How to pause a thread at once in C#? - c#

I use ManualResetEnvent to pause/continue a thread. The code example is below.
private _rstEvent = new ManualResetEvent(true);
public void DoSomeWork()
{
while(judgementValue)
{
_rstEvent.WaitOne();
...
}
}
public void Pause()
{
_rstEvent.Reset();
}
public void Continue()
{
_rstEvent.Set();
}
The problem is what if the while loop is large, which means every loop in the while statement has many operations to do. The thread will keep going until meet the next _rstEvent.WaitOne();. Is there a way to pause the thread at once except the deprecated suspend?

The problem with Suspend, Abort and the like is that you're trying to do something to another thread which could literally be doing anything at the time.
It has been repeatedly observed that this can lead to difficult to diagnose bugs where e.g. none of your current threads can obtain a lock because the thread that held the lock has been suspended or aborted and thus will never release the lock (substitute in any other resource also).
This is why modern threading mechanisms are built around cooperative primitives where the thread itself checks (at appropriate moments) whether it's being asked to suspend or abort and can ensure that it only does so when not holding any resources.
There's no reason your loop code cannot check the event multiple times per iteration, whenever it is appropriate.

Related

How can I make a interrupt in C#? [duplicate]

I understand Thread.Abort() is evil from the multitude of articles I've read on the topic, so I'm currently in the process of ripping out all of my abort's in order to replace it for a cleaner way; and after comparing user strategies from people here on stackoverflow and then after reading "How to: Create and Terminate Threads (C# Programming Guide)" from MSDN both which state an approach very much the same -- which is to use a volatile bool approach checking strategy, which is nice, but I still have a few questions....
Immediately what stands out to me here, is what if you do not have a simple worker process which is just running a loop of crunching code? For instance for me, my process is a background file uploader process, I do in fact loop through each file, so that's something, and sure I could add my while (!_shouldStop) at the top which covers me every loop iteration, but I have many more business processes which occur before it hits it's next loop iteration, I want this cancel procedure to be snappy; don't tell me I need to sprinkle these while loops every 4-5 lines down throughout my entire worker function?!
I really hope there is a better way, could somebody please advise me on if this is in fact, the correct [and only?] approach to do this, or strategies they have used in the past to achieve what I am after.
Thanks gang.
Further reading: All these SO responses assume the worker thread will loop. That doesn't sit comfortably with me. What if it is a linear, but timely background operation?
Unfortunately there may not be a better option. It really depends on your specific scenario. The idea is to stop the thread gracefully at safe points. That is the crux of the reason why Thread.Abort is not good; because it is not guaranteed to occur at safe points. By sprinkling the code with a stopping mechanism you are effectively manually defining the safe points. This is called cooperative cancellation. There are basically 4 broad mechanisms for doing this. You can choose the one that best fits your situation.
Poll a stopping flag
You have already mentioned this method. This a pretty common one. Make periodic checks of the flag at safe points in your algorithm and bail out when it gets signalled. The standard approach is to mark the variable volatile. If that is not possible or inconvenient then you can use a lock. Remember, you cannot mark a local variable as volatile so if a lambda expression captures it through a closure, for example, then you would have to resort to a different method for creating the memory barrier that is required. There is not a whole lot else that needs to be said for this method.
Use the new cancellation mechanisms in the TPL
This is similar to polling a stopping flag except that it uses the new cancellation data structures in the TPL. It is still based on cooperative cancellation patterns. You need to get a CancellationToken and the periodically check IsCancellationRequested. To request cancellation you would call Cancel on the CancellationTokenSource that originally provided the token. There is a lot you can do with the new cancellation mechanisms. You can read more about here.
Use wait handles
This method can be useful if your worker thread requires waiting on an specific interval or for a signal during its normal operation. You can Set a ManualResetEvent, for example, to let the thread know it is time to stop. You can test the event using the WaitOne function which returns a bool indicating whether the event was signalled. The WaitOne takes a parameter that specifies how much time to wait for the call to return if the event was not signaled in that amount of time. You can use this technique in place of Thread.Sleep and get the stopping indication at the same time. It is also useful if there are other WaitHandle instances that the thread may have to wait on. You can call WaitHandle.WaitAny to wait on any event (including the stop event) all in one call. Using an event can be better than calling Thread.Interrupt since you have more control over of the flow of the program (Thread.Interrupt throws an exception so you would have to strategically place the try-catch blocks to perform any necessary cleanup).
Specialized scenarios
There are several one-off scenarios that have very specialized stopping mechanisms. It is definitely outside the scope of this answer to enumerate them all (never mind that it would be nearly impossible). A good example of what I mean here is the Socket class. If the thread is blocked on a call to Send or Receive then calling Close will interrupt the socket on whatever blocking call it was in effectively unblocking it. I am sure there are several other areas in the BCL where similiar techniques can be used to unblock a thread.
