c# - SemaphoreSlim.WaitAsync 延续代码

Question

我对 await 关键字的理解是，await 限定语句之后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!

完整程序如下:

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 针对您正在等待的任务已经完成的时间进行优化(当您将信号量设置为 10 而只有 1 个线程使用它时就是这种情况)。在那种情况下，线程只是同步进行。

您可以看到，通过向 RunAsync 添加一个实际的异步操作并查看它如何改变正在使用的线程池线程(这将是当您的信号量为空并且调用者实际需要异步等待):

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

您还可以对 Run 进行此更改，并让它同步执行延续并获得与 RunAsync 相同的结果(线程计数方面):

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 ...  

---

重要说明:当说 async-await 充当延续时，它更像是一个类比。这些概念之间存在几个关键差异，尤其是关于 SynchronizationContext。 async-await 自动保留当前上下文(除非您指定 ConfigureAwait(false))，因此您可以在重要的环境(UI、ASP.Net 等)中安全地使用它.).关于同步上下文的更多信息 here .

此外，await Task.Delay(1000); 可以替换为 await Task.Yield();说明时间无关紧要，只是方法异步等待这一事实很重要。 Task.Yield() 在异步代码的单元测试中通常很有用。