I have a timer event that fires every second. Sometimes when I exit the program (in the VS debugger), it tells me that the event thread is trying to access an object that no longer exists (because the main thread is over). I tried disabling the event before I exit (UpdateTime.aTimer.Enabled = false;). This cut down the number of times this problem occurs, but it still happens sometimes because the event fires before I can disable it.
Is this a serious problem? Will is haunt me if I don't deal with it?
If yes to the above, how do I kill it?
I ask the second question because I have no reference to the event thread, so I don't know how I can tell it to stop or wait for it to finish.
EDIT: More context. This is a Winform.
Also, I'm not explicitly creating a thread. It's my understanding that a thread is automatically created to handle events.
Creating the timer:
public static void Update(){
System.Timers.Timer aTimer = new System.Timers.Timer(1000);
aTimer.Elapsed += new ElapsedEventHandler(OnTimedEvent);
aTimer.Enabled = true;
}
Event handler:
private static void OnTimedEvent(object source,ElapsedEventArgs e) {
Form1obj.updateLabel(String.Format("{0}", DateTime.Now.Second),Label1);
}
Closing program handler:
private void Form1_FormClosing(object sender,FormClosingEventArgs e) {
aTimer.Enabled = false;
}
Serious Problem? Probably not, but I don't think a problem needs to be serious to need to be fixed. Warnings should be treated as errors in compilation, for example. In addition, if this is an app sent to clients, showing ugly errors on shutdown is not very professional.
How to solve this depends on how you are setting up the threads ("event thread" does not give enough info about the mechanics). One easy method might be to stall the main thread for a bit when the application shuts down until the threads all return. You have to also stop issuing new threads during this time.
Another possible solution would be to handle how the threads are created so you can shut down the process that fires them. Concepts like ThreadPool come to mind. Also ensuring threads are background threads, explicitly, can help in some situations.
The short answer is nobody will be able to give you a firm "this will fix your issue" answer without some context of what you are doing in your code.
ADDED:
There are a couple of "quick, down and dirty" ways to handle this. Don't have time for full analysis, so see if they work.
Just cure the error by waiting
Add a counter and wait until incremented down
The first thing I would consider is adding a safety net to not update the label when in a shutdown condition. That is regardless of anything else, as that is where your errors are firing. I don't think "main thread is not present" is the core of the issue, but rather this line:
Form1obj.updateLabel(String.Format("{0}", DateTime.Now.Second),Label1);
How can you update something that no longer exists? Yes, it is on the main thread, so technically ...
A simple wait would be something like:
private void Form1_FormClosing(object sender,FormClosingEventArgs e)
{
aTimer.Enabled = false;
Thread.Sleep(5000);
}
Hiding the form is also not a bad idea, so the user does not see this?
If you want to use a more "COM like approach", you can add a counter. Increment on Update() (when the event is fired) and decrement on OnTimedEvent(). Make sure you lock the counter when changing it so you do not end up with two threads changing it at the same millisecond. You can then wait until the counter is 0 to finish form close or application unload.
Once again, these are quick, down and dirty, approaches, but they can save you from the error. I am sure someone with more time can come up with a more elegant solution.
You can close the window as suggested in MSDN - when you set the timer to be disabled during shutdown processing, set a flag that your Elapsed event handler can check to know that no more work is needed.
Elapsed events can occur after the
Dispose or Stop method has been called
or after the Enabled property has been
set to false, because the signal to
raise the Elapsed event is always
queued for execution on a thread pool
thread. One way to resolve this race
condition
is to set a flag that tells the event
handler for the Elapsed event to
ignore subsequent events.
Its hard to give a general awnser to the question if its serious or not, it depends entiry on what the timer is doing. what kind of timer is it? a system.Threading one or one of the UI timers?
If possible try and refactor your code so that you can tell the timer to stop firing, if only for the reason not to confuse the users with an error message. it could be as simple as sharing a variable or (preferably) using a CancellationToken
Related
I have a WPF application which used to shutdown via Environment.Exit. Since this caused problems with my automated UI tests, I changed the exiting application to Application.Current.ShutDown.
This works fine, except there is a thread which is waiting to be pulsed (Monitor.Wait), which keeps the process running since it never gets pulsed anymore.