Interrupt the thread via Thread.Interrupt
The advantage here is that it is simple and you do not have to focus on sprinkling your code with anything really. The disadvantage is that you have little control over where the safe points are in your algorithm. The reason is because Thread.Interrupt works by injecting an exception inside one of the canned BCL blocking calls. These include Thread.Sleep, WaitHandle.WaitOne, Thread.Join, etc. So you have to be wise about where you place them. However, most the time the algorithm dictates where they go and that is usually fine anyway especially if your algorithm spends most of its time in one of these blocking calls. If you algorithm does not use one of the blocking calls in the BCL then this method will not work for you. The theory here is that the ThreadInterruptException is only generated from .NET waiting call so it is likely at a safe point. At the very least you know that the thread cannot be in unmanaged code or bail out of a critical section leaving a dangling lock in an acquired state. Despite this being less invasive than Thread.Abort I still discourage its use because it is not obvious which calls respond to it and many developers will be unfamiliar with its nuances.
Well, unfortunately in multithreading you often have to compromise "snappiness" for cleanliness... you can exit a thread immediately if you Interrupt it, but it won't be very clean. So no, you don't have to sprinkle the _shouldStop checks every 4-5 lines, but if you do interrupt your thread then you should handle the exception and exit out of the loop in a clean manner.
Update
Even if it's not a looping thread (i.e. perhaps it's a thread that performs some long-running asynchronous operation or some type of block for input operation), you can Interrupt it, but you should still catch the ThreadInterruptedException and exit the thread cleanly. I think that the examples you've been reading are very appropriate.
Update 2.0
Yes I have an example... I'll just show you an example based on the link you referenced:
public class InterruptExample
{
private Thread t;
private volatile boolean alive;
public InterruptExample()
{
alive = false;
t = new Thread(()=>
{
try
{
while (alive)
{
/* Do work. */
}
}
catch (ThreadInterruptedException exception)
{
/* Clean up. */
}
});
t.IsBackground = true;
}
public void Start()
{
alive = true;
t.Start();
}
public void Kill(int timeout = 0)
{
// somebody tells you to stop the thread
t.Interrupt();
// Optionally you can block the caller
// by making them wait until the thread exits.
// If they leave the default timeout,
// then they will not wait at all
t.Join(timeout);
}
}
If cancellation is a requirement of the thing you're building, then it should be treated with as much respect as the rest of your code--it may be something you have to design for.
Lets assume that your thread is doing one of two things at all times.
Something CPU bound
Waiting for the kernel
If you're CPU bound in the thread in question, you probably have a good spot to insert the bail-out check. If you're calling into someone else's code to do some long-running CPU-bound task, then you might need to fix the external code, move it out of process (aborting threads is evil, but aborting processes is well-defined and safe), etc.
If you're waiting for the kernel, then there's probably a handle (or fd, or mach port, ...) involved in the wait. Usually if you destroy the relevant handle, the kernel will return with some failure code immediately. If you're in .net/java/etc. you'll likely end up with an exception. In C, whatever code you already have in place to handle system call failures will propagate the error up to a meaningful part of your app. Either way, you break out of the low-level place fairly cleanly and in a very timely manner without needing new code sprinkled everywhere.
A tactic I often use with this kind of code is to keep track of a list of handles that need to be closed and then have my abort function set a "cancelled" flag and then close them. When the function fails it can check the flag and report failure due to cancellation rather than due to whatever the specific exception/errno was.
You seem to be implying that an acceptable granularity for cancellation is at the level of a service call. This is probably not good thinking--you are much better off cancelling the background work synchronously and joining the old background thread from the foreground thread. It's way cleaner becasue:
It avoids a class of race conditions when old bgwork threads come back to life after unexpected delays.
It avoids potential hidden thread/memory leaks caused by hanging background processes by making it possible for the effects of a hanging background thread to hide.
There are two reasons to be scared of this approach:
You don't think you can abort your own code in a timely fashion. If cancellation is a requirement of your app, the decision you really need to make is a resource/business decision: do a hack, or fix your problem cleanly.
You don't trust some code you're calling because it's out of your control. If you really don't trust it, consider moving it out-of-process. You get much better isolation from many kinds of risks, including this one, that way.
The best answer largely depends on what you're doing in the thread.
Like you said, most answers revolve around polling a shared boolean every couple lines. Even though you may not like it, this is often the simplest scheme. If you want to make your life easier, you can write a method like ThrowIfCancelled(), which throws some kind of exception if you're done. The purists will say this is (gasp) using exceptions for control flow, but then again cacelling is exceptional imo.
If you're doing IO operations (like network stuff), you may want to consider doing everything using async operations.
If you're doing a sequence of steps, you could use the IEnumerable trick to make a state machine. Example:
<
abstract class StateMachine : IDisposable
{
public abstract IEnumerable<object> Main();
public virtual void Dispose()
{
/// ... override with free-ing code ...
}
bool wasCancelled;
public bool Cancel()
{
// ... set wasCancelled using locking scheme of choice ...
}
public Thread Run()
{
var thread = new Thread(() =>
{
try
{
if(wasCancelled) return;
foreach(var x in Main())
{
if(wasCancelled) return;
}
}
finally { Dispose(); }
});
thread.Start()
}
}
class MyStateMachine : StateMachine
{
public override IEnumerabl<object> Main()
{
DoSomething();
yield return null;
DoSomethingElse();
yield return null;
}
}
// then call new MyStateMachine().Run() to run.
