BC42358：因為未等候此呼叫，所以在呼叫完成之前會繼續執行目前的方法

因為未等候此呼叫，所以在呼叫完成之前會繼續執行目前的方法。 請考慮將 Await 運算子套用至呼叫的結果。

目前的方法會呼叫傳回 Task 或 Task<TResult> 且不會將 Await 運算子套用至結果的非同步方法。 非同步方法的呼叫會啟動非同步工作。 不過，由於不會套用任何 Await 運算子，因此程式會繼續執行，而不等候工作完成。 在大部分情況下，不應該有此行為。 通常呼叫方法的其他方面取決於呼叫的結果，或至少被呼叫的方法必須完成，才能從包含呼叫的方法傳回。

同樣重要的問題是，在呼叫的非同步方法中引發的例外狀況會發生什麼情形。 在傳回 Task 或 Task<TResult> 的方法中引發的例外狀況會儲存到傳回的工作中。 如果您未等候工作，也未明確檢查例外狀況，例外狀況就會遺失。 如果您等候工作，則其例外狀況會再次擲回。

因此最佳做法是一律等候呼叫。

根據預設，這個訊息是一個警告。 如需隱藏警告或將警告視為錯誤的詳細資訊，請參閱 Configuring Warnings in Visual Basic。

錯誤 ID︰ BC42358

解決這個警告

只有在您確定不要等候非同步呼叫完成，而且被呼叫的方法不會引發任何例外狀況時，才應考慮隱藏警告。 在這種情況下，您可以藉由將呼叫的工作結果指定至變數來隱藏警告。

下列範例將示範如何產生警告、如何隱藏警告，以及如何等候呼叫：

Async Function CallingMethodAsync() As Task

    ResultsTextBox.Text &= vbCrLf & "  Entering calling method."

    ' Variable delay is used to slow down the called method so that you
    ' can distinguish between awaiting and not awaiting in the program's output.
    ' You can adjust the value to produce the output that this topic shows
    ' after the code.
    Dim delay = 5000

    ' Call #1.
    ' Call an async method. Because you don't await it, its completion isn't
    ' coordinated with the current method, CallingMethodAsync.
    ' The following line causes the warning.
    CalledMethodAsync(delay)

    ' Call #2.
    ' To suppress the warning without awaiting, you can assign the
    ' returned task to a variable. The assignment doesn't change how
    ' the program runs. However, the recommended practice is always to
    ' await a call to an async method.
    ' Replace Call #1 with the following line.
    'Task delayTask = CalledMethodAsync(delay)

    ' Call #3
    ' To contrast with an awaited call, replace the unawaited call
    ' (Call #1 or Call #2) with the following awaited call. The best
    ' practice is to await the call.

    'Await CalledMethodAsync(delay)

    ' If the call to CalledMethodAsync isn't awaited, CallingMethodAsync
    ' continues to run and, in this example, finishes its work and returns
    ' to its caller.
    ResultsTextBox.Text &= vbCrLf & "  Returning from calling method."
End Function

Async Function CalledMethodAsync(howLong As Integer) As Task

    ResultsTextBox.Text &= vbCrLf & "    Entering called method, starting and awaiting Task.Delay."
    ' Slow the process down a little so you can distinguish between awaiting
    ' and not awaiting. Adjust the value for howLong if necessary.
    Await Task.Delay(howLong)
    ResultsTextBox.Text &= vbCrLf & "    Task.Delay is finished--returning from called method."
End Function

在此範例中，如果您選擇呼叫 #1 或呼叫 #2，則未等候的非同步方法 (CalledMethodAsync) 會在其呼叫端 (CallingMethodAsync) 和呼叫端的呼叫端 (StartButton_Click) 都完成之後才完成。 下列輸出的最後一行將顯示被呼叫的方法完成的時間。 輸入中會標記在完整範例中進入和結束呼叫 CallingMethodAsync 的事件處理常式。

Entering the Click event handler.
  Entering calling method.
    Entering called method, starting and awaiting Task.Delay.
  Returning from calling method.
Exiting the Click event handler.
    Task.Delay is finished--returning from called method.

