Timer是.NET中定时触发一个事件处理方法的类
作者:互联网
ystem.Threading.Timer是.NET中定时触发一个事件处理方法的类(本文后面简称Timer),它背后依靠的是.NET的线程池(ThreadPool),所以当Timer在短时间内触发了过多的事件处理方法后,可能会造成事件处理方法在线程池(ThreadPool)中排队,可以参考这篇文章。
我们启动Timer后,如果我们想停止它,必须要用到Timer.Dispose方法,该方法会让Timer停止启动新的线程去执行事件处理方法,但是已经在线程池(ThreadPool)中处理和排队的事件处理方法还是会被继续执行,而Timer.Dispose方法会立即返回,它并不会被阻塞来等待剩下在线程池(ThreadPool)中处理和排队的事件处理方法都执行完毕。
所以这个时候我们需要一个机制来知道当Timer.Dispose方法被调用后,剩下在线程池(ThreadPool)中处理和排队的事件处理方法,是否都已经被执行完毕了。这个时候我们需要用到Timer的bool Dispose(WaitHandle notifyObject)重载方法,这个Dispose方法会传入一个WaitHandle notifyObject参数,当Timer剩下在线程池(ThreadPool)中处理和排队的事件处理方法都执行完毕后,Timer会给Dispose方法传入的WaitHandle notifyObject参数发出一个信号,而我们可以通过WaitHandle.WaitOne()方法来等待该信号,在收到信号前WaitHandle notifyObject会被一直阻塞,代码如下所示(基于.NET Core控制台项目):
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using System;
using System.Threading;
namespace TimerDispose
{
class Program
{
static Timer timer = null;
static ManualResetEvent timerDisposed = null;//ManualResetEvent继承WaitHandle
static int timeCount = 0;
static void CreateAndStartTimer()
{
//初始化Timer,设置触发间隔为2000毫秒
timer = new Timer(TimerCallBack, null, 0, 2000);
}
/// <summary>
/// TimerCallBack方法是Timer每一次触发后的事件处理方法
/// </summary>
static void TimerCallBack(object state)
{
//模拟做一些处理逻辑的事情
timeCount++;//每一次Timer触发调用TimerCallBack方法后,timeCount会加1
//当timeCount为100的时候,调用Timer.Change方法来改变Timer的触发间隔为1000毫秒
if (timeCount == 100)
{
timer.Change(0, 1000);
}
}
static void Main(string[] args)
{
CreateAndStartTimer();
Console.WriteLine("按任意键调用Timer.Dispose方法...");
Console.ReadKey();
timerDisposed = new ManualResetEvent(false);
timer.Dispose(timerDisposed);//调用Timer的bool Dispose(WaitHandle notifyObject)重载方法,来结束Timer的触发,当线程池中的所有TimerCallBack方法都执行完毕后,Timer会发一个信号给timerDisposed
timerDisposed.WaitOne();//WaitHandle.WaitOne()方法会等待收到一个信号,否则一直被阻塞
timerDisposed.Dispose();
Console.WriteLine("Timer已经结束,按任意键结束整个程序...");
Console.ReadKey();
}
}
}
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但是我们上面的代码中的TimerCallBack事件处理方法有一个逻辑,也就是当timeCount变量增加到100的时候,我们会调用Timer.Change方法,更改Timer的触发间隔为1000毫秒。而Timer.Change方法是不能够在Timer.Dispose方法后调用的,也就是说当一个Timer调用了Dispose方法后,就不能再调用Timer.Change方法了,否则Timer.Change方法会抛出ObjectDisposedException异常,对此MSDN上的解释如下:
If the callback uses the Change method to set the dueTime parameter to zero, a race condition can occur when the Dispose(WaitHandle) method overload is called: If the timer queues a new callback before the Dispose(WaitHandle) method overload detects that there are no callbacks queued, Dispose(WaitHandle) continues to block; otherwise, the timer is disposed while the new callback is being queued, and an ObjectDisposedException is thrown when the new callback calls the Change method.