>
Overengineering? It depends how many state machines you use. If you just have 1, yes. If you have 100, then maybe not. Too tricky? Well, it depends. Another bonus of this approach is that it lets you (with minor modifications) move your operation into a Timer.tick callback and void threading altogether if it makes sense.
and do everything that blucz says too.
Perhaps the a piece of the problem is that you have such a long method / while loop. Whether or not you are having threading issues, you should break it down into smaller processing steps. Let's suppose those steps are Alpha(), Bravo(), Charlie() and Delta().
You could then do something like this:
public void MyBigBackgroundTask()
{
Action[] tasks = new Action[] { Alpha, Bravo, Charlie, Delta };
int workStepSize = 0;
while (!_shouldStop)
{
tasks[workStepSize++]();
workStepSize %= tasks.Length;
};
}
So yes it loops endlessly, but checks if it is time to stop between each business step.
You don't have to sprinkle while loops everywhere. The outer while loop just checks if it's been told to stop and if so doesn't make another iteration...
If you have a straight "go do something and close out" thread (no loops in it) then you just check the _shouldStop boolean either before or after each major spot inside the thread. That way you know whether it should continue on or bail out.
for example:
public void DoWork() {
RunSomeBigMethod();
if (_shouldStop){ return; }
RunSomeOtherBigMethod();
if (_shouldStop){ return; }
//....
}
Instead of adding a while loop where a loop doesn't otherwise belong, add something like if (_shouldStop) CleanupAndExit(); wherever it makes sense to do so. There's no need to check after every single operation or sprinkle the code all over with them. Instead, think of each check as a chance to exit the thread at that point and add them strategically with this in mind.
All these SO responses assume the worker thread will loop. That doesn't sit comfortably with me
There are not a lot of ways to make code take a long time. Looping is a pretty essential programming construct. Making code take a long time without looping takes a huge amount of statements. Hundreds of thousands.
Or calling some other code that is doing the looping for you. Yes, hard to make that code stop on demand. That just doesn't work.

ManualResetEvent vs while loop

ManualResetEvent basically says to other threads "you can only proceed when you receive a signal to continue" and is used to pause execution for certain threads until certain condition has been fulfilled. What I want to ask is that why ManualResetEvent when we could easily achieve what we want by using a while loop? Consider the following context:
public class BackgroundService {
ManualResetEvent mre;
public BackgroundService() {
mre = new ManualResetEvent(false);
}
public void Initialize() {
// Initialization
mre.Set();
}
public void Start() {
mre.WaitOne();
// The rest of execution
}
}
is somewhat similar to
public class BackgroundService {
bool hasInitialized;
public BackgroundService() {
}
public void Initialize() {
// Initialization
hasInitialized = true;
}
public void Start() {
while (!hasInitialized)
Thread.Sleep(100);
// The rest of execution
}
}
Is there any particular context where ManualResetEvent is more suitable than a while loop?
Is there any particular context where ManualResetEvent is more suitable than a while loop?
Absolutely. There are two primary reasons: latency and efficiency.
Context-switching the thread to start it running again is relatively expensive, when it's just going to go back to sleep, and the approach you've given will take an average of 50ms to respond to the hasInitialized variable being set - assuming it responds at all. (You don't have any explicit memory barriers, so it's possible that the thread won't actually see a change to the variable at all. I suspect that calling Thread.Sleep effectively adds a memory barrier, but it's not guaranteed.) With OS/CLR-level synchronization primitives, a thread can respond much faster.
Using signals such as that provided ManualResetEvent is more efficient. Using a while loop as you have means after roughly every 100 milliseconds, i.e. 10 times a second, other threads have to stop running so your thread to check the condition has to run, this context switch when the condition is mostly false is less efficient.
However something smells very fishy in your code, why would you have such code polling when something is initialized? If the initialisation is asynchronous there will already be some notification mechanism, e.g. a callback, when it is done so the polling is unnecessary.

C# program using up too much CPU?

I have a program that starts constantly juggles between 3 separate timers.
The main thread of my application has a while loop which constantly checks if a global variable has been set to true and if it has, it will stop one timer and launch two other ones - one continuously, another to stop it automatically if it isn't commanded to stop for whatever reason.
This while loop has a condition of (1==1) so that it runs forever.
In the task manager (XP) I see that my program is using up 50% of cpu on a more or less idle system.
Is there a way to decrease that number by decreasing the speed of the while loop or something?
Thanks.
Is there a way to decrease that number by decreasing the speed of the while loop or something?
Just stop doing a busy loop. There are better ways of coordinating events between threads. Consider using Monitor.Wait/Pulse, or AutoResetEvent / ManualResetEvent. Basically, the thread that sets the global variable should signal that it's done so. Alternatively, if your main thread wouldn't be doing anything else, why not add a normal C# event so that whenever the variable is changed, the event is raised and the appropriate action can be taken?
Your program performs busy waiting, which is a bad practice. You should change your logic so that instead of looping, you block on some kind of synchronization primitive (also known as wait handle).
Blocking on a wait handle is not an option for the UI thread, so you would have to create three threads in total and implement the scheme like this:
The UI thread does not concern itself at all with what other threads to. No looping, no sleeping, no blocking.