I thought that would be a no brainer to fix, but from the point where my application exits (the Application.Current.ShutDown) it's rather hard to retrieve a reference to the object which holds the waiting thread (in order to force pulse it, so that it can exit).
I tried to google an appropriate answer, but not much luck yet. Is there an "easy way" out of this? Or should I start refactoring already? :)
Some snippets:
Thread is created like this
workerThread = new Thread(Worker) { Name = logName, IsBackground = true};
In the Worker method, Monitor.Wait is called
while ((action = GetNextTask(out task)) == ProductPreparationAction.None)
{
Monitor.Wait(preparationTasks);
}
Nevermind my comment. Start refactoring :).
First of all, there should be a way for the while loop to end when before the app stops. Perhaps you can use and propagate a CancellationToken all the way down to the Worker method.
If you want to keep your loose coupling, you should be able to pulse by creating an event in the class that calls Application.Current.ShutDown and by subscribing to it in the class where the Worker method is (and call Pulse in the event handler).
If you do this, then you can store the CancellationToken in this class and flag it when the event is received.
That event should be raised before calling Application.Current.ShutDown.
I want to use a System.Windows.Forms.Timer to ensure that an event fires on the UI thread of an excel addin I'm creating. I construct the timer as follows:
private System.Windows.Forms.Timer _timer;
private void ThisAddIn_Startup(object sender, System.EventArgs e)
{
Debug.WriteLine("ThisAddIn_Startup:" + Thread.CurrentThread.ManagedThreadId);
_timer = new System.Windows.Forms.Timer();
_timer.Tick += new EventHandler(TimerEventHandler);
_timer.Interval = 500;
}
The timer is fired by a COM event from a library I am using:
private void OnEvent()
{
_timer.Start();
}
I then expect the _timer to call the following method when it ticks:
public void TimerEventHandler(object sender, EventArgs args)
{
_timer.Stop();
Debug.WriteLine("Tick: " + Thread.CurrentThread.ManagedThreadId);
}
As I understand, when I create the timer in the Addin thread, even though it is started from another thread (COM event in this case), it should fire on the thread that it was created on, i.e. the addin thread. However, this doesn't happen.
I have implemented this exact mechanism in an RTDServer I wrote in the past (as outlined by Kenny Kerr) and it works as expected but the _timer in this scenario never ticks.
I have also read other SO articles that point to the same behavior and can't figure out what is different about my addin setup?
EDIT:
The OnEvent() method is fired.
The winforms timer is a control and must be used by placing it on a form. You never add it to a control-collection, so I would not expect it to work properly. The documentation says the following
Implements a timer that raises an event at user-defined intervals. This timer is optimized for use in Windows Forms applications and must be used in a window.
Therefore, I would suggest that you use an instance of the System.Timers.Timer class. This class can be used anywhere.
Note that the Tick-event you use above, is called by another name in the System.Timer.Timer class, namely the Elapsed-event.
I initially meant to post this as comment, but it turned to be too long.
Firstly, your thread structure is a bit confusing to me, the way you described it. Put Debug.WriteLine("OnEvent:" + Thread.CurrentThread.ManagedThreadId) inside OnEvent and let us know all thread IDs you see from your debug output.
That said, the rules are:
You should create WinForms' Timer object on an STA thread, and the thread should be configured as STA before it starts.
This thread may or may not be the main UI thread (where your main form was created), but it still should execute a message loop (with Application.Run) for timer events to fire. There are other ways of pumping messages, but generally you do not control them from .NET code.
You should handle the events sourced by WinForms' Timer on the same thread it was created. You can then 'forward' these events to another thread context if you like (using SynchronizationContext Send or Post) but I can't think of any reasons for such complexity.
The answer by #Maarten actually suggests the right way of doing it, in my opinion.
I don't yet understand why the Forms.Timer doesn't operate as expected but the following excellent article explains in detail how to marshal work onto the UI thread: http://www.codeproject.com/Articles/31971/Understanding-SynchronizationContext-Part-I
In a Silverlight app, I have a block of code that has to run every 500ms. I am planning o use a DispatcherTimer to achieve this (see code below).
