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SyncOverAsync

  • Task.Result or Task.Wait on asynchronous operations is much worse than calling truly synchronous APIs. Here is what happens
    • An asynchronous operation is kicked off.
    • The calling thread is blocked waiting for that operation to complete. When the asynchronous operation completes, it unblocks the code waiting on that operation. This takes place on another thread.
  • This leads to thread-pool starvation and service outages due to 2 threads being used instead of 1 to complete synchronous operations.
  • If a synchronization context is available it can even lead to deadlocks

GoodCitizen

  • Is async all the way and embraces virality of the API where it makes sense (IO-bound path)
  • Async void methods will crash the process if an exception is thrown
  • Task-returning methods are better since unhandled exceptions trigger the TaskScheduler.UnobservedTaskException
  • Be ware that only when the finalizers are run the unobserved exception is thrown
  • Does not explicitely offload to the worker thread pool by using Task.Run or Task.Factory.StartNew (offloading is a concern of the caller)
  • Returns Task, Task<TResult>, ValueTask or ValueTask<TResult>
  • Accepts CancellationToken if cancellation is appropriate. It is OK to add it but not respect it. Cancellation is cooperative.
  • Tries to respect token where it can and makes sense
  • Disposes owned CancellationTokenSource (due to allocated Timer instances when used with CancelAfter)
  • Disposes owned CancelationTokenRegistration to not leak memory
  • For high-perf scenarios avoid closure capturing in token registrations
  • For library or framework code uses ConfigureAwait(false) to opt-out from context capturing if not needed
  • Can used linked token sources for internal SLAs and cancellation scenarios
  • Favours simple sequential async execution over explicit concurrency (concurrency is hard)
  • No silver bullet that magically makes your DB query fasters ;)

ShortcutStatemachine

  • For highperf scenario async keyword can be omitted
  • Apply carefully and only after measuring
  • For most scenarios apply the keyword since it prevents mistakes because
    • Asynchronous and synchronous exceptions are normalized to always be asynchronous.
    • The code is easier to modify (consider adding a using, for example).
    • Diagnostics of asynchronous methods are easier (debugging hangs etc).
    • Exceptions thrown will be automatically wrapped in the returned Task instead of surprising the caller with an actual exception.
  • NET Core 2.2:
Method Allocated
Return 376 B
Await 488 B
  • NET Core 3.0 RC1:
Method Allocated
Return 344 B
Await 456 B

StackTracesOhMy

  • With .NET Core 2.1 and later finally readable stack traces
   at StackTracesOhMy.Level6() in C:\p\Async.Netcore\StackTracesOhMy.cs:line 19
   at StackTracesOhMyExtensions.Level5(StackTracesOhMy runnable) in C:\p\Async.Netcore\StackTracesOhMyExtensions.cs:line 41
   at StackTracesOhMyExtensions.Level4(StackTracesOhMy runnable) in C:\p\Async.Netcore\StackTracesOhMyExtensions.cs:line 36
   at StackTracesOhMyExtensions.Level3(StackTracesOhMy runnable) in C:\p\Async.Netcore\StackTracesOhMyExtensions.cs:line 31
   at StackTracesOhMyExtensions.Level2(StackTracesOhMy runnable) in C:\p\Async.Netcore\StackTracesOhMyExtensions.cs:line 26
   at StackTracesOhMyExtensions.Level2to6(StackTracesOhMy runnable) in C:\p\Async.Netcore\StackTracesOhMyExtensions.cs:line 21

AsyncDisposable

  • Stream, Utf8JsonWriter, System.Threading.Timer, CancellationTokenRegistration, BinaryWriter, TextWriter and IAsyncEnumerator<T> implement IAsyncDisposable
  • Stream, BinaryWriter, TextWriter calls .Dispose synchronously
  • Stream FlushAsync calls Flush on another thread which is bad behavior that should be overwritten

AsyncEnumerable

  • IAsyncEnumerable<T> allows to write asynchronous pull based streams, similar to regular enumerables with yield return and yield break
  • WithCancellation only adds the token to the enumerator but doesn't influence the state machine
  • WithCancellation in combination with [EnumeratorCancellation] can be used to create a combined token

CustomAsyncEnumerable

  • WithCancellation instructs the compiler generated statemachine to call GetAsyncEnumerator with the provided token. Otherwise default(CancellationToken) will be used.

PowerfulAsyncEnumerable

  • Async Enumerable can be combined in powerful ways with other async and TPL constructs such as Task.WhenAny