The new "controller" thread would start the existing "worker" thread and then immediately block (e.g. on an event that is not signaled). It will remain in this state, without consuming CPU, until the event is signaled (i.e. the "worker" completes).
The "worker" thread would run its course and then signal the event.
Is there a way to decrease that number by decreasing the speed of the while loop or something?
Yes, you could insert a call to Thread.Sleep(n). With a granularity of ~20 ms.
But the far better option would be using a Waithandle.
Your main thread would Wait on the handle and the end of the timer code would signal it to wake up.
You need to sleep the threat for a given number of ms. Look at the Thread.sleep() function and place it within your while loop.
The easiest way to slow down a loop like this is to just add a System.Threading.Thread.Sleep(100); For every iteration the process will sleep for 100 ms and it will not use 50% cpu anymore.
You can use Threads instead of Timer it costlier than Thread. Or Please check the thread state of your time which stopped before start another. You can improve performance by cutting down your code logic.
Hope this will helps you. :)
While the answers here aren't wrong per-say, They don't really address a lot of issues with doing while(true) loops which is what while(1==1) is.
First of, even if the loop is running the entire time your application is in use, you will want to shit it down at some point, say when the user exits your application, because if you have a thread with a constant loop, even if the user closes the UI window, the process will remain until the thread exits (which is never in a while true loop) or until the user becomes wise and closes it from the task manager.
You COULD solve this by putting a true conditional in the while conditional check that references a accessible property outside the loop.
Example:
private bool RequestExit = false;
private Thread MyLoopThread;
private void MyLoop()
{
while (!RequestExit)
{
//Do stuff!
Sleep(1);
}
}
public void StartLoop()
{
RequestExit = false;
MyLoopThread = new Thread(MyLoop);
MyLoopThread.Start();
}
public void StopLoop()
{
RequestExit = true;
}
That is the bare-bones and doesn't even get into avoiding double launches or double shutdown events.
A much cleaner way would be to set an arbitrary interval that you want to pool at, 10ms or so should do just fine for pretty much any real time event, and trigger an method to fire at that interval.
private Timer DoStuffTimer;
private void DoStuffMethod(object obj = null)
{
//Do stuff!
}
public void StartLoop()
{
DoStuffTimer = new Timer(DoStuffMethod,null,10,10);
}
public void StopLoop()
{
DoStuffTimer.Dispose();
}

Question about terminating a thread cleanly in .NET

I understand Thread.Abort() is evil from the multitude of articles I've read on the topic, so I'm currently in the process of ripping out all of my abort's in order to replace it for a cleaner way; and after comparing user strategies from people here on stackoverflow and then after reading "How to: Create and Terminate Threads (C# Programming Guide)" from MSDN both which state an approach very much the same -- which is to use a volatile bool approach checking strategy, which is nice, but I still have a few questions....
Immediately what stands out to me here, is what if you do not have a simple worker process which is just running a loop of crunching code? For instance for me, my process is a background file uploader process, I do in fact loop through each file, so that's something, and sure I could add my while (!_shouldStop) at the top which covers me every loop iteration, but I have many more business processes which occur before it hits it's next loop iteration, I want this cancel procedure to be snappy; don't tell me I need to sprinkle these while loops every 4-5 lines down throughout my entire worker function?!
I really hope there is a better way, could somebody please advise me on if this is in fact, the correct [and only?] approach to do this, or strategies they have used in the past to achieve what I am after.
Thanks gang.
Further reading: All these SO responses assume the worker thread will loop. That doesn't sit comfortably with me. What if it is a linear, but timely background operation?
Unfortunately there may not be a better option. It really depends on your specific scenario. The idea is to stop the thread gracefully at safe points. That is the crux of the reason why Thread.Abort is not good; because it is not guaranteed to occur at safe points. By sprinkling the code with a stopping mechanism you are effectively manually defining the safe points. This is called cooperative cancellation. There are basically 4 broad mechanisms for doing this. You can choose the one that best fits your situation.
Poll a stopping flag
You have already mentioned this method. This a pretty common one. Make periodic checks of the flag at safe points in your algorithm and bail out when it gets signalled. The standard approach is to mark the variable volatile. If that is not possible or inconvenient then you can use a lock. Remember, you cannot mark a local variable as volatile so if a lambda expression captures it through a closure, for example, then you would have to resort to a different method for creating the memory barrier that is required. There is not a whole lot else that needs to be said for this method.
Use the new cancellation mechanisms in the TPL
This is similar to polling a stopping flag except that it uses the new cancellation data structures in the TPL. It is still based on cooperative cancellation patterns. You need to get a CancellationToken and the periodically check IsCancellationRequested. To request cancellation you would call Cancel on the CancellationTokenSource that originally provided the token. There is a lot you can do with the new cancellation mechanisms. You can read more about here.
Use wait handles
This method can be useful if your worker thread requires waiting on an specific interval or for a signal during its normal operation. You can Set a ManualResetEvent, for example, to let the thread know it is time to stop. You can test the event using the WaitOne function which returns a bool indicating whether the event was signalled. The WaitOne takes a parameter that specifies how much time to wait for the call to return if the event was not signaled in that amount of time. You can use this technique in place of Thread.Sleep and get the stopping indication at the same time. It is also useful if there are other WaitHandle instances that the thread may have to wait on. You can call WaitHandle.WaitAny to wait on any event (including the stop event) all in one call. Using an event can be better than calling Thread.Interrupt since you have more control over of the flow of the program (Thread.Interrupt throws an exception so you would have to strategically place the try-catch blocks to perform any necessary cleanup).
