Cancel a library async method like Dns.GetHostEntryAsync()

Question

Is it possible to cancel an async library method like Dns.GetHostEntryAsync() ?

I have hundreds of these tasks running in parallel (DNS blacklist lookups) and every now and then I need to cancel the entire list of tasks.

Example:

List<Task<IPHostEntry>> tasks = new List<Task<IPHostEntry>>();
foreach (string s in BlackLists)
{
    tasks.Add(Dns.GetHostEntryAsync(s));
}

Edit: This question is about library async methods which does not take a CancellationToken.

Answer 1

While it is possible to cancel some async Library method, it is not possible for Dns.GetHostEntryAsync as you can't provide a CancellationToken to it.

For more information about how to cancel an async method that support cancellation, see Cancellation in Managed Threads.

var cts = new CancellationTokenSource();
//Task.Delay allows me to provide a cancellation token
var task = Task.Delay(new TimeSpan(1, 0, 0), cts.Token);
cts.Cancel();
try
{
  await task;
}
  catch (OperationCanceledException)
{
  Console.WriteLine("Cancelled");
}