监视器

更新： 2008 年 7 月

Monitor 对象通过使用 Monitor.Enter、Monitor.TryEnter 和 Monitor.Exit 方法对特定对象获取锁和释放锁来公开同步访问代码区域的能力。在对代码区域获取锁后，就可以使用 Monitor.Wait、Monitor.Pulse 和 Monitor.PulseAll 方法了。如果锁被暂挂，则 Wait 释放该锁并等待通知。当 Wait 接到通知后，它将返回并再次获取该锁。Pulse 和 PulseAll 都会发出信号以便等待队列中的下一个线程继续执行。

Visual Basic SyncLock 和 C# lock 语句使用 Monitor.Enter 获取锁，使用 Monitor.Exit 释放锁。使用语言语句的优点在于 lock 或 SyncLock 块中的所有内容都包含在 Try 语句中。Try 语句有一个 Finally 块，用以保证锁得以释放。

Monitor 将锁定对象（即引用类型），而非值类型。尽管可以向 Enter 和 Exit 传递值类型，但对于每次调用它都是分别装箱的。因为每次调用都创建一个独立的对象，所以 Enter 永远不会阻止，而且它要保护的代码并没有真正同步。另外，传递给 Exit 的对象不同于传递给 Enter 的对象，所以 Monitor 引发 SynchronizationLockException，并显示以下消息：“从不同步的代码块中调用了对象同步方法”。下面的示例演示这些问题。

Private x As Integer
' The next line creates a generic object containing the value of 
' x each time the code is executed, so that Enter never blocks.
Monitor.Enter(x)
Try
    ' Code that needs to be protected by the monitor.
Finally
    ' Always use Finally to ensure that you exit the Monitor.
    ' The following line creates another object containing 
    ' the value of x, and throws SynchronizationLockException
    ' because the two objects do not match.
    Monitor.Exit(x)
End Try

private int x;
// The next line creates a generic object containing the value of
// x each time the code is executed, so that Enter never blocks.
Monitor.Enter(x);
try {
    // Code that needs to be protected by the monitor.
}
finally {
    // Always use Finally to ensure that you exit the Monitor.
    // The following line creates another object containing 
    // the value of x, and throws SynchronizationLockException
    // because the two objects do not match.
    Monitor.Exit(x);
}

尽管您可以如下面的示例所示，在调用 Enter 和 Exit 之前将值类型变量装箱，并将同一个装箱的对象传递给这两个方法，但这样做并没有什么特别的用处。对变量的更改不能在装箱的变量中体现出来，也没有办法更改已装箱的变量的值。

Private o As Object = x

private Object o = x;

注意到 Monitor 和 WaitHandle 对象在使用上的区别是非常重要的。Monitor 对象是完全托管、完全可移植的，并且在操作系统资源要求方面可能更为有效。WaitHandle 对象表示操作系统可等待对象，对于在托管和非托管代码之间进行同步非常有用，并公开一些高级操作系统功能（如同时等待许多对象的能力）。

下面的代码示例演示 Monitor 类（使用 lock 和 SyncLock 编译器语句实现）、Interlocked 类和 AutoResetEvent 类的结合使用。

Imports System
Imports System.Threading
Imports Microsoft.VisualBasic

' Note: The class whose internal public member is the synchronizing method
' is not public; none of the client code takes a lock on the Resource object.
' The member of the nonpublic class takes the lock on itself. Written this 
' way, malicious code cannot take a lock on a public object.
Class SyncResource
   
   Public Sub Access(threadNum As Int32)
      ' Uses Monitor class to enforce synchronization.
      SyncLock Me
         ' Synchronized: Despite the next conditional, each thread 
         ' waits on its predecessor.
         If threadNum Mod 2 = 0 Then
            Thread.Sleep(2000)
         End If
         Console.WriteLine("Start Synched Resource access (Thread={0})", threadNum)
         Thread.Sleep(200)
         Console.WriteLine("Stop Synched Resource access (Thread={0})", threadNum)
      End SyncLock
   End Sub 'Access
End Class 'SyncResource

' Without the lock, the method is called in the order in which 
' threads reach it.
Class UnSyncResource
   
   Public Sub Access(threadNum As Int32)
      ' Does not use Monitor class to enforce synchronization.
      ' The next call throws the thread order.
      If threadNum Mod 2 = 0 Then
         Thread.Sleep(2000)
      End If
      Console.WriteLine("Start UnSynched Resource access (Thread={0})", threadNum)
      Thread.Sleep(200)
      Console.WriteLine("Stop UnSynched Resource access (Thread={0})", threadNum)
   End Sub 'Access
End Class 'UnSyncResource

Public Class App
   Private Shared numAsyncOps As Int32 = 5
   Private Shared asyncOpsAreDone As New AutoResetEvent(False)
   Private Shared SyncRes As New SyncResource()
   Private Shared UnSyncRes As New UnSyncResource()
   Private Shared threadNum As Int32
   Public Shared Sub Main()
      
      For threadNum = 0 To 4
         ThreadPool.QueueUserWorkItem(AddressOf SyncUpdateResource, threadNum)
      Next threadNum
      
      ' Wait until this WaitHandle is signaled.
      asyncOpsAreDone.WaitOne()
      Console.WriteLine(ControlChars.Tab + ControlChars.Lf + "All synchronized operations have completed." + ControlChars.Lf)
      
      ' Reset the thread count for unsynchronized calls.
      numAsyncOps = 5
      
      For threadNum = 0 To 4
         ThreadPool.QueueUserWorkItem(AddressOf UnSyncUpdateResource, threadNum)
      Next threadNum
      