Specialized scenarios
There are several one-off scenarios that have very specialized stopping mechanisms. It is definitely outside the scope of this answer to enumerate them all (never mind that it would be nearly impossible). A good example of what I mean here is the Socket class. If the thread is blocked on a call to Send or Receive then calling Close will interrupt the socket on whatever blocking call it was in effectively unblocking it. I am sure there are several other areas in the BCL where similiar techniques can be used to unblock a thread.
Interrupt the thread via Thread.Interrupt
The advantage here is that it is simple and you do not have to focus on sprinkling your code with anything really. The disadvantage is that you have little control over where the safe points are in your algorithm. The reason is because Thread.Interrupt works by injecting an exception inside one of the canned BCL blocking calls. These include Thread.Sleep, WaitHandle.WaitOne, Thread.Join, etc. So you have to be wise about where you place them. However, most the time the algorithm dictates where they go and that is usually fine anyway especially if your algorithm spends most of its time in one of these blocking calls. If you algorithm does not use one of the blocking calls in the BCL then this method will not work for you. The theory here is that the ThreadInterruptException is only generated from .NET waiting call so it is likely at a safe point. At the very least you know that the thread cannot be in unmanaged code or bail out of a critical section leaving a dangling lock in an acquired state. Despite this being less invasive than Thread.Abort I still discourage its use because it is not obvious which calls respond to it and many developers will be unfamiliar with its nuances.
Well, unfortunately in multithreading you often have to compromise "snappiness" for cleanliness... you can exit a thread immediately if you Interrupt it, but it won't be very clean. So no, you don't have to sprinkle the _shouldStop checks every 4-5 lines, but if you do interrupt your thread then you should handle the exception and exit out of the loop in a clean manner.
Update
Even if it's not a looping thread (i.e. perhaps it's a thread that performs some long-running asynchronous operation or some type of block for input operation), you can Interrupt it, but you should still catch the ThreadInterruptedException and exit the thread cleanly. I think that the examples you've been reading are very appropriate.
Update 2.0
Yes I have an example... I'll just show you an example based on the link you referenced:
public class InterruptExample
{
private Thread t;
private volatile boolean alive;
public InterruptExample()
{
alive = false;
t = new Thread(()=>
{
try
{
while (alive)
{
/* Do work. */
}
}
catch (ThreadInterruptedException exception)
{
/* Clean up. */
}
});
t.IsBackground = true;
}
public void Start()
{
alive = true;
t.Start();
}
public void Kill(int timeout = 0)
{
// somebody tells you to stop the thread
t.Interrupt();
// Optionally you can block the caller
// by making them wait until the thread exits.
// If they leave the default timeout,
// then they will not wait at all
t.Join(timeout);
}
}
If cancellation is a requirement of the thing you're building, then it should be treated with as much respect as the rest of your code--it may be something you have to design for.
Lets assume that your thread is doing one of two things at all times.
Something CPU bound
Waiting for the kernel
If you're CPU bound in the thread in question, you probably have a good spot to insert the bail-out check. If you're calling into someone else's code to do some long-running CPU-bound task, then you might need to fix the external code, move it out of process (aborting threads is evil, but aborting processes is well-defined and safe), etc.
If you're waiting for the kernel, then there's probably a handle (or fd, or mach port, ...) involved in the wait. Usually if you destroy the relevant handle, the kernel will return with some failure code immediately. If you're in .net/java/etc. you'll likely end up with an exception. In C, whatever code you already have in place to handle system call failures will propagate the error up to a meaningful part of your app. Either way, you break out of the low-level place fairly cleanly and in a very timely manner without needing new code sprinkled everywhere.
A tactic I often use with this kind of code is to keep track of a list of handles that need to be closed and then have my abort function set a "cancelled" flag and then close them. When the function fails it can check the flag and report failure due to cancellation rather than due to whatever the specific exception/errno was.
You seem to be implying that an acceptable granularity for cancellation is at the level of a service call. This is probably not good thinking--you are much better off cancelling the background work synchronously and joining the old background thread from the foreground thread. It's way cleaner becasue:
It avoids a class of race conditions when old bgwork threads come back to life after unexpected delays.
It avoids potential hidden thread/memory leaks caused by hanging background processes by making it possible for the effects of a hanging background thread to hide.
There are two reasons to be scared of this approach:
You don't think you can abort your own code in a timely fashion. If cancellation is a requirement of your app, the decision you really need to make is a resource/business decision: do a hack, or fix your problem cleanly.
You don't trust some code you're calling because it's out of your control. If you really don't trust it, consider moving it out-of-process. You get much better isolation from many kinds of risks, including this one, that way.
The best answer largely depends on what you're doing in the thread.
Like you said, most answers revolve around polling a shared boolean every couple lines. Even though you may not like it, this is often the simplest scheme. If you want to make your life easier, you can write a method like ThrowIfCancelled(), which throws some kind of exception if you're done. The purists will say this is (gasp) using exceptions for control flow, but then again cacelling is exceptional imo.
If you're doing IO operations (like network stuff), you may want to consider doing everything using async operations.
If you're doing a sequence of steps, you could use the IEnumerable trick to make a state machine. Example:
<
abstract class StateMachine : IDisposable
{
public abstract IEnumerable<object> Main();
public virtual void Dispose()
{
/// ... override with free-ing code ...
}
bool wasCancelled;
public bool Cancel()
{
// ... set wasCancelled using locking scheme of choice ...
}
public Thread Run()
{
var thread = new Thread(() =>
{
try
{
if(wasCancelled) return;
foreach(var x in Main())
{
if(wasCancelled) return;
}
}
finally { Dispose(); }
});
thread.Start()
}
}
class MyStateMachine : StateMachine
{
public override IEnumerabl<object> Main()
{
DoSomething();
yield return null;
DoSomethingElse();
yield return null;
}
}
// then call new MyStateMachine().Run() to run.
>
Overengineering? It depends how many state machines you use. If you just have 1, yes. If you have 100, then maybe not. Too tricky? Well, it depends. Another bonus of this approach is that it lets you (with minor modifications) move your operation into a Timer.tick callback and void threading altogether if it makes sense.
and do everything that blucz says too.
Perhaps the a piece of the problem is that you have such a long method / while loop. Whether or not you are having threading issues, you should break it down into smaller processing steps. Let's suppose those steps are Alpha(), Bravo(), Charlie() and Delta().
You could then do something like this:
public void MyBigBackgroundTask()
{
Action[] tasks = new Action[] { Alpha, Bravo, Charlie, Delta };
int workStepSize = 0;
while (!_shouldStop)
{
tasks[workStepSize++]();
workStepSize %= tasks.Length;
};
}
So yes it loops endlessly, but checks if it is time to stop between each business step.
You don't have to sprinkle while loops everywhere. The outer while loop just checks if it's been told to stop and if so doesn't make another iteration...
If you have a straight "go do something and close out" thread (no loops in it) then you just check the _shouldStop boolean either before or after each major spot inside the thread. That way you know whether it should continue on or bail out.
for example:
public void DoWork() {
RunSomeBigMethod();
if (_shouldStop){ return; }
RunSomeOtherBigMethod();
if (_shouldStop){ return; }
//....
}
Instead of adding a while loop where a loop doesn't otherwise belong, add something like if (_shouldStop) CleanupAndExit(); wherever it makes sense to do so. There's no need to check after every single operation or sprinkle the code all over with them. Instead, think of each check as a chance to exit the thread at that point and add them strategically with this in mind.
All these SO responses assume the worker thread will loop. That doesn't sit comfortably with me
There are not a lot of ways to make code take a long time. Looping is a pretty essential programming construct. Making code take a long time without looping takes a huge amount of statements. Hundreds of thousands.
Or calling some other code that is doing the looping for you. Yes, hard to make that code stop on demand. That just doesn't work.

Using lock statement within a loop in C#

Lets take the sample class SomeThread where we are attempting to prevent the DoSomething methods from being called after the Running property is set to false and Dispose is called by the OtherThread class because if they are called after the Dispose method is the world would end as we know it.
It feels like there is a chance for something evil to happen because of the loop. That at the point where it starts the next loop and before the lock is taken before calling the DoSomething methods, Running could be changed to false, and Disposed called before it hits the lock. In this scenario life would not be good.
I was looking at ways to handle this when using a loop in a simple easy to maintain method. For the record I did considered the Double Lock Check patterned, however it is does not seem to be recommend for C#.
Warning: This is a simplified example to try to make it easy to focus on the issue with the loop and locking within one. If I didn't elaborate enough some place please let me know and I will do my best to fill in any details.
public class SomeThread : IDisposable
{
private object locker = new object();
private bool running = false;
public bool Running
{
get
{
lock(locker)
{
return running;
}
}
set
{
lock(locker)
{
running = value;
}
}
}
public void Run()
{
while (Running)
{
lock(locker)
{
DoSomething1();
DoSomething2();
}
}
}
private void DoSomething1()
{
// something awesome happens here
}
private void DoSomething2()
{
// something more awesome happens here
}
public void Dispose()
{
lock (locker)
{
Dispose1();
Dispose2();
}
}
private void Dispose1()
{
// something awesome happens here
}
private void Dispose2()
{
// something more awesome happens here
}
}
public class OtherThread
{
SomeThread st = new SomeThread();
public void OnQuit()
{
st.Running = false;
st.Dispose();
Exit();
}
}
Take a step back.
Start by specifying all the desirable and undesirable characteristics before you start to write a solution. A few that come immediately to mind:
The "work" is done on thread W. The "UI" is done on thread U.
The work is done in "units of work". Each unit of work is "short" in duration, for some definition of "short". Let's call the method that does the work M().
The work is done continuously by W, in a loop, until U tells it to stop.
U calls a cleanup method, D(), when all the work is done.
D() must not ever run before or while M() is running.
Exit() must be called after D(), on thread U.
U must never block for a "long" time; it is acceptable for it to block for a "short" time.
No deadlocks, and so on.
Does this sum up the problem space?