範例

下列 Windows Presentation Foundation (WPF) 應用程式包含了前述範例的方法。 下列步驟將會設定應用程式：

1. 建立 WPF 應用程式，並將其命名為 AsyncWarning。

2. 在 Visual Studio 程式碼編輯器中，選擇 [ MainWindow.xaml ] 索引標籤。

   如果未顯示索引標籤，請在 [ 方案總管] 中開啟 MainWindow.xaml 的捷徑功能表，然後選擇 [ 檢視程式碼]。

3. 在 MainWindow.xaml 的 [ XAML ] 檢視中，將程式碼取代為下列程式碼：

   <Window x:Class="MainWindow"
           xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
           xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
           Title="MainWindow" Height="350" Width="525">
       <Grid>
           <Button x:Name="StartButton" Content="Start" HorizontalAlignment="Left" Margin="214,28,0,0" VerticalAlignment="Top" Width="75" HorizontalContentAlignment="Center" FontWeight="Bold" FontFamily="Aharoni" Click="StartButton_Click" />
           <TextBox x:Name="ResultsTextBox" Margin="0,80,0,0" TextWrapping="Wrap" FontFamily="Lucida Console"/>
       </Grid>
   </Window>
   

   包含按鈕和文字方塊的簡單視窗會出現在 MainWindow.xaml 的 [ 設計 ] 檢視中。

   如需 XAML 設計工具的詳細資訊，請參閱使用 XAML 設計工具建立 UI。 如需如何建置您自己的簡單 UI 的資訊，請參閱逐步解說：使用 Async 和 Await 存取 Web的＜建立 WPF 應用程式＞和＜設計簡單的 WPF MainWindow＞章節。

4. 以下列程式碼取代 MainWindow.xaml.vb 中的程式碼。

   Class MainWindow
   
       Private Async Sub StartButton_Click(sender As Object, e As RoutedEventArgs)
   
           ResultsTextBox.Text &= vbCrLf & "Entering the Click event handler."
           Await CallingMethodAsync()
           ResultsTextBox.Text &= vbCrLf & "Exiting the Click event handler."
       End Sub
   
       Async Function CallingMethodAsync() As Task
   
           ResultsTextBox.Text &= vbCrLf & "  Entering calling method."
   
           ' Variable delay is used to slow down the called method so that you
           ' can distinguish between awaiting and not awaiting in the program's output.
           ' You can adjust the value to produce the output that this topic shows
           ' after the code.
           Dim delay = 5000
   
           ' Call #1.
           ' Call an async method. Because you don't await it, its completion isn't
           ' coordinated with the current method, CallingMethodAsync.
           ' The following line causes the warning.
           CalledMethodAsync(delay)
   
           ' Call #2.
           ' To suppress the warning without awaiting, you can assign the
           ' returned task to a variable. The assignment doesn't change how
           ' the program runs. However, the recommended practice is always to
           ' await a call to an async method.
   
           ' Replace Call #1 with the following line.
           'Task delayTask = CalledMethodAsync(delay)
   
           ' Call #3
           ' To contrast with an awaited call, replace the unawaited call
           ' (Call #1 or Call #2) with the following awaited call. The best
           ' practice is to await the call.
   
           'Await CalledMethodAsync(delay)
   
           ' If the call to CalledMethodAsync isn't awaited, CallingMethodAsync
           ' continues to run and, in this example, finishes its work and returns
           ' to its caller.
           ResultsTextBox.Text &= vbCrLf & "  Returning from calling method."
       End Function
   
       Async Function CalledMethodAsync(howLong As Integer) As Task
   
           ResultsTextBox.Text &= vbCrLf & "    Entering called method, starting and awaiting Task.Delay."
           ' Slow the process down a little so you can distinguish between awaiting
           ' and not awaiting. Adjust the value for howLong if necessary.
           Await Task.Delay(howLong)
           ResultsTextBox.Text &= vbCrLf & "    Task.Delay is finished--returning from called method."
       End Function
   
   End Class
   
   ' Output
   
   ' Entering the Click event handler.
   '   Entering calling method.
   '     Entering called method, starting and awaiting Task.Delay.
   '   Returning from calling method.
   ' Exiting the Click event handler.
   '     Task.Delay is finished--returning from called method.
   
   ' Output
   
   ' Entering the Click event handler.
   '   Entering calling method.
   '     Entering called method, starting and awaiting Task.Delay.
   '     Task.Delay is finished--returning from called method.
   '   Returning from calling method.
   ' Exiting the Click event handler.
   

5. 選擇 F5 鍵以執行程式，然後選擇 [ 開始 ] 按鈕。

   預期的輸出會出現在程式碼結尾。

另請參閱

• Await 運算子
• 使用 Async 和 Await 進行非同步程式設計