在异步方法中省略 async 和 await [英] eliding async and await in async methods
问题描述
一个简单的问题;阅读这篇文章:http://blog.stephencleary.com/2016/12/eliding-async-await.html
a quick question; reading this article: http://blog.stephencleary.com/2016/12/eliding-async-await.html
它通常告诉我,使用 async/await.已经在这样做了.但是,他还说在代理任务时不必使用异步部分.
it generally tells me, use async/await. Allready doing that. However, he is also saying that you don't have to use the async part when you are proxying the task.
// 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.
}
为什么在通过时不应该使用 async/await?这不是不太方便吗,这甚至有意义吗?
why should be not use async/await when passing through? Isn't that less convenient, and does this even make sense?
有什么想法吗?
推荐答案
为什么在通过时不应该使用 async/await?
why should be not use async/await when passing through?
因为在您键入 await
的那一刻,编译器会添加 吨 的实现胶水,对您来说绝对没有任何作用 - 调用者已经可以只是等待代理的任务.
because the moment you type await
, the compiler adds a ton of implementation glue that does absolutely nothing for you - the caller can already just await the proxied task.
如果我添加类似于您的 PassthroughAsync
的内容,但使用 async
/await
:
If I add something like your PassthroughAsync
, but with the async
/await
:
async Task<string> AwaitedAsync(int x) => await DoSomethingPrettyAsync(x);
然后我们可以通过编译和反编译IL来看到巨大但完全冗余的代码:
then we can see the huge but completely redundant code by compiling it and decompiling the IL:
[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);
}
}
现在对比非async
passthru 编译成的内容:
Now contrast to what the non-async
passthru compiles to:
private Task<string> PassthroughAsync(int x)
{
return DoSomethingPrettyAsync(x);
}
除了绕过大量的 struct
初始化和方法调用之外,如果它实际上是异步的,则可能在堆上装箱"(它不会在已经完成的情况下装箱")-同步情况),这个 PassthroughAsync
也将是 JIT 内联的一个很好的候选者,所以在实际 CPU 操作码中,PassthroughAsync
可能不会甚至存在.
In addition to bypassing a huge amount of struct
initialization and method calls, a possible "box" onto the heap if it is actually async (it doesn't "box" in the already-completed-synchronously case), this PassthroughAsync
will also be a great candidate for JIT-inlining, so in the actual CPU opcodes, PassthroughAsync
will probably not even exist.
这篇关于在异步方法中省略 async 和 await的文章就介绍到这了,希望我们推荐的答案对大家有所帮助,也希望大家多多支持IT屋!