When to dispose CancellationTokenSource?

问题:

The class CancellationTokenSource is disposable. A quick look in Reflector proves usage of KernelEvent, a (very likely) unmanaged resource. Since CancellationTokenSource has no finalizer, if we do not dispose it, the GC won't do it.

On the other hand, if you look at the samples listed on the MSDN article Cancellation in Managed Threads, only one code snippet disposes of the token.

What is the proper way to dispose of it in code?

1. You cannot wrap code starting your parallel task with using if you do not wait for it. And it makes sense to have cancellation only if you do not wait.
2. Of course you can add ContinueWith on task with a Dispose call, but is that the way to go?
3. What about cancelable PLINQ queries, which do not synchronize back, but just do something at the end? Let's say .ForAll(x => Console.Write(x))?
4. Is it reusable? Can the same token be used for several calls and then dispose it together with the host component, let's say UI control?

Because it does not have something like a Reset method to clean-up IsCancelRequested and Token field I would suppose it's not reusable, thus every time you start a task (or a PLINQ query) you should create a new one. Is it true? If yes, my question is what is the correct and recommended strategy to deal with Dispose on those many CancellationTokenSource instances?

回答1:

Speaking about whether it's really necessary to call Dispose on CancellationTokenSource... I had a memory leak in my project and it turned out that CancellationTokenSource was the problem.

My project has a service, that is constantly reading database and fires off different tasks, and I was passing linked cancellation tokens to my workers, so even after they had finished processing data, cancellation tokens weren't disposed, which caused a memory leak.

MSDN Cancellation in Managed Threads states it clearly:

Notice that you must call Dispose on the linked token source when you are done with it. For a more complete example, see How to: Listen for Multiple Cancellation Requests.

I used ContinueWith in my implementation.

回答2:

I didn't think any of the current answers were satisfactory. After researching I found this reply from Stephen Toub (reference):

It depends. In .NET 4, CTS.Dispose served two primary purposes. If the CancellationToken's WaitHandle had been accessed (thus lazily allocating it), Dispose will dispose of that handle. Additionally, if the CTS was created via the CreateLinkedTokenSource method, Dispose will unlink the CTS from the tokens it was linked to. In .NET 4.5, Dispose has an additional purpose, which is if the CTS uses a Timer under the covers (e.g. CancelAfter was called), the Timer will be Disposed.

It's very rare for CancellationToken.WaitHandle to be used, so cleaning up after it typically isn't a great reason to use Dispose. If, however, you're creating your CTS with CreateLinkedTokenSource, or if you're using the CTS' timer functionality, it can be more impactful to use Dispose.

The bold part I think is the important part. He uses "more impactful" which leaves it a bit vague. I'm interpreting it as meaning calling Dispose in those situations should be done, otherwise using Dispose is not needed.

回答3:

I took a look in ILSpy for the CancellationTokenSource but I can only find m_KernelEvent which is actually a ManualResetEvent, which is a wrapper class for a WaitHandle object. This should be handled properly by the GC.

回答4:

You should always dispose CancellationTokenSource.

How to dispose it depends exactly on the scenario. You propose several different scenarios.

1. using only works when you're using CancellationTokenSource on some parallel work that you're waiting. If that's your senario, then great, it's the easiest method.

2. When using tasks, use a ContinueWith task as you indicated to dispose of CancellationTokenSource.

3. For plinq you can use using since you're running it in parallel but waiting on all of the parallel running workers to finish.

4. For UI, you can create a new CancellationTokenSource for each cancellable operation that is not tied to a single cancel trigger. Maintain a List<IDisposable> and add each source to the list, disposing all of them when your component is disposed.

5. For threads, create a new thread that joins all the worker threads and closes the single source when all of the worker threads finished. See CancellationTokenSource, When to dispose?

There's always a way. IDisposable instances should always be disposed. Samples often don't because they're either quick samples to show core usage or because adding in all aspects of the class being demonstrated would be overly complex for a sample. The sample is just that a sample, not necessarily (or even usually) production quality code. Not all samples are acceptable to be copied into production code as is.

回答5:

This answer is still coming up in Google searches, and I believe the voted up answer is does not give the full story. After looking over the source code for CancellationTokenSource (CTS) and CancellationToken (CT) I believe that for most use cases the following code sequence is fine:

if (cancelTokenSource != null)
{
    cancelTokenSource.Cancel();
    cancelTokenSource.Dispose();
    cancelTokenSource = null;
}

The m_kernelHandle internal field mentioned above is the synchronization object backing the WaitHandle property in both the CTS and CT classes. It is only instantiated if you access that property. So, unless you are using WaitHandle for some old-school thread synchronization in your Task calling dispose will have no effect.

Of course, if you are using it you should do what is suggested by the other answers above and delay calling Dispose until any WaitHandle operations using the handle are complete, because, as is described in the Windows API documentation for WaitHandle, the results are undefined.

回答6:

Create a new Windows Forms application from the project template. Drop a button on the form and double click it. Make it look like this:

    private void button1_Click(object sender, EventArgs e) {
        var t = new System.Threading.Thread(() => { });
        t.Start();
    }

Press Ctrl+F5 to start it. Start + Run, TaskMgr.exe, Processes tab. View + Select Columns and tick "Handles". Observe the value of this column for the WindowsFormsApplication1.exe process while you click the button repeatedly.

The Thread class doesn't have a Dispose() method.

Let's work from the assumption that it had one. When would you call it?

---

Read more about the wisdom of trying to dispose hard-to-dispose objects in this blog post by Stephen Toub.

回答7:

It has been a long time since I asked this and got many helpful answers but I came across an interesting issue related to this and thought I would post it here as another answer of sorts:

You should call CancellationTokenSource.Dispose() only when you are sure that nobody is going to try to get the CTS's Token property. Otherwise you should not call it, because it is a race. For instance, see here:

https://github.com/aspnet/AspNetKatana/issues/108

In the fix for this issue, code which previously did cts.Cancel(); cts.Dispose(); was edited to just do cts.Cancel(); because anyone so unlucky as to try to get the cancellation token in order to observe its cancellation state after Dispose has been called will unfortunately also need to handle ObjectDisposedException - in addition to the OperationCanceledException that they were planning for.

Another key observation related to this fix is made by Tratcher: "Disposal is only required for tokens that won't be cancelled, as cancellation does all of the same cleanup." i.e. just doing Cancel() instead of disposing is really good enough!