SemaphoreSlim.WaitAsync继续码

Question

我的await关键字的理解是，继await合格声明代码为声明的延续运行，一旦它完成。

因此，下面的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！

下面是完整的程序：

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); 
     } 
    } 
} 

这里是输出：

[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 ... 

我缺少什么？

Answer 1

async-await优化您正在等待的任务何时已完成（当信号量设置为10时，只有一个线程正在使用它）。在那种情况下，线程只是同步进行。

你可以看到，通过增加一个实际的异步操作RunAsync，看看它是如何改变使用（这将是行为时，你的信号是空的，调用者实际需要的等待异步）线程池中的线程：

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

您也可以进行此更改Run并使其同步执行的延续，并得到了相同的结果。你RunAsync（线数明智）：

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充当延续它更多的比喻。这些概念之间有几个关键的区别，特别是关于SynchronizationContext s。 async-await自动地保留当前的背景下（除非你指定ConfigureAwait(false)），这样你就可以在环境中的事项（UI，ASP.Net等），安全地使用它。关于同步上下文的更多信息here。

Answer 2

他们不会因为当你调用异步方法，它是立即开始。所以，只要你的信号没有被锁定，WaitAsync()甚至不会开始，将没有上下文切换（这是一种优化的，这同样适用于取消的任务），所以你异步方法将是同步的。

同时延续版本实际上将并行线程上开始延续。