Question

我对await关键字的理解是，await限定语句后面的代码在完成后作为该语句的延续运行。

因此，以下两个版本应该产生相同的输出：

    public static Task Run(SemaphoreSlim sem)
    {
        TraceThreadCount();
        return sem.WaitAsync().ContinueWith(t =>
        {
            TraceThreadCount();
            sem.Release();
        });
    }

    public static async Task RunAsync(SemaphoreSlim sem)
    {
        TraceThreadCount();
        await sem.WaitAsync();
        TraceThreadCount();
        sem.Release();
    }

但他们没有！

这是完整的程序：

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

namespace CDE
{
    class Program
    {
        static void Main(string[] args)
        {
            try
            {
                var sem = new SemaphoreSlim(10);
                var task = Run(sem);

                Trace("About to wait for Run.");

                task.Wait();

                Trace("--------------------------------------------------");
                task = RunAsync(sem);

                Trace("About to wait for RunAsync.");

                task.Wait();
            }
            catch (Exception exc)
            {
                Console.WriteLine(exc.Message);
            }
            Trace("Press any key ...");
            Console.ReadKey();
        }

        public static Task Run(SemaphoreSlim sem)
        {
            TraceThreadCount();
            return sem.WaitAsync().ContinueWith(t =>
            {
                TraceThreadCount();
                sem.Release();
            });
        }

        public static async Task RunAsync(SemaphoreSlim sem)
        {
            TraceThreadCount();
            await sem.WaitAsync();
            TraceThreadCount();
            sem.Release();
        }

        private static void Trace(string fmt, params object[] args)
        {
            var str = string.Format(fmt, args);
            Console.WriteLine("[{0}] {1}", Thread.CurrentThread.ManagedThreadId, str);
        }
        private static void TraceThreadCount()
        {
            int workerThreads;
            int completionPortThreads;
            ThreadPool.GetAvailableThreads(out workerThreads, out completionPortThreads);
            Trace("Available thread count: worker = {0}, completion port = {1}", workerThreads, completionPortThreads);
        }
    }
}

这是输出：

[9] Available thread count: worker = 1023, completion port = 1000
[9] About to wait for Run.
[6] Available thread count: worker = 1021, completion port = 1000
[9] --------------------------------------------------
[9] Available thread count: worker = 1023, completion port = 1000
[9] Available thread count: worker = 1023, completion port = 1000
[9] About to wait for RunAsync.
[9] Press any key ...

我错过了什么？

Answer 1

async-await优化您正在等待的任务何时已经完成（当您将信号量设置为10且只有1个线程使用它时）。在这种情况下，线程只是同步进行。

您可以通过向RunAsync添加实际的异步操作来查看它是如何更改正在使用的线程池线程的（当您的信号量为空且调用者实际需要时，这将是行为等待异步）：

public static async Task RunAsync(SemaphoreSlim sem)
{
    TraceThreadCount();
    await Task.Delay(1000);
    await sem.WaitAsync();
    TraceThreadCount();
    sem.Release();
}

您也可以对Run进行此更改，并让它同步执行延续，并获得与RunAsync中相同的结果（线程计数）：

public static Task Run(SemaphoreSlim sem)
{
    TraceThreadCount();
    return sem.WaitAsync().ContinueWith(t =>
    {
        TraceThreadCount();
        sem.Release();
    }, TaskContinuationOptions.ExecuteSynchronously);
}

输出：

[1] Available thread count: worker = 1023, completion port = 1000  
[1] Available thread count: worker = 1023, completion port = 1000  
[1] About to wait for Run.  
[1] --------------------------------------------------  
[1] Available thread count: worker = 1023, completion port = 1000  
[1] Available thread count: worker = 1023, completion port = 1000  
[1] About to wait for RunAsync.  
[1] Press any key ...

重要提示：当它说async-await作为延续时，它更像是一个类比。这些概念之间存在一些重要的区别，特别是关于SynchronizationContext s。 async-await自动保留当前上下文（除非您指定ConfigureAwait(false)），以便您可以在重要的环境（UI，ASP.Net等）中安全地使用它。有关同步上下文的更多信息here。

Answer 2

他们赢了，因为当您调用异步方法时，它立即启动。因此，只要您的信号量未被锁定，WaitAsync()甚至无法启动，将无法进行上下文切换（它的优化，同样适用于已取消的任务），因此您的异步方法将是同步的。

同时，continuation版本实际上会在并行线程上开始延续。

SemaphoreSlim.WaitAsync延续代码

2 个答案: