Question about terminating a thread cleanly in .NET

Question

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?

Answer 1

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.