SemaphoreSlim.WaitAsync繼續碼

Question

我的await關鍵字的理解是，繼await合格聲明代碼爲聲明的延續運行，一旦它完成。

因此，下面的兩個版本應該產生相同的輸出：

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(); 
    } 

但他們沒有！

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); 
     } 
    } 
} 

這裏是輸出：

[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 ... 

我缺少什麼？

Answer 1

async-await優化您正在等待的任務何時已完成（當信號量設置爲10時，只有一個線程正在使用它）。在那種情況下，線程只是同步進行。

你可以看到，通過增加一個實際的異步操作RunAsync，看看它是如何改變使用（這將是行爲時，你的信號是空的，調用者實際需要的等待異步）線程池中的線程：

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

您也可以進行此更改Run並使其同步執行的延續，並得到了相同的結果。你RunAsync（線數明智）：

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

輸出：

[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：當它說，async-await充當延續它更多的比喻。這些概念之間有幾個關鍵的區別，特別是關於SynchronizationContext s。 async-await自動地保留當前的背景下（除非你指定ConfigureAwait(false)），這樣你就可以在環境中的事項（UI，ASP.Net等），安全地使用它。關於同步上下文的更多信息here。

Answer 2

他們不會因爲當你調用異步方法，它是立即開始。所以，只要你的信號沒有被鎖定，WaitAsync()甚至不會開始，將沒有上下文切換（這是一種優化的，這同樣適用於取消的任務），所以你異步方法將是同步的。

同時延續版本實際上將並行線程上開始延續。