      ' Wait until this WaitHandle is signaled.
      asyncOpsAreDone.WaitOne()
      Console.WriteLine(ControlChars.Tab + ControlChars.Cr + "All unsynchronized thread operations have completed.")
   End Sub 'Main
   
   
   
   ' The callback method's signature MUST match that of 
   ' a System.Threading.TimerCallback delegate
   ' (it takes an Object parameter and returns void).
   Shared Sub SyncUpdateResource(state As Object)
      ' This calls the internal synchronized method, passing 
      ' a thread number.
      SyncRes.Access(CType(state, Int32))
      
      ' Count down the number of methods that the threads have called.
      ' This must be synchronized, however; you cannot know which thread 
      ' will access the value **before** another thread's incremented 
      ' value has been stored into the variable.
      If Interlocked.Decrement(numAsyncOps) = 0 Then
         asyncOpsAreDone.Set() 
         ' Announce to Main that in fact all thread calls are done.
      End If
   End Sub 'SyncUpdateResource
    
   ' The callback method's signature MUST match that of 
   ' a System.Threading.TimerCallback delegate
   ' (it takes an Object parameter and returns void).
   Shared Sub UnSyncUpdateResource(state As [Object])
      ' This calls the unsynchronized method, passing 
      ' a thread number.
      UnSyncRes.Access(CType(state, Int32))
      
      ' Count down the number of methods that the threads have called.
      ' This must be synchronized, however; you cannot know which thread 
      ' will access the value **before** another thread's incremented 
      ' value has been stored into the variable.
      If Interlocked.Decrement(numAsyncOps) = 0 Then
         asyncOpsAreDone.Set() 
         ' Announce to Main that in fact all thread calls are done.
      End If
   End Sub 'UnSyncUpdateResource 
End Class 'App

using System;
using System.Threading;

// Note: The class whose internal public member is the synchronizing 
// method is not public; none of the client code takes a lock on the 
// Resource object.The member of the nonpublic class takes the lock on 
// itself. Written this way, malicious code cannot take a lock on 
// a public object.
class SyncResource {
   public void Access(Int32 threadNum) {
      // Uses Monitor class to enforce synchronization.
      lock (this) {
       // Synchronized: Despite the next conditional, each thread 
       // waits on its predecessor.
       if (threadNum % 2 == 0)
         Thread.Sleep(2000);
         Console.WriteLine("Start Synched Resource access (Thread={0})", threadNum);
         Thread.Sleep(200);
         Console.WriteLine("Stop Synched Resource access (Thread={0})", threadNum);
      }
   }
}

// Without the lock, the method is called in the order in which threads reach it.
class UnSyncResource {
   public void Access(Int32 threadNum) {
    // Does not use Monitor class to enforce synchronization.
    // The next call throws the thread order.
    if (threadNum % 2 == 0)
      Thread.Sleep(2000);
     Console.WriteLine("Start UnSynched Resource access (Thread={0})", threadNum);
     Thread.Sleep(200);
     Console.WriteLine("Stop UnSynched Resource access (Thread={0})", threadNum);
   }
}

public class App {
   static Int32 numAsyncOps = 5;
   static AutoResetEvent asyncOpsAreDone = new AutoResetEvent(false);
   static SyncResource SyncRes = new SyncResource();
   static UnSyncResource UnSyncRes = new UnSyncResource();

   public static void Main() {

      for (Int32 threadNum = 0; threadNum < 5; threadNum++) {
         ThreadPool.QueueUserWorkItem(new WaitCallback(SyncUpdateResource), threadNum);
      }

      // Wait until this WaitHandle is signaled.
     asyncOpsAreDone.WaitOne();
      Console.WriteLine("\t\nAll synchronized operations have completed.\t\n");

     // Reset the thread count for unsynchronized calls.
     numAsyncOps = 5;

      for (Int32 threadNum = 0; threadNum < 5; threadNum++) {
         ThreadPool.QueueUserWorkItem(new WaitCallback(UnSyncUpdateResource), threadNum);
      }

      // Wait until this WaitHandle is signaled.
     asyncOpsAreDone.WaitOne();
      Console.WriteLine("\t\nAll unsynchronized thread operations have completed.");
   }


   // The callback method's signature MUST match that of a 
   // System.Threading.TimerCallback delegate (it takes an Object 
   // parameter and returns void).
   static void SyncUpdateResource(Object state) {
     // This calls the internal synchronized method, passing 
     // a thread number.
      SyncRes.Access((Int32) state);

     // Count down the number of methods that the threads have called.
     // This must be synchronized, however; you cannot know which thread 
     // will access the value **before** another thread's incremented 
     // value has been stored into the variable.
      if (Interlocked.Decrement(ref numAsyncOps) == 0)
         asyncOpsAreDone.Set(); 
         // Announce to Main that in fact all thread calls are done.
   }

   // The callback method's signature MUST match that of a 
   // System.Threading.TimerCallback delegate (it takes an Object 
   // parameter and returns void).
   static void UnSyncUpdateResource(Object state) {
     // This calls the unsynchronized method, passing a thread number.
      UnSyncRes.Access((Int32) state);

     // Count down the number of methods that the threads have called.
     // This must be synchronized, however; you cannot know which thread 
     // will access the value **before** another thread's incremented 
     // value has been stored into the variable.
      if (Interlocked.Decrement(ref numAsyncOps) == 0)
         asyncOpsAreDone.Set(); 
         // Announce to Main that in fact all thread calls are done.
   }
}

请参见

参考

Monitor

其他资源

线程处理对象和功能

修订记录