簡介

創建線程是昂貴的操作，為每一個短暫的異步操作創建線程會產生顯著的開銷。

在線程池中調用委托

 1 using System;
 2 using System.Threading;
 3 
 4 namespace Chapter3.Recipe1
 5 {
 6     class Program
 7     {
 8         static void Main(string[] args)
 9         {
10             int threadId = 0;
11 
12             RunOnThreadPool poolDelegate = Test;
 
13 
14             var t = new Thread(() => Test(out threadId));
15             t.Start();
16             t.Join();
17 
18             Console.WriteLine("Thread id: {0}", threadId);
19 
20             //調用BeginInvoke()運行委托，回調函數在異步操作完成後被調用
21             //BeginInvoke()調用後會立即返回,線程池中的工作線程在執行異步操作時，仍允許繼續其他工作。
 

22             IAsyncResult r = poolDelegate.BeginInvoke(out threadId, Callback, "a delegate asynchronous call"); 
23             r.AsyncWaitHandle.WaitOne(); //等待異步操作完成
24             //事實上EndInvoke()會等待異步操作的完成，所以r.AsyncWaitHandle.WaitOne();並不是必要的
25             string result = poolDelegate.EndInvoke(out
 
 threadId, r);
26             //異步操作完成後，回調函數會被放置到線程池中，確切的說是一個工作線程中。
27             Console.WriteLine("Thread pool worker thread id: {0}", threadId);
28             Console.WriteLine(result);
29 
30             Thread.Sleep(TimeSpan.FromSeconds(2));
31         }
32 
33         private delegate string RunOnThreadPool(out int threadId);
34 
35         private static void Callback(IAsyncResult ar)//回調函數
36         {
37             Console.WriteLine("Starting a callback...");
38             Console.WriteLine("State passed to a callbak: {0}", ar.AsyncState);
39             Console.WriteLine("Is thread pool thread: {0}", Thread.CurrentThread.IsThreadPoolThread);
40             Console.WriteLine("Thread pool worker thread id: {0}", Thread.CurrentThread.ManagedThreadId);
41         }
42 
43 
44         private static string Test(out int threadId)
45         {
46             Console.WriteLine("Starting...");
47             Console.WriteLine("Is thread pool thread: {0}", Thread.CurrentThread.IsThreadPoolThread);
48             Thread.Sleep(TimeSpan.FromSeconds(2));
49             threadId = Thread.CurrentThread.ManagedThreadId;
50             return string.Format("Thread pool worker thread id was: {0}", threadId);
51         }
52     }
53 }

使用BeginOperationName/EndOperation方法和.NET中的IAsyncResult對象等方式被稱為異步編程模型，這樣的方法對被稱為異步方法。

使用線程池