Task.Yield()与Task.Delay(0)

Question

难道Delay(0)总要得到内联？根据我的经验,它确实:

using System;
using System.Threading;
using System.Threading.Tasks;

namespace ConsoleApplication
{
    class Program
    {
        static async Task Test()
        {
            await Task.Yield();
            Console.WriteLine("after Yield(), thread: {0}", Thread.CurrentThread.ManagedThreadId);
            await Task.Delay(0);
            Console.WriteLine("after Delay(0), thread: {0}", Thread.CurrentThread.ManagedThreadId);
            await Task.Delay(100);
            Console.WriteLine("after Delay(100), thread: {0}", Thread.CurrentThread.ManagedThreadId);
        }
        static void Main(string[] args)
        {
            Console.WriteLine("Main thread: {0}", Thread.CurrentThread.ManagedThreadId);
            Test().Wait();
        }
    }
}

这是一个控制台应用程序,因此线程池用于继续.输出:

Main thread: 11
after Yield(), thread: 7
after Delay(0), thread: 7
after Delay(100), thread: 6

Answer 1

在里面Task.Delay,它看起来像这样(单个参数(int)版本只调用以下版本):

[__DynamicallyInvokable]
public static Task Delay(int millisecondsDelay, CancellationToken cancellationToken)
{
    if (millisecondsDelay < -1)
    {
        throw new ArgumentOutOfRangeException("millisecondsDelay", Environment.GetResourceString("Task_Delay_InvalidMillisecondsDelay"));
    }
    if (cancellationToken.IsCancellationRequested)
    {
        return FromCancellation(cancellationToken);
    }
    if (millisecondsDelay == 0)
    {
        return CompletedTask;
    }
    DelayPromise state = new DelayPromise(cancellationToken);
    if (cancellationToken.CanBeCanceled)
    {
        state.Registration = cancellationToken.InternalRegisterWithoutEC(delegate (object state) {
            ((DelayPromise) state).Complete();
        }, state);
    }
    if (millisecondsDelay != -1)
    {
        state.Timer = new Timer(delegate (object state) {
            ((DelayPromise) state).Complete();
        }, state, millisecondsDelay, -1);
        state.Timer.KeepRootedWhileScheduled();
    }
    return state;
}

你可以希望看到:

    if (millisecondsDelay == 0)
    {
        return CompletedTask;
    }

这意味着它始终返回已完成的任务,因此您的代码将始终继续运行该特定await行.

Answer 2

是的,它确实.检查反射器中的IL(以及其他逻辑):

if (millisecondsDelay == 0)
{
    return CompletedTask;
}

所以,是的,在这种情况下,它将把你已经完成的任务交还给你.

请注意,await包含检查的实现包括确保已完成的任务不会导致额外的上下文切换,所以是的:您的代码将继续运行而不会在此处暂停.

当答案已经同步已知/可用时,返回已完成的任务是推荐的技巧; Task为常见结果值缓存s 也很常见.