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Part V
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Threading
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Internally, the thread pool has just one thread that it uses for all Timer objects . This thread knows when the next Timer object s time is due . When the next Timer object is due, the thread wakes up, and internally calls ThreadPool s QueueUserWorkItem to enter an entry into the thread pool s queue, causing your callback method to get called . If your callback method takes a long time to execute, the timer could go off again . This could cause multiple thread pool threads to be executing your callback method simultaneously . To work around this problem, I recommend the following: Construct the Timer specifying Timeout.Infinite for the period parameter . Now, the timer will fire only once . Then, in your callback method, call the Change method specifying a new due time and again specify Timeout.Infinite for the period parameter . Here is what the Change method overloads look like:
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public sealed class Timer : MarshalByRefObject, IDisposable { public Boolean Change(Int32 dueTime, Int32 period); public Boolean Change(UInt32 dueTime, UInt32 period); public Boolean Change(Int64 dueTime, Int64 period); public Boolean Change(TimeSpan dueTime, TimeSpan period); }
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The Timer class also offers a Dispose method which allows you to cancel the timer altogether and optionally signal the kernel object identified by the notifyObject parameter when all pending callbacks for the time have completed . Here is what the Dispose method overloads look like:
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public sealed class Timer : MarshalByRefObject, IDisposable { public Boolean Dispose(); public Boolean Dispose(WaitHandle notifyObject); }
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Important When a Timer object is garbage collected, its finalization code tells the thread pool
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to cancel the timer so that it no longer goes off . So when using a Timer object, make sure that a variable is keeping the Timer object alive or else your callback method will stop getting called . This is discussed and demonstrated in the Garbage Collections and Debugging section in 21, Automatic Memory Management (Garbage Collection) .
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The following code demonstrates how to have a thread pool thread call a method starting immediately and then every 2 seconds thereafter:
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internal static class TimerDemo { private static Timer s_timer; public static void Go() { Console.WriteLine("Main thread: starting a timer"); using (s_timer = new Timer(ComputeBoundOp, 5, 0, Timeout.Infinite)) { Console.WriteLine("Main thread: Doing other work here..."); Thread.Sleep(10000); // Simulating other work (10 seconds) } // Calls Dispose to cancel the timer now }
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26
Compute-Bound Asynchronous Operations
// This method's signature must match the TimerCallback delegate private static void ComputeBoundOp(Object state) { // This method is executed by a thread pool thread Console.WriteLine("In ComputeBoundOp: state={0}", state); Thread.Sleep(1000); // Simulates other work (1 second) // Have the Timer call this method again in 2 seconds s_timer.Change(2000, Timeout.Infinite); // When this method returns, the thread goes back // to the pool and waits for another work item } }
So Many Timers, So Little Time
Unfortunately, the FCL actually ships with several timers, and it is not clear to most programmers what makes each timer unique . Let me attempt to explain:
This is the timer discussed in the previous section, and it is the best timer to use when you want to perform periodic background tasks on a thread pool thread .
System.Threading s Timer class
Constructing an instance of this class tells Windows to associate a timer with the calling thread (see the Win32 SetTimer function) . When this timer goes off, Windows injects a timer message ( WM_TIMER) into the thread s message queue . The thread must execute a message pump that extracts these messages and dispatches them to the desired callback method . Notice that all of the work is done by just one thread the thread that sets the timer is guaranteed to be the thread that executes the callback method . This also means that your timer method will not be executed by multiple threads concurrently .
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