Question

How to cancel an asynchronous call? The .NET APM doesn t seem to support this operation.

I have the following loop in my code which spawns multiple threads on the ThreadPool. When I click a button on my UI, I would like these threads (or asynchronous calls) to end.

foreach (var sku in skus)
{
    loadSku.BeginInvoke(...
}

Is there any elegant solution other than creating a global "Cancel flag" and having the asynchronous methods to look for it?

Answer 1

A "cancel flag" is the way to do it, though not a global one, necessarily. The unavoidable point is that you need some way to signal to the thread that it should stop what it s doing.

In the case of BeginInvoke, this is hard to do with anything but a global flag, because the work is carried out on the threadpool, and you don t know which thread. You have a couple of options (in order of preference):

Use the BackgroundWorker instead of BeginInvoke. This has cancellation functionality baked-in. This has other benefits, like progress monitoring, and "Work complete" callbacks. It also nicely handles exceptions.
Use ThreadPool.QueueUserWorkItem, passing in an object as the state that has a Cancel() method that sets a Cancelled flag that the executing code can check. Of course you ll need to keep a reference to the state object so you can call Cancel() on it (which is something the BackgroundWorker does for you - you have a component on your form. (Thanks to Fredrik for reminding about this).
Create your own ThreadStart delegate, passing in a state object as with option 2.

Answer 2

If you re lookin for a "TerminateAsnyc" method, you won t find one. Therefore, no, there s probably no elegant way while using Control.BeginInvoke/EndInvoke. Thus, I d put the boolean flag on the UI thread and have the delegate being executed asynchronously check that flag periodically while it s executing.

However, you might check into using background worker threads.

Answer 3

There are definitely other solutions, although I don t know that I would call them "elegant".

you could call Abort or Interrupt on the thread but these can have some negative side effects. Personally, for something like this I prefer to use BackgroundWorker if possible. It has a Cancel feature but it is similar to what you mentioned - a bool flag in the class that you have to periodically check for in the executing code (at least it s not a global flag). This post on stopping threads in .NET is a bit old but goes over some of the pitfalls of the other options I mentioned above.

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