在异步方法中省略async和await的含义是什么？

Question

一个快速的问题；阅读这篇文章：http://blog.stephencleary.com/2016/12/eliding-async-await.html

它通常告诉我，使用 async/await。我已经在做了。然而，他也说当你代理任务时不必使用 async 部分。

// Simple passthrough to next layer: elide.
Task<string> PassthroughAsync(int x) => _service.DoSomethingPrettyAsync(x);

// Simple overloads for a method: elide.
async Task<string> DoSomethingPrettyAsync(CancellationToken cancellationToken)
{
    ... // Core implementation, using await.
}

在通过时为什么不应该使用async/await？这难道不是更不方便吗，这样做有意义吗？

大家有什么想法吗？

- Roelant M

---

你链接的文章已经描述了所有内容。省略是一种优化。 - bommelding

在这种情况下，如何使用async await更加“方便”？ - Evk

Answer 1

在代理时为什么不应该使用async/await？

因为一旦输入了await关键字，编译器就会添加大量的实现细节，而这些对你来说完全没有任何作用——调用者可以直接等待被代理的任务。

如果我加入像您的PassthroughAsync这样的代码，但带有async/await：

async Task<string> AwaitedAsync(int x) => await DoSomethingPrettyAsync(x);

那么我们可以通过编译和反汇编IL代码来看到巨大但完全冗余的代码：

[AsyncStateMachine(typeof(<AwaitedAsync>d__1))]
private Task<string> AwaitedAsync(int x)
{
    <AwaitedAsync>d__1 <AwaitedAsync>d__ = default(<AwaitedAsync>d__1);
    <AwaitedAsync>d__.<>4__this = this;
    <AwaitedAsync>d__.x = x;
    <AwaitedAsync>d__.<>t__builder = AsyncTaskMethodBuilder<string>.Create();
    <AwaitedAsync>d__.<>1__state = -1;
    AsyncTaskMethodBuilder<string> <>t__builder = <AwaitedAsync>d__.<>t__builder;
    <>t__builder.Start(ref <AwaitedAsync>d__);
    return <AwaitedAsync>d__.<>t__builder.Task;
}
[StructLayout(LayoutKind.Auto)]
[CompilerGenerated]
private struct <AwaitedAsync>d__1 : IAsyncStateMachine
{
    public int <>1__state;

    public AsyncTaskMethodBuilder<string> <>t__builder;

    public C <>4__this;

    public int x;

    private TaskAwaiter<string> <>u__1;

    private void MoveNext()
    {
        int num = <>1__state;
        C c = <>4__this;
        string result;
        try
        {
            TaskAwaiter<string> awaiter;
            if (num != 0)
            {
                awaiter = c.DoSomethingPrettyAsync(x).GetAwaiter();
                if (!awaiter.IsCompleted)
                {
                    num = (<>1__state = 0);
                    <>u__1 = awaiter;
                    <>t__builder.AwaitUnsafeOnCompleted(ref awaiter, ref this);
                    return;
                }
            }
            else
            {
                awaiter = <>u__1;
                <>u__1 = default(TaskAwaiter<string>);
                num = (<>1__state = -1);
            }
            result = awaiter.GetResult();
        }
        catch (Exception exception)
        {
            <>1__state = -2;
            <>t__builder.SetException(exception);
            return;
        }
        <>1__state = -2;
        <>t__builder.SetResult(result);
    }

    void IAsyncStateMachine.MoveNext()
    {
        //ILSpy generated this explicit interface implementation from .override directive in MoveNext
        this.MoveNext();
    }

    [DebuggerHidden]
    private void SetStateMachine(IAsyncStateMachine stateMachine)
    {
        <>t__builder.SetStateMachine(stateMachine);
    }

    void IAsyncStateMachine.SetStateMachine(IAsyncStateMachine stateMachine)
    {
        //ILSpy generated this explicit interface implementation from .override directive in SetStateMachine
        this.SetStateMachine(stateMachine);
    }
}

现在来对比一下非async传递编译成的代码：

private Task<string> PassthroughAsync(int x)
{
    return DoSomethingPrettyAsync(x);
}

除了跳过大量的struct初始化和方法调用，如果它实际上是异步的（在已同步完成的情况下不会"box"），这个PassthroughAsync也将成为JIT内联的绝佳选择，因此在实际的CPU操作码中，PassthroughAsync可能甚至不存在。

- Marc Gravell