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For a discussion as to why aborting .NET Framework code is
not safe, see the topic “Aborting Threads” at http://www.alba
hari.com/threading/.
There are two cases, though, where you can safely use Abort. One is if you are willing
to tear down a thread’s application domain after it is aborted. A good example of
when you might do this is in writing a unit-testing framework. (We discuss application
domains fully in Chapter 24.) Another case where you can call Abort safely
is on your own thread (because you know exactly where you are). Aborting your
own thread throws an “unswallowable” exception: one that gets rethrown after each
catch block. ASP.NET does exactly this when you call Redirect.
Safe Cancellation
LINQPad aborts threads when you cancel a runaway query. After
aborting, it dismantles and re-creates the query’s application
domain to avoid the potentially polluted state that could
otherwise occur.
As we saw in the preceding section, calling Abort on a thread is dangerous in most
scenarios. The alternative, then, is to implement a cooperative pattern whereby the
worker periodically checks a flag that indicates whether it should abort (like in
BackgroundWorker). To cancel, the instigator simply sets the flag, and then waits for
the worker to comply. This BackgroundWorker helper class implements such a flagbased
cancellation pattern, and you easily implement one yourself.
The obvious disadvantage is that the worker method must be written explicitly to
support cancellation. Nonetheless, this is one of the few safe cancellation patterns.
To illustrate this pattern, we’ll first write a class to encapsulate the cancellation flag:
class RulyCanceler
{
object _cancelLocker = new object();
bool _cancelRequest;
public bool IsCancellationRequested
{
get { lock (_cancelLocker) return _cancelRequest; }
}
public void Cancel() { lock (_cancelLocker) _cancelRequest = true; }
public void ThrowIfCancellationRequested()
{
if (IsCancellationRequested) throw new OperationCanceledException();
}
}
Safe Cancellation | 857
Threading