然而在我们的代码中调用Timer.Dispose方法和TimerCallBack事件处理方法是并行的,因为Timer.Dispose方法是在程序主线程上执行的,而TimerCallBack事件处理方法是在线程池(ThreadPool)中的线程上执行的,所以Timer.Dispose方法执行后,很有可能会再执行TimerCallBack事件处理方法,这时候如果恰好timeCount变量也增加到100了,会导致Timer.Change方法在Timer.Dispose方法后执行,抛出ObjectDisposedException异常。
对此我们要对我们的代码稍作更改,在TimerCallBack事件处理方法中来捕捉ObjectDisposedException异常:
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using System;
using System.Threading;
namespace TimerDispose
{
class Program
{
static Timer timer = null;
static ManualResetEvent timerDisposed = null;//ManualResetEvent继承WaitHandle
static int timeCount = 0;
static void CreateAndStartTimer()
{
//初始化Timer,设置触发间隔为2000毫秒
timer = new Timer(TimerCallBack, null, 0, 2000);
}
/// <summary>
/// TimerCallBack方法是Timer每一次触发后的事件处理方法
/// </summary>
static void TimerCallBack(object state)
{
//模拟做一些处理逻辑的事情
timeCount++;//每一次Timer触发调用TimerCallBack方法后,timeCount会加1
//当timeCount为100的时候,调用Timer.Change方法来改变Timer的触发间隔为1000毫秒
if (timeCount == 100)
{
//添加try catch代码块,来捕捉Timer.Change方法抛出的ObjectDisposedException异常
try
{
timer.Change(0, 1000);
}
catch (ObjectDisposedException)
{
//当Timer.Change方法抛出ObjectDisposedException异常后的处理逻辑
Console.WriteLine("在Timer.Dispose方法执行后,再调用Timer.Change方法已经没有意义");
}
}
}
static void Main(string[] args)
{
CreateAndStartTimer();
Console.WriteLine("按任意键调用Timer.Dispose方法...");
Console.ReadKey();
timerDisposed = new ManualResetEvent(false);
timer.Dispose(timerDisposed);//调用Timer的bool Dispose(WaitHandle notifyObject)重载方法,来结束Timer的触发,当线程池中的所有TimerCallBack方法都执行完毕后,Timer会发一个信号给timerDisposed
timerDisposed.WaitOne();//WaitHandle.WaitOne()方法会等待收到一个信号,否则一直被阻塞
timerDisposed.Dispose();
Console.WriteLine("Timer已经结束,按任意键结束整个程序...");
Console.ReadKey();
}
}
}
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所以这样我们可以防止Timer.Change方法在Timer.Dispose方法后意外抛出ObjectDisposedException异常,至少异常抛出时我们是有代码去处理的。
而国外的一位高手不仅考虑到了Timer.Change方法会抛出ObjectDisposedException异常,他还给WaitHandle.WaitOne方法添加了超时限制(_disposalTimeout),并且还加入了逻辑来防止Timer.Dispose方法被多次重复调用,注意Timer的bool Dispose(WaitHandle notifyObject)重载方法是会返回一个bool值的,如果它返回了false,那么表示Timer.Dispose方法已经被调用过了,代码如下所示:
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using System;
using System.Threading;
namespace TimerDispose
{
class SafeTimer
{
private readonly TimeSpan _disposalTimeout;
private readonly System.Threading.Timer _timer;
private bool _disposeEnded;
public SafeTimer(TimeSpan disposalTimeout)
{
_disposalTimeout = disposalTimeout;
_timer = new System.Threading.www.078881.cn/ Timer(HandleTimerElapsed);
}
public void TriggerOnceIn(TimeSpan time)
{
try
{
_timer.Change(time, Timeout.InfiniteTimeSpan);
}
catch (ObjectDisposedException)
{
// race condition with Dispose can cause trigger to be called when underlying
// timer is being disposed -www.tkcyl1.com/ and a change will fail in this case.
// see
// https://msdn.microsoft.com/en-us/library/b97tkt95(v=vs.110).aspx#Anchor_2
if (_disposeEnded)
{
// we still want to throw the exception in case someone really tries
// to change the timer after disposal has finished
// of course there's a slight race condition here where we might not
// throw even though disposal is already done.
// since the offending code would most likely already be "failing"
// unreliably i personally can live with increasing the
// "unreliable failure"www.365soke.com time-window slightly
throw;
}
}
}
//Timer每一次触发后的事件处理方法
private void HandleTimerElapsed(object state)
{
//Do something
}
public void Dispose()
{
using (var waitHandle = new ManualResetEvent(false))
{
// returns false on second dispose
if (_timer.Dispose(waitHandle))
{
if (!waitHandle.WaitOne(_disposalTimeout))
{
throw new TimeoutException(
"Timeout waiting for www.shengchangyule.net timer to stop. (...)");
}
_disposeEnded = true;
}
}
}
}
}
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可以参考这个链接查看详情,需要注意的是里面有说到几点:
Timer.Dispose(WaitHandle) can return false. It does so in case it's already been disposed (i had to look at the source code). In that case it won't set the WaitHandle - so don't wait on it! (Note: multiple disposal should be supported)
也就是说如果Timer的bool Dispose(WaitHandle notifyObject)重载方法返回了false,Timer是不会给WaitHandle notifyObject参数发出信号的,所以当Dispose方法返回false时,不要去调用WaitHandle.WaitOne方法。
Timer.Dispose(WaitHandle) does not work properly with -Slim waithandles, or not as one would expect. For example, the following does not work (it blocks www.shengchangyule.net forever):
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using (var manualResetEventSlim = new ManualResetEventSlim())
{
timer.Dispose(manualResetEventSlim.WaitHandle);
manualResetEventSlim.Wait();
}
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也就是说不要用ManualResetEventSlim,否则WaitHandle.WaitOne方法会一直阻塞下去。
标签:事件处理,Dispose,Timer,timer,NET,方法,WaitHandle 来源: https://blog.csdn.net/li123128/article/details/89376309