屏障類c＃

Question

我明白了在C＃中使用的Barrier類。但是，在下面的代碼中，我不明白爲什麼SignalAndWait()被調用兩次？任務中的呼叫不夠嗎？代碼基本上模擬了三個朋友（或任務）從A到B，B到C以及一些從B到A而不會轉到C的情形。 請幫我解決。順便說一句，這段代碼來自於這本書：MCSD Certification Exam Toolkit（70-483）。非常感謝你！

static void Main(string[] args) 
{ 
    var participants = 5; 
    Barrier barrier = new Barrier(participants + 1, 
     b => { // This method is only called when all the paricipants arrived. 
      Console.WriteLine("{0} paricipants are at rendez-vous point {1}.", 
       b.ParticipantCount -1, // We substract the main thread. 
       b.CurrentPhaseNumber); 
     }); 
    for (int i = 0; i < participants; i++) 
    { 
     var localCopy = i; 
     Task.Run(() => { 
      Console.WriteLine("Task {0} left point A!", localCopy); 
      Thread.Sleep(1000 * localCopy + 1); // Do some "work" 
      if (localCopy % 2 == 0) { 
       Console.WriteLine("Task {0} arrived at point B!", localCopy); 
       barrier.SignalAndWait(); 
      } 
      else 
      { 
       Console.WriteLine("Task {0} changed its mind and went back!", localCopy); 
       barrier.RemoveParticipant(); 
       return; 
      } 
      Thread.Sleep(1000 * (participants - localCopy)); // Do some "morework" 
      Console.WriteLine("Task {0} arrived at point C!", localCopy); 
      barrier.SignalAndWait(); 
     }); 
    } 

    Console.WriteLine("Main thread is waiting for {0} tasks!", 
    barrier.ParticipantCount - 1); 
    barrier.SignalAndWait(); // Waiting at the first phase 
    barrier.SignalAndWait(); // Waiting at the second phase 
    Console.WriteLine("Main thread is done!"); 
} 

Answer 1

您還會看到行Console.WriteLine("{0} paricipants are at rendez-vous point {1}.",...)執行兩次。

單個Barrier實例用於在B和C處渲染。（剩餘的）taks調用SignalAndWait()來標記它們到達B和C兩個調用。

穿着下來代碼：

if (localCopy % 2 == 0) 
    { 
     ... 
     barrier.SignalAndWait();  // arrival at B 
    } 
    else 
    { 
     ... 
     barrier.RemoveParticipant(); // return to A 
     return; 
    } 
    ... 
    barrier.SignalAndWait();   // arrival at C