DispatcherTimer dt = new DispatcherTimer();
dt.Interval = new TimeSpan(0, 0, 0, 0, 500); // 500 Milliseconds
dt.Tick += new EventHandler(dt_Tick);
dt.Start();
However, it may happen that the block of code takes longer than 500ms to execute (the block of code does some webservice calls). How do I make sure that if a call is currently in progress, the DispatcherTimer doesn't trigger another event? What are the options and what is the best way? Using locks?
The DispatcherTimer only runs on the dispatcher thread - so there's no way you could have two handlers running at the same time. It's possible they'll be queued up and run one directly after another, of course - you should check.
However, you shouldn't be making a web service call in a DispatcherTimer anyway. Do it in a background thread, otherwise you're blocking the UI for updating all the time that you're waiting for the web service. Basically you shouldn't do any long-running work in the UI thread. Use one of the various other timers (e.g. System.Timers.Timer) to regularly perform work on a thread pool thread and use the dispatcher to call back to the UI thread when you've got some data which needs to be displayed on the UI.
Of course, now you've got the potential problem of the new kind of timer firing multiple times concurrently, on multiple threads. One option to avoid this is to set the AutoReset property to false, and just schedule the next timer tick at the end of the current one.
I would say you skip a tick if it takes too long, otherwise you will get a huge queue because of the lock.
So in the eventhandler say:
if(!busy) {
busy = true;
// some code which could take longer than 500 ms
busy = false;
}
In order make the event run successfull without getting a call from your DispatcherTimer again with in the previous tick completes stop the dispatcher timer after entering in to dt_Tick event and at the end of the tick event call the start again which will initializes the IsEnabled of DispatcherTimer again to true.
I don't know if DispatchTimer has any clever way to do this but what I would do in this situation is not to try to get the timer to not fire the event but to get the event to do nothing if it has not finished the previous run.
You can use locks to do this by getting a lock at the beginning of your event handler. If the lock is not available then exit the function (its already running) if you get the lock do the work and then once you've finished the work release the lock.
The method you want is Monitor.TryEnter and you'll want to make sure that you do your error trapping correctly as with any use of locks.
Can anybody suggest any reasons why a C# timer (created in a code-behind class) would stop without being told to?
My timer starts on page load and then stops when I click a button. I don't need to click the button for it to sometimes stop. IIS is not being restarted to my knowledge and no errors are being thrown either.
This is confusing me quite a bit...
Thanks.
// This gets called on page_load
private void checkTimer()
{
if (!parentTimer.Enabled) // If parent timer is not enabled then it is probably the start of a new day (after a night time server backup which kills timers)
{
parentTimer.Interval = 60000; // One minute
parentTimer.Elapsed += new ElapsedEventHandler(parentTimer_Elapsed); // Define what happens when elapsed
parentTimer.AutoReset = true; // Set timer to repeat
parentTimer.Enabled = true; // Start the timer
}
}
protected void btnCancel_Click(object sender, System.EventArgs e)
{
parentTimer.Stop();
...etc...
}
Note: I do not change ParentTimer at all in its elapsed method.
Basically ParentTimer governs a list of ChildTimers. If ParentTimer elapses it checks if one or more of the ChildTimers have elapsed too, if so, there is an event, if not then it resets the ChildTimer and carries on.
My suspicion is it's because the worker process for the page is stopping at the end of the request.
You could try increasing the request time out, but a better question is Can you explain why you're trying to do this ? What is the problem you're trying to solve ?
Remember, that regardless of all the fluff that ASP.Net puts around your code to make you feel comfortable (session state, viewstate etc), a web request is stateless and should be considered as a distinct pass of logic, it's not like a windows application where a background thread of code in your void Main(...) function is constantly running.
A timer is tied to the thread that created it and in the case of ASP.net the thread that handles each page request issued by a given user will change frequently due to the use of worker threads and the thread pool.
Using a timer at page-level simply won't work; you need to be tracking the state at Session-level (tied to the particular user) as your starting point.
In fact, I just wouldn't use timers at all in a web application, because their execution is simply not guaranteed.
If you're using this to run a background task - consider firing up your own worker thread in Application_Start or something like that. The thread will be terminated when the app pool recycles. You should also look at manually shutting the thread down the application is being shut down too.