First off, I note that it seems at first glance that the problem is that U must be the caller of D(). If W were the caller of D(), then you wouldn't have to worry; you'd just signal W to break out of the loop, and then W would call D() after the loop. But that just trades one problem for another; presumably in this scenario, U must wait for W to call D() before U calls Exit(). So moving the call to D() from U to W doesn't actually make the problem easier.
You've said that you don't want to use double-checked locking. You should be aware that as of CLR v2, the double-checked locking pattern is known to be safe. The memory model guarantees were strengthened in v2. So it is probably safe for you to use double-checked locking.
UPDATE: You asked for information on (1) why is double-checked locking safe in v2 but not in v1? and (2) why did I use the weasel-word "probably"?
To understand why double-checked locking is unsafe in the CLR v1 memory model but safe in the CLR v2 memory model, read this:
http://web.archive.org/web/20150326171404/https://msdn.microsoft.com/en-us/magazine/cc163715.aspx
I said "probably" because as Joe Duffy wisely says:
once you venture even slightly outside
of the bounds of the few "blessed"
lock-free practices [...] you are
opening yourself up to the worst kind
of race conditions.
I do not know if you are planning on using double-checked locking correctly, or if you're planning on writing your own clever, broken variation on double-checked locking that in fact dies horribly on IA64 machines. Hence, it will probably work for you, if your problem is actually amenable to double checked locking and you write the code correctly.
If you care about this you should read Joe Duffy's articles:
http://www.bluebytesoftware.com/blog/2006/01/26/BrokenVariantsOnDoublecheckedLocking.aspx
and
http://www.bluebytesoftware.com/blog/2007/02/19/RevisitedBrokenVariantsOnDoubleCheckedLocking.aspx
And this SO question has some good discussion:
The need for volatile modifier in double checked locking in .NET
Probably it is best to find some other mechanism other than double-checked locking.
There is a mechanism for waiting for one thread which is shutting down to complete -- thread.Join. You could join from the UI thread to the worker thread; when the worker thread is shut down, the UI thread wakes up again and does the dispose.
UPDATE: Added some information on Join.
"Join" basically means "thread U tells thread W to shut down, and U goes to sleep until that happens". Brief sketch of the quit method:
// do this in a thread-safe manner of your choosing
running = false;
// wait for worker thread to come to a halt
workerThread.Join();
// Now we know that worker thread is done, so we can
// clean up and exit
Dispose();
Exit();
Suppose you didn't want to use "Join" for some reason. (Perhaps the worker thread needs to keep running in order to do something else, but you still need to know when it is done using the objects.) We can build our own mechanism that works like Join by using wait handles. What you need now are two locking mechanisms: one that lets U send a signal to W that says "stop running now" and then another that waits while W finishes off the last call to M().
What I would do in this circumstance is:
make a thread-safe flag "running". Use whatever mechanism you are comfortable with to make it thread safe. I would personally start with a lock dedicated to it; if you decide later that you can go with lock-free interlocked operations on it then you can always do that later.
make an AutoResetEvent to act as a gate on the dispose.
So, brief sketch:
UI thread, startup logic:
running = true
waithandle = new AutoResetEvent(false)
start up worker thread
UI thread, quit logic:
running = false; // do this in a thread-safe manner of your choosing
waithandle.WaitOne();
// WaitOne is robust in the face of race conditions; if the worker thread
// calls Set *before* WaitOne is called, WaitOne will be a no-op. (However,
// if there are *multiple* threads all trying to "wake up" a gate that is
// waiting on WaitOne, the multiple wakeups will be lost. WaitOne is named
// WaitOne because it WAITS for ONE wakeup. If you need to wait for multiple
// wakeups, don't use WaitOne.
Dispose();
waithandle.Close();
Exit();
worker thread:
while(running) // make thread-safe access to "running"
M();
waithandle.Set(); // Tell waiting UI thread it is safe to dispose
Notice that this relies on the fact that M() is short. If M() takes a long time then you can wait a long time to quit the application, which seems bad.
Does that make sense?
Really though, you shouldn't be doing this. If you want to wait for the worker thread to shut down before you dispose an object it is using, just join it.
UPDATE: Some additional questions raised:
is it a good idea to wait without a timeout?
Indeed, note that in my example with Join and my example with WaitOne, I do not use the variants on them that wait for a specific amount of time before giving up. Rather, I call out that my assumption is that the worker thread shuts down cleanly and quickly. Is this the correct thing to do?
It depends! It depends on just how badly the worker thread behaves and what it is doing when it is misbehaving.
If you can guarantee that the work is short in duration, for whatever 'short' means to you, then you don't need a timeout. If you cannot guarantee that, then I would suggest first rewriting the code so that you can guarantee that; life becomes much easier if you know that the code will terminate quickly when you ask it to.
If you cannot, then what's the right thing to do? The assumption of this scenario is that the worker is ill-behaved and does not terminate in a timely manner when asked to. So now we've got to ask ourselves "is the worker slow by design, buggy, or hostile?"
