什麼是本地varibale（在CountdownEvent類的MSDN代碼示例中）？

Question

在MSDN文章CountdownEvent Class的代碼示例（如下）中，
什麼是local變量？

using System; 
using System.Collections.Concurrent; 
using System.Linq; 
using System.Threading; 
using System.Threading.Tasks; 
class CDESample 
{ 
    // Demonstrates: 
    //  CountdownEvent construction 
    //  CountdownEvent.AddCount() 
    //  CountdownEvent.Signal() 
    //  CountdownEvent.Wait() 
    //  CountdownEvent.Wait() w/ cancellation 
    //  CountdownEvent.Reset() 
    //  CountdownEvent.IsSet 
    //  CountdownEvent.InitialCount 
    //  CountdownEvent.CurrentCount 
    static void Main() 
    { 
     // Initialize a queue and a CountdownEvent 
     ConcurrentQueue<int> queue = new ConcurrentQueue<int>(Enumerable.Range(0, 10000)); 
     CountdownEvent cde = new CountdownEvent(10000); // initial count = 10000 

     // This is the logic for all queue consumers 
     Action consumer =() => 
     { 
      int local; 
      // decrement CDE count once for each element consumed from queue 
      while (queue.TryDequeue(out local)) cde.Signal(); 
     }; 

     // Now empty the queue with a couple of asynchronous tasks 
     Task t1 = Task.Factory.StartNew(consumer); 
     Task t2 = Task.Factory.StartNew(consumer); 

     // And wait for queue to empty by waiting on cde 
     cde.Wait(); // will return when cde count reaches 0 

     Console.WriteLine("Done emptying queue. InitialCount={0}, CurrentCount={1}, IsSet={2}", 
      cde.InitialCount, cde.CurrentCount, cde.IsSet); 

     // Proper form is to wait for the tasks to complete, even if you that their work 
     // is done already. 
     Task.WaitAll(t1, t2); 

     // Resetting will cause the CountdownEvent to un-set, and resets InitialCount/CurrentCount 
     // to the specified value 
     cde.Reset(10); 

     // AddCount will affect the CurrentCount, but not the InitialCount 
     cde.AddCount(2); 

     Console.WriteLine("After Reset(10), AddCount(2): InitialCount={0}, CurrentCount={1}, IsSet={2}", 
      cde.InitialCount, cde.CurrentCount, cde.IsSet); 

     // Now try waiting with cancellation 
     CancellationTokenSource cts = new CancellationTokenSource(); 
     cts.Cancel(); // cancels the CancellationTokenSource 
     try 
     { 
      cde.Wait(cts.Token); 
     } 
     catch (OperationCanceledException) 
     { 
      Console.WriteLine("cde.Wait(preCanceledToken) threw OCE, as expected"); 
     } 

     // It's good for to release a CountdownEvent when you're done with it. 
     cde.Dispose(); 

    } 
} 

Answer 1

我想這只是因爲該方法TryDequeue()有int類型的參數，你必須給它一個，即使你不使用它。

Answer 2

TryDequeue需要一個類型爲T的輸出參數.T是隊列的類型。本地參數將填滿您剛剛爲隊列移除的對象。它可以用於進一步處理。見http://msdn.microsoft.com/en-us/library/dd287208.aspx

Answer 3

從ConcurrentQueue<T>.TryDequeue

嘗試刪除，並在 併發隊列的開頭返回的對象。

public bool TryDequeue(
    out T result 
) 

Parameters 
result 
Type: T 
When this method returns, if the operation was successful, result contains the object removed. If no object was available to be removed, the value is unspecified. 

T是您的隊列的類型。 如果您想知道out是做什麼的，請從MSDN退房。

雖然queue.TryDequeue(out local)方法返回true，您的cde.Signal()方法將繼續處理。