通过

由于不等待此调用，因此在调用完成之前，当前方法将继续运行

  • 项目

错误消息

由于此调用不会等待，因此在调用完成前将继续执行当前方法。应用调用的结果的考虑“Await运算符。

当前方法调用返回 Task 或 Task<TResult>，不应用 等待 运算符应用于该结果的异步方法。对异步方法的调用将启动异步任务。但是，在中，因为 Await 运算符不是应用的，程序将继续运行，而不等待任务完成。在大多数情况下，此行为不需要。通常被调用的方法的其他方面依赖于调用的结果，或者最低限度上，调用方法预计完成，在从包含调用的方法之前返回。

同样重要的问题是发生在调用异步方法引发的异常。在方法引发返回 Task 或 Task<TResult> 的异常在返回的任务存储。如果不等待任务也不显式检查异常，则丢失。如果您等待任务，其异常来重新引发。

作为最佳做法，您应始终等待调用。

默认情况下，此消息是一个警告。有关隐藏警告或将警告视为错误的更多信息，请参见在 Visual Basic 中配置警告。

**错误 ID：**BC42358

处理此警告

  • 应考虑禁止显示该警告，只有在确信您不希望等待异步调用完成，然后调用方法不会引发任何异常。在这种情况，您可以通过分配调用的任务结果禁止显示该警告给变量。

    下面的示例演示如何导致该警告，如何禁止它以及如何等待调用。

        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

    在此示例中，因此，如果选择调用#1或调用#2，在其调用方(CallingMethodAsync)之后的unawaited异步方法(CalledMethodAsync)完成，并且调用方的调用方(StartButton_Click)已完成。当调用方法完成时，以下输出的最后一行显示您。项以及从输出中对该完整示例中的 CallingMethodAsync 标记的事件处理程序退出。

    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.

示例

以下Windows presentation foundation (WPF)应用程序包含前面示例中的方法。以下步骤将的应用程序。

  1. 创建一个WPF应用程序，并将其命名为 AsyncWarning。

  2. 在Visual Studio代码编辑器"中，选择 *** MainWindow.xaml *** 选项。

    如果看不到选项卡，打开MainWindow.xaml的快捷菜单在 *** 解决方案资源管理器 ***，然后选择 *** 查看代码 ***。

  3. 用下面的代码替换在MainWindow.xaml *** XAML *** 视图中的代码。

    <Window x:Class="MainWindow"
        xmlns="https://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="https://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>

    包含一个按钮和文本框的简单窗口显示MainWindow.xaml *** 设计 *** 视图。

    有关XAML设计器的更多信息，请参见 使用 XAML 设计器创建 UI。有关如何生成简单的UI的信息，请参见 演练：使用 Async 和 Await 访问 Web（C# 和 Visual Basic）“创建WPF应用程序”和“设计简单的WPF的”部分。

  4. 用下面的代码替换在MainWindow.xaml.vb的代码。

    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. 选择F5键运行程序，然后选择 *** 启动 *** 按钮。

    预期的输出显示在代码的结尾。

请参见

参考

Await 运算符 (Visual Basic)

概念

使用 Async 和 Await 的异步编程（C# 和 Visual Basic）