What would be a good way to Cancel long running IO/Network operation using Tasks?

Question

I've been studying Tasks in .net 4.0 and their cancellation. I like the fact that TPL tries to deal with cancellation correctly in cooperative manner.

However, what should one do in situation where a call inside a task is blocking and takes a long time? For examle IO/Network.

Obviously cancelling writes would be dangerous. But those are examples.

Example: How would I cancel this? DownloadFile can take a long time.

Task.Factory.StartNew(() =>
    WebClient client = new WebClient();
    client.DownloadFile(url, localPath);
);

Answer 1

Task supports cancellation tokens. You can create an instance of CancellationTokenSource and pass it's Token property to your DownloadFile method. Then at points in your code where you can stop, check the tokens, IsCancellationRequested property to see if a cancel was requested.

You should also pass the token to StartNew (after the method).

To actually cancel the operation you can call the Cancel method on the cancellation token.

Check out this MSDN article on cancellation