Be careful with this, however, this thread can't assume it's always the only one running - due to IIS overlapped recycling, when a new one fires up the old one could still be running in the old AppDomain.
In my form's class, I've added a method to "fade" it out. This makes use of System.Timers.Timer and the Elapsed event uses a delegate to change the form's opacity. This was the code:
public void FadeOut()
{
// Timer for transition
Timer fade = new Timer();
// Transition code
fade.Elapsed += delegate
{
this.Opacity += 0.05;
if (this.Opacity >= .95)
{
this.Opacity = 1;
fade.Enabled = false;
fade.Dispose();
}
};
fade.Interval = 100;
fade.Enabled = true;
}
This caused a "Cross-thread operation not valid" error, which is a common hurdle I see. So I looked around for solutions and the first ones to come up involved using .BeginInvoke and blocks of code to keep the call to the same thread as the control. But I found this looked really bulky, so I kept looking and then discovered the SynchronizingObject property of System.Timers.Timer. This seems better because it only needed one extra line of code:
// Timer for transition
Timer fade = new Timer();
fade.SynchronizingObject = this;
The code runs fine now. But I'm really confused, how come a lot of solutions are suggesting the use of BeginInvoke/Invoke when all that's needed is to set SynchronizingObject to the form control?
Mostly because it is pointless to use the property. Yes, it ensures that the Elapsed event handler runs on the UI thread. But now it just does the same thing as a System.Windows.Forms.Timer.
Not quite though, it is worse. Because it doesn't guarantee that Elapsed won't be called after you disable it. Disabling it doesn't flush any pending invokes nor TP threads that aren't ready to run yet. There could be hundreds if the Interval is small compared to the amount of work done by the handler.
You absolutely want a System.Windows.Forms.Timer here. You are not doing any useful work on the threadpool thread.
I'm not sure, but I believe that the Timer will internally use Invoke or BeginInvoke as well on the SynchronizingObject property.
Let us say that this property just gives some abstraction to the developer; to make his life easier.
My guess was indeed correct, this is what Reflector tells us about the MyTimerCallback private member method of System.Timers.Timer:
ElapsedEventHandler onIntervalElapsed = this.onIntervalElapsed;
if (onIntervalElapsed != null)
{
if ((this.SynchronizingObject != null) && this.SynchronizingObject.InvokeRequired)
{
this.SynchronizingObject.BeginInvoke(onIntervalElapsed, new object[] { this, e });
}
else
{
onIntervalElapsed(this, e);
}
}
Why don't you use the WinForms timer? This is based on window messages and will always run in the UI thread; since you want to perform updates where the UI thread needs to pump messages anyways this may be a better solution (no synchronization/blocking required).
Timer solutions like this are really a bit of a hack.
You'd be better off writing a proper asynchronous thread and doing the callback either with
BeginInvoke or SynchronizationContext.
As you rightly observe, it's far from a one-liner, but multi-tasking, done properly, never is.
The point is the SCOPE of code. The whole event handler? or just the UI changing code.
timer.SynchronizingObject make the event handler to be called on the thread of given object. If you set this to 'this' in 'Form1' class, This means that all of your code for processing timer event is run by that same thread of 'Form1' instance even though you created a thread based timer. So this does NOTHING. SAME as windows form timer.
The reason you want to do this is because you want to access UI controls and make some changes in Form1, but the whole UI hangs if you use windows form timer. Because they all get executed on the same thread.
To avoid this, you use a threading based timer. Your event handler is called on another thread that comes from some system thread pool. This solves the problem of hanging UI. However, this brings up another problem. Cross-thread accessing UI controls exception. On some versions of Visual Studio, you can disable this checking on debug builds. But there is no way you can just bypass this checking on release builds. This thread checking is just by design to safeguard from crashes caused by multi-threading. THEN you need to use all those invoke and delegation stuff.
The important part here is that it is the only few lines of code that is redirected to be run on the Form1 thread. Not the whole event handler. Most of the event handling code is run on another thread. And that includes code like doing something on network or disk.
This makes the DIFFERENCE.
BUT, all these apply to windows form apps only. For WPF, just use dispatcher timer. (This is why you cannot find Windows.Threading in WinForm because you can't used dispatcher timer in WinForm, but available in WPF)