpDown1.Enabled = True

    ' Enable the Start button.
    startAsyncButton.Enabled = True

    ' Disable the Cancel button.
    cancelAsyncButton.Enabled = False
End Sub

備註

當 DoWork 事件處理者返回時，此事件會被提出。

如果操作成功完成且其結果被指派到 DoWork 事件處理器中，你可以透過屬性 RunWorkerCompletedEventArgs.Result 存取該結果。

Error屬性System.ComponentModel.RunWorkerCompletedEventArgs表示該操作拋出了例外。

Cancelled屬性 表示System.ComponentModel.RunWorkerCompletedEventArgs取消請求是否被背景操作處理。 如果你在事件處理程式中DoWork偵測到取消請求，並CancellationPending檢查旗標並將 的System.ComponentModel.DoWorkEventArgs旗標設Cancel為 true，Cancelled則 的System.ComponentModel.RunWorkerCompletedEventArgs旗標也會被設為 true。

謹慎

請注意，事件處理程式中的 DoWork 程式碼可能會在取消請求中完成工作，輪詢迴圈可能會錯過 CancellationPending 設定為 true。 在這種情況下，Cancelled你的RunWorkerCompleted事件處理器中的 旗System.ComponentModel.RunWorkerCompletedEventArgs標不會被設為 true，儘管已經提出取消請求。 這種情況稱為 競賽條件 ，是多執行緒程式設計中常見的問題。 欲了解更多多執行緒設計問題，請參閱管理執行任務最佳實務。

你的RunWorkerCompleted事件處理員應該在存取RunWorkerCompletedEventArgs.Result屬性前先檢查 AsyncCompletedEventArgs.Error and AsyncCompletedEventArgs.Cancelled 屬性。 若提出例外或操作被取消，存取 RunWorkerCompletedEventArgs.Result 該屬性會產生例外。

另請參閱

• 如何：在背景執行操作
• 如何：在背景下載檔案