In the first scenario, the worker is simply doing something that takes a long time and for whatever reason, cannot be interrupted. What's the right thing to do here? I have no idea. This is a terrible situation to be in. Presumably the worker is not shutting down quickly because doing so is dangerous or impossible. In that case, what are you going to do when the timeout times out??? You've got something that is dangerous or impossible to shut down, and its not shutting down in a timely manner. Your choices seem to be (1) do nothing, (2) do something dangerous, or (3) do something impossible. Choice three is probably out. Choice one is equivalent to waiting forever, whcih we've already rejected. That leaves "do something dangerous".
Knowing what the right thing to do in order to minimize harm to user data depends upon the exact circumstances that are causing the danger; analyse it carefully, understand all the scenarios, and figure out the right thing to do.
Now suppose the worker is supposed to be able to shut down quickly, but does not because it has a bug. Obviously, if you can, fix the bug. If you cannot fix the bug -- perhaps it is in code you do not own -- then again, you are in a terrible fix. You have to understand what the consequences are of not waiting for already-buggy-and-therefore-unpredictable code to finish before disposing of the resources that you know it is using right now on another thread. And you have to know what the consequences are of terminating an application while a buggy worker thread is still busy doing heaven only knows what to operating system state.
If the code is hostile and is actively resisting being shut down then you have already lost. You cannot halt the thread by normal means, and you cannot even thread abort it. There is no guarantee whatsoever that aborting a hostile thread actually terminates it; the owner of the hostile code that you have foolishly started running in your process could be doing all of its work in a finally block or other constrained region which prevents thread abort exceptions.
The best thing to do is to never get into this situation in the first place; if you have code that you think is hostile, either do not run it at all, or run it in its own process, and terminate the process, not the thread when things go badly.
In short, there's no good answer to the question "what do I do if it takes too long?" You are in a terrible situation if that happens and there is no easy answer. Best to work hard to ensure you don't get into it in the first place; only run cooperative, benign, safe code that always shuts itself down cleanly and rapidly when asked.
What if the worker throws an exception?
OK, so what if it does? Again, better to not be in this situation in the first place; write the worker code so that it does not throw. If you cannot do that, then you have two choices: handle the exception, or don't handle the exception.
Suppose you don't handle the exception. As of I think CLR v2, an unhandled exception in a worker thread shuts down the whole application. The reason being, in the past what would happen is you'd start up a bunch of worker threads, they'd all throw exceptions, and you'd end up with a running application with no worker threads left, doing no work, and not telling the user about it. It is better to force the author of the code to handle the situation where a worker thread goes down due to an exception; doing it the old way effectively hides bugs and makes it easy to write fragile applications.
Suppose you do handle the exception. Now what? Something threw an exception, which is by definition an unexpected error condition. You now have no clue whatsoever that any of your data is consistent or any of your program invariants are maintained in any of your subsystems. So what are you going to do? There's hardly anything safe you can do at this point.
The question is "what is best for the user in this unfortunate situation?" It depends on what the application is doing. It is entirely possible that the best thing to do at this point is to simply aggressively shut down and tell the user that something unexpected failed. That might be better than trying to muddle on and possibly making the situation worse, by, say, accidentally destroying user data while trying to clean up.
Or, it is entirely possible that the best thing to do is to make a good faith effort to preserve the user's data, tidy up as much state as possible, and terminate as normally as possible.
Basically, both your questions are "what do I do when my subsystems do not behave themselves?" If your subsystems are unreliable, either make them reliable, or have a policy for how you deal with an unreliable subsystem, and implement that policy. That's a vague answer I know, but that's because dealing with an unreliable subsystem is an inherently awful situation to be in. How you deal with it depends on the nature of its unreliability, and the consequences of that unreliability to the user's valuable data.
Check Running again inside the lock:
while (Running)
{
lock(locker)
{
if(Running) {
DoSomething1();
DoSomething2();
}
}
}
You could even rewrite this as a while(true)...break, which would probably be preferable.
Instead of using a bool for Running, why not use an Enum with states of Stopped, Starting, Running, and Stopping?
That way, you break out of the loop when Running gets set to Stopping, and do your Disposing. Once that's done, Running gets set to Stopped. When OnQuit() sees Running set to Stopped, it will go ahead and exit.
Edit: Here's code, quick and dirty, not tested, etc.
public class SomeThread : IDisposable
{
private object locker = new object();
private RunState running = RunState.Stopped;
public enum RunState
{
Stopped,
Starting,
Running,
Stopping,
}
public RunState Running
{
get
{
lock(locker)
{
return running;
}
}
set
{
lock(locker)
{
running = value;
}
}
}
public void Run()
{
while (Running == RunState.Running)
{
lock(locker)
{
DoSomething1();
DoSomething2();
}
}
Dispose();
}
private void DoSomething1()
{
// something awesome happens here
}
private void DoSomething2()
{
// something more awesome happens here
}
public void Dispose()
{
lock (locker)
{
Dispose1();
Dispose2();
}
Running = RunState.Stopped;
}
private void Dispose1()
{
// something awesome happens here
}
private void Dispose2()
{
// something more awesome happens here
}
}
public class OtherThread
{
SomeThread st = new SomeThread();
public void OnQuit()
{
st.Running = SomeThread.RunState.Stopping;
while (st.Running == SomeThread.RunState.Stopping)
{
// Do something while waiting for the other thread.
}
Exit();
}
}

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