Dev Blog

Summary ​

Conclusions ​

A TPL task describes an operation that will complete successfully, with an exception, or with cancellation. It is not a thread: computations run in the thread pool, while waiting for I/O does not occupy a thread. Task.Run starts computation; Task.WhenAll, WhenAny, and WhenEach combine tasks without blocking, while Wait and Result block the thread and may cause deadlocks. Conditional ContinueWith continuations build task graphs. Task exceptions accumulate in an AggregateException; await throws the first one. Cancellation in .NET is cooperative: a CancellationTokenSource sends a request, and the operation checks the token and throws OperationCanceledException. The compiler transforms an async method into a state machine whose continuations run in the captured synchronization context or in the pool. ValueTask, IAsyncEnumerable<T>, PeriodicTimer, and TaskCompletionSource<T> extend the model to synchronous results, data streams, periodic actions, and events. SemaphoreSlim limits concurrent operations, while async void and synchronous waiting on asynchronous code are the most common antipatterns.

Self-check questions ​

  1. How does a task differ from a thread?
  2. When do you use Task.Factory.StartNew instead of Task.Run? What does LongRunning provide?
  3. What states can a task have? Which are final states?
  4. How do Task.WaitAll and Task.WhenAll differ?
  5. Why can accessing Result cause a deadlock?
  6. What is a continuation? How do you run a continuation only if its antecedent fails?
  7. How does a child task differ from a nested task?
  8. Why is a task's Exception property of type AggregateException? What are Flatten and Handle for?
  9. How do you retrieve all exceptions after await Task.WhenAll(...)?
  10. Describe cooperative cancellation. What are the roles of CancellationTokenSource and CancellationToken?
  11. Under what condition does a task enter the Canceled state rather than Faulted?
  12. How does the compiler transform an async method? What does the state machine's state field store?
  13. What is a synchronization context? When do you use ConfigureAwait(false)?
  14. How do compute operations differ from I/O operations in terms of async/await?
  15. When is ValueTask appropriate? What are its limitations?
  16. What is TaskCompletionSource<T> for? Why use RunContinuationsAsynchronously?
  17. How do you limit the number of concurrent asynchronous operations?
  18. Why are async void and .Result considered antipatterns?