編譯器警告 (層級 1) CS4014

發行項
05/22/2023

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，該行為並非您所預期。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。

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。

static async Task CallingMethodAsync(int millisecondsDelay)
{
    Console.WriteLine("  Entering calling method.");

    // 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(millisecondsDelay);

    // 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(millisecondsDelay);

    // 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(millisecondsDelay);

    Console.WriteLine("  Returning from calling method.");
}

static async Task CalledMethodAsync(int millisecondsDelay)
{
    Console.WriteLine("    Entering called method, starting and awaiting Task.Delay.");

    await Task.Delay(millisecondsDelay);

    Console.WriteLine("    Task.Delay is finished--returning from called method.");
}

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.

您也可以使用 #pragma warning 指示詞隱藏編譯器警告。

範例

下列主控台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。

建立主控台應用程式，並命名為 AsyncWarning。
在 Visual Studio 程式碼編輯器中，選擇 [Program.cs] 索引標籤。

將 Program.cs 中的程式碼取代為下列程式碼。

using System;
using System.Threading.Tasks;

namespace AsyncWarning
{
    class Program
    {
        static async Task Main()
        {
            Console.WriteLine("Entering Main() application entry point.");

            int millisecondsDelay = 2000;
            await CallingMethodAsync(millisecondsDelay);

            Console.WriteLine("Exiting Main() application entry point.");

            await Task.Delay(millisecondsDelay + 500);
        }

        static async Task CallingMethodAsync(int millisecondsDelay)
        {
            Console.WriteLine("  Entering calling method.");

            // 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(millisecondsDelay);

            // 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(millisecondsDelay);

            // 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(millisecondsDelay);

            Console.WriteLine("  Returning from calling method.");
        }

        static async Task CalledMethodAsync(int millisecondsDelay)
        {
            Console.WriteLine("    Entering called method, starting and awaiting Task.Delay.");

            await Task.Delay(millisecondsDelay);

            Console.WriteLine("    Task.Delay is finished--returning from called method.");
        }
    }

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

    // Entering Main() application entry point.
    //   Entering calling method.
    //     Entering called method, starting and awaiting Task.Delay.
    //   Returning from calling method.
    // Exiting Main() application entry point.
    //     Task.Delay is finished--returning from called method.

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

    // Entering Main() application entry point.
    //   Entering calling method.
    //     Entering called method, starting and awaiting Task.Delay.
    //     Task.Delay is finished--returning from called method.
    //   Returning from calling method.
    // Exiting Main() application entry point.
}

選取 F5 鍵以執行程式。

預期的輸出會出現在程式碼結尾。

共用方式為

編譯器警告 (層級 1) CS4014

範例

另請參閱

意見反應

其他資源