Task.Yield() versus Task.Delay(0)

问题

Does Delay(0) always get inlined? In my experience, it does:

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();
        }
    }
}

This is a console app, thus the thread pool is used for continuation. The output:

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

回答1:

Inside Task.Delay, it looks like this (the single parameter (int) version just calls the below version):

[__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;
}

As you can hopefully see:

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

Which means it always returns a completed task, and therefore your code will always continue running past that particular await line.

回答2:

Yes, it does. A check of the IL in reflector shows (among other logic):

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

So yes, it will hand you back an already-completed task in this case.

Note that the implementation of await includes checks that ensure that an already-completed task doesn't cause an additional context switch, so yes: your code will keep running without pausing for breath here.

Returning an already completed task is a recommended trick when the answer is already known / available synchronously; it is also common to cache Tasks for common result values.

来源：https://stackoverflow.com/questions/18527149/task-yield-versus-task-delay0