What is preferred in the while loop of the RunAsync method in a Service: use IsCancellationRequested or throw exception

Question

Why is the sample RunAsync for new Service Fabric code structured like this

protected override async Task RunAsync(CancellationToken cancellationToken)
{
  while(true)
  {
    cancellationToken.ThrowIfCancellationRequested();

    await Task.Delay(TimeSpan.FromSeconds(1), cancellationToken);
  }
} 

instead of this

protected override async Task RunAsync(CancellationToken cancellationToken)
{
  while(cancellationToken.IsCancellationRequested)
  {
    await Task.Delay(TimeSpan.FromSeconds(1), cancellationToken);
  }
} 

Wouldn't the version without the throw be preferred? The docs state that both implementations are correct: "The system will wait for your task to end (by successful completion, cancellation, or fault)".

Answer 1

Short answer - both are fine. I'd use ThrowIfCancellationRequested because IMO it's the safer option. It's also more consistent - methods lower in the call chain can propagate cancellation via the exception.

Whenever an exception occurs in RunAsync, Service Fabric reports a transient fault (which means that the service is restarted without the instance/replica being recreated).

It gives special treatment for OperationCanceledException - if it was thrown by the cancellation token passed to the method, then the method is considered to have been successfully cancelled and no fault will be reported.

You can try this yourself by monitoring the Microsoft-ServiceFabric-Services ETW events.