编译器警告（等级 1）CS4014

项目
03/12/2013

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。请考虑将“await”运算符应用于调用结果。

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，此行为不是预期。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。

作为一个最优方法，应始终等待调用。

应考虑禁止显示该警告，只有 + 当确保将不要等待异步调用完成，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 Task CallingMethodAsync()
{
    resultsTextBox.Text += "\r\n  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.
    var 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 warning CS4014.
    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, 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. 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 += "\r\n  Returning from calling method.";
}


async Task CalledMethodAsync(int howLong)
{
    resultsTextBox.Text += 
        "\r\n    Entering called method, starting and awaiting Task.Delay.";

    // Slow the process down a little so that you can distinguish between
    // awaiting and not awaiting in the program's output. Adjust the value
    // for howLong if necessary.
    await Task.Delay(howLong);
    resultsTextBox.Text += 
        "\r\n    Task.Delay is finished--returning from called method.";
}

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.

使用 #pragma warning（C# 参考）指令，还可以禁止编译器警告。

示例

以下 windows 演示基础 (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。

创建一个 WPF 应用程序，并将其命名为 AsyncWarning。
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。

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

<Window x:Class="AsyncWarning.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。

用下面的代码替换在 MainWindow.xaml.cs 的代码。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Data;
using System.Windows.Documents;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Imaging;
using System.Windows.Navigation;
using System.Windows.Shapes;

namespace AsyncWarning
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private async void startButton_Click(object sender, RoutedEventArgs e)
        {
            resultsTextBox.Text += "\r\nEntering the Click event handler.";
            await CallingMethodAsync();
            resultsTextBox.Text += "\r\nExiting the Click event handler.";
        }


        async Task CallingMethodAsync()
        {
            resultsTextBox.Text += "\r\n  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.
            var 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 warning CS4014.
            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, 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. 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 += "\r\n  Returning from calling method.";
        }


        async Task CalledMethodAsync(int howLong)
        {
            resultsTextBox.Text += 
                "\r\n    Entering called method, starting and awaiting Task.Delay.";

            // Slow the process down a little so that you can distinguish between
            // awaiting and not awaiting in the program's output. Adjust the value
            // for howLong if necessary.
            await Task.Delay(howLong);
            resultsTextBox.Text += 
                "\r\n    Task.Delay is finished--returning from called method.";
        }
    }

    // Output with Call #1 or Call #2. (Wait for the last line to appear.)

    // 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 with Call #3, which awaits the call to CalledMethodAsync.

    // 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.
}

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。

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编译器警告（等级 1）CS4014

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