Execute .NET Core 3.1 background worker tasks simultaneously

Question

I have following code:

public class Worker : BackgroundService
{
 public Worker()
 {
 }
 protected async override Task ExecuteAsync(CancellationToken stoppingToken)
 {
 while (!stoppingToken.IsCancellationRequested)
 {
 var task1 = Task.Run(() =>
 MethodOneAsync(stoppingToken)
 ).ContinueWith(_ => Debug.WriteLine($"MethodOneAsync finished @ {DateTime.Now:dd.MM.yyyy HH:mm:ss.ffff}"));
 var task2 = Task.Run(() =>
 MethodTwoAsync(stoppingToken)
 ).ContinueWith(_ => Debug.WriteLine($"MethodTwoAsync finished @ {DateTime.Now:dd.MM.yyyy HH:mm:ss.ffff}"));
 await Task.WhenAll(task1, task2);
 await Task.Delay(10000);
 }
 }
 private async Task<bool> MethodOneAsync(CancellationToken stoppingToken)
 {
 Debug.WriteLine($"MethodOneAsync started @ {DateTime.Now:dd.MM.yyyy HH:mm:ss.ffff}");
 await Task.Delay(1000); // emulating some long work
 return true;
 }
 private async Task<bool> MethodTwoAsync(CancellationToken stoppingToken)
 {
 Debug.WriteLine($"MethodTwoAsync started @ {DateTime.Now:dd.MM.yyyy HH:mm:ss.ffff}");
 await Task.Delay(1000); // emulating some long work
 return true;
 }
}

What I want to achieve is:

• to create background worker that will call MethodOneAsync and MethodTwoAsync on separate threads (start both tasks simultaneously)
• log time each task started and finished
• wait for them both to finish
• then wait another 10 seconds
• repeat all over again

The code I wrote does its job as expected. Since I am still trying to get a grasp of .NET Core tasks, I would like to know is this the right way to do it?

• Peter Csala: Are MethodOneAsync and MethodTwoAsync CPU-bound or IO-bound?
• leroy: They get a list of objects from SQL, map them to MongoDB objects, and update MongoDB with new objects. So, mostly IO-bound.
• Peter Csala: Then you don't need Task.Run calls in the ExecuteAsync. You could simply call await Task.WhenAll(MethodOneAsync(stoppingToken), MethodTwoAsync(stoppingToken));
• leroy: @PeterCsala can you please, in short, tell why?
• Peter Csala: Task.Run was designed for CPU-bound work, so it pulls a thread from the thread pool to run the method and returns a Task which represents the completion. Async non-blocking I/O operation does not need a dedicated thread.

Answer 1

• Async method out-of-the-box is using multiple threads for continuations. (Everything after await is continuation). Exception of this rule if existing SynchronizationContext. Don't Task.Run asynchronous I/O-bound operation, do it only for synchronous CPU-bound one. Asynchronous programming, SynchrinizationContext & TaskScheduler.
• Mixing .ContinueWith and await is not recommended as it doing the same but using different coding pattern. Mixing patterns with no reason makes the code more complicated.

If I were you, the code could look like this:

public class Worker : BackgroundService
{
 protected async override Task ExecuteAsync(CancellationToken stoppingToken)
 {
 // all methods will throw OperationCanceledException if cancellation was happened,
 // then checking if it was canceled is a kind of redundancy here.
 while (true) 
 {
 var task1 = MethodOneAsync(stoppingToken);
 var task2 = MethodTwoAsync(stoppingToken);
 await Task.WhenAll(task1, task2, Task.Delay(10000, stoppingToken)); // inlining Task.Delay here changes the logic: 10 seconds counted between starts of two iterations not between finish of previous and start of next.
 }
 }
 private async Task MethodOneAsync(CancellationToken stoppingToken)
 {
 Debug.WriteLine($"MethodOneAsync started @ {DateTime.Now:dd.MM.yyyy HH:mm:ss.ffff}");
 await Task.Delay(1000, stoppingToken);
 Debug.WriteLine($"MethodOneAsync finished @ {DateTime.Now:dd.MM.yyyy HH:mm:ss.ffff}");
 }
 private async Task MethodTwoAsync(CancellationToken stoppingToken)
 {
 Debug.WriteLine($"MethodTwoAsync started @ {DateTime.Now:dd.MM.yyyy HH:mm:ss.ffff}");
 await Task.Delay(1000, stoppingToken);
 Debug.WriteLine($"MethodTwoAsync finished @ {DateTime.Now:dd.MM.yyyy HH:mm:ss.ffff}");
 }
}

About Worker class purpose. It looks like Scheduler not Worker because it repeats some work continuously. Consider to change its name.