EventWaitHandle类

EventWaitHandle类

表示一个线程同步事件。该类存在于mscorlib.DLL中。

EventWaitHandle类允许线程通过发信号的方式互相通信。 通常，一个或多个线程在EventWaitHandle上被阻止，直到一个未阻止的线程通过调用Set（）方法，来释放一个或多个被阻止的线程。线程可以通过调用SignalAndWait()方法（该方法为静态函数）以原子操作的方式，向EventWaitHandle发出信号，然后在它上面阻止。

以终止的EventWaitHandle的行为取决于它的重置模式，在释放单个等待线程后，用EventResetMode.AutoReset标志创建的EventWaitHandle在终止时会自动地重置，用EventResetMode.ManualReset标志创建的EventWaitHandle一直保持终止状态，直到它的Reset函数被调用。

自动重置时间提供对资源的独占访问，如果没有线程等待时，自动重置事件处于终止状态。则该事件一直保持终止状态，直到某个线程尝试在该事件上等待，该事件释放线程，并立即重置，以阻止后面的线程。

手动重置事件类似于入口，当事件不处于终止状态时，在该事件上等待的线程则被阻止，当事件处于终止状态时，所有等待的线程均被释放，而事件一直保持终止状态（即后边的线程不阻止），直到它的Reset方法被调用，如果一个线程必须完成某项活动时，其他线程才能继续执行，则使用手动重置事件。

EventWaitHandle对象可以与其静态函数：WaitAll和WaitAny方法一起使用。

MSDN示例：

1 using System;
2 using System.Threading;
3
4 public class Example
5 {
6 // The EventWaitHandle used to demonstrate the difference
7 // between AutoReset and ManualReset synchronization events.
8 //
9 private static EventWaitHandle ewh;
10
11 // A counter to make sure all threads are started and
12 // blocked before any are released. A Long is used to show
13 // the use of the 64-bit Interlocked methods.
14 //
15 private static long threadCount = 0;
16
17 // An AutoReset event that allows the main thread to block
18 // until an exiting thread has decremented the count.
19 //
20 private static EventWaitHandle clearCount =
21 new EventWaitHandle(false, EventResetMode.AutoReset);
22
23 [MTAThread]
24 public static void Main()
25 {
26 // Create an AutoReset EventWaitHandle.
27 //
28 ewh = new EventWaitHandle(false, EventResetMode.AutoReset);
29
30 // Create and start five numbered threads. Use the
31 // ParameterizedThreadStart delegate, so the thread
32 // number can be passed as an argument to the Start
33 // method.
34 for (int i = 0; i <= 4; i++)
35 {
36 Thread t = new Thread(
37 new ParameterizedThreadStart(ThreadProc)
38 );
39 t.Start(i);
40 }
41
42 // Wait until all the threads have started and blocked.
43 // When multiple threads use a 64-bit value on a 32-bit
44 // system, you must access the value through the
45 // Interlocked class to guarantee thread safety.
46 //
47 while (Interlocked.Read(ref threadCount) < 5)
48 {
49 Thread.Sleep(500);
50 }
51
52 // Release one thread each time the user presses ENTER,
53 // until all threads have been released.
54 //
55 while (Interlocked.Read(ref threadCount) > 0)
56 {
57 Console.WriteLine("Press ENTER to release a waiting thread.");
58 Console.ReadLine();
59
60 // SignalAndWait signals the EventWaitHandle, which
61 // releases exactly one thread before resetting,
62 // because it was created with AutoReset mode.
63 // SignalAndWait then blocks on clearCount, to
64 // allow the signaled thread to decrement the count
65 // before looping again.
66 //
67 WaitHandle.SignalAndWait(ewh, clearCount);
68 }
69 Console.WriteLine();
70
71 // Create a ManualReset EventWaitHandle.
72 //
73 ewh = new EventWaitHandle(false, EventResetMode.ManualReset);
74
75 // Create and start five more numbered threads.
76 //
77 for(int i=0; i<=4; i++)
78 {
79 Thread t = new Thread(
80 new ParameterizedThreadStart(ThreadProc)
81 );
82 t.Start(i);
83 }
84
85 // Wait until all the threads have started and blocked.
86 //
87 while (Interlocked.Read(ref threadCount) < 5)
88 {
89 Thread.Sleep(500);
90 }
91
92 // Because the EventWaitHandle was created with
93 // ManualReset mode, signaling it releases all the
94 // waiting threads.
95 //
96 Console.WriteLine("Press ENTER to release the waiting threads.");
97 Console.ReadLine();
98 ewh.Set();
99
100 }
101
102 public static void ThreadProc(object data)
103 {
104 int index = (int) data;
105
106 Console.WriteLine("Thread {0} blocks.", data);
107 // Increment the count of blocked threads.
108 Interlocked.Increment(ref threadCount);
109
110 // Wait on the EventWaitHandle.
111 ewh.WaitOne();
112
113 Console.WriteLine("Thread {0} exits.", data);
114 // Decrement the count of blocked threads.
115 Interlocked.Decrement(ref threadCount);
116
117 // After signaling ewh, the main thread blocks on
118 // clearCount until the signaled thread has
119 // decremented the count. Signal it now.
120 //
121 clearCount.Set();
122 }
123 }