Working with CancellationToken and Dispose

CancellationToken, and its owner CancellationTokenSource (CTS), were introduced in .NET 4.0 as a general purpose cancellation framework. It is often associated with Task as that was the first API to it. However, it is, in fact, independent of Task and should be used wherever you are supporting the concept of cancellation as async APIs you may call with be designed with it in mind.

One thing I only noticed recently is that CTS implements IDisposable. It previously hadn’t occurred to me that I would need to Dispose a CTS. Looking at the implementation in DotPeek I saw there were a few things that would require cleanup. So, dutifully, I put the calls in my code – only to see it sometimes blow up spectacularly with an ObjectDisposedException. To understand what was happening consider the following code

public class CancellableExecutor
{
    CancellationTokenSource cts = new CancellationTokenSource();
    public Task Execute(Action<CancellationToken> action)
    {
        return Task.Run(() => action(cts.Token), cts.Token);
    }

    public void Cancel()
    {
       cts.Cancel();
       cts.Dispose();
       cts = new CancellationTokenSource();
    }
}

Here we have a class whose job it is to be able to run a series of actions and cancel any outstanding on request. As you can imagine this type of object is intended to be used in asynchronous execution and therefore new pieces of work could be getting requested for execution while someone else is going to cancel.

The problem is that cts.Token throws an ObjectDisposedException if accessed after Dispose has been called. Now, I fully understand why I shouldn’t be able to, say, call Cancel on the CTS once it is Disposed but accessing a value that allows no more interaction than if I had accessed it just before Dispose was called seems very odd. Because of this, let’s call it a, “feature” we have to write the above code in the following non-obvious way

public class CancellableExecutor
{
    CancellationTokenSource cts = new CancellationTokenSource();
    CancellationToken token = CancellationToken.None;

    public CancellableExecutor()
    {
        token = cts.Token;
    }

    public Task Execute(Action<CancellationToken> action)
    {
        return Task.Run(() => action(token), token);
    }

    public void Cancel()
    {
        cts.Cancel();
        cts.Dispose();
        cts = new CancellationTokenSource();
        token = cts.Token;
    }
}

As you can see, we have to cache the CancellationToken and pass that around. Although this works, I really wish we could access the Token property after Dispose is called.

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