C#: Understanding ConfigureAwait(false)
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This slides explains why ConfigureAwait(false) needs to be called when we use async await. This slides is based on the following blog by Microsoft. https://devblogs.microsoft.com/dotnet/configureawait-faq/
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C#: Understanding ConfigureAwait(false)
- 2. If you’ve already read this blog, Finish reading this slides. Do not waste your time!
- 3. Agenda • Understanding SynchronizationContext and TaskScheduler • How do SynchronizationContext and TaskScheduler relate to await? • Problems of Posting callbacks • What does ConfigureAwait(false) do?
- 5. Official definition of SynchronizationContext Official document says… “Provides the basic functionality for propagating a synchronization context in various synchronization models.” Too abstract!!!
- 6. Easier explanation of SynchronizationContext • Provides Post method which passes a delegate(callback, action..) from one thread to another • Base method just enqueues a callback into ThreadPool Current Thread ThreadPool Thread Post
- 7. Sample code of SynchronizationContext Output ThreadId: 1 Action Called ThreadId: 11
- 8. Derived-Type of SynchronizationContext • Some frameworks have a specific thread • E.X. Winforms sets Derived-Type of SynchronizationContext to allow the context to be brought onto the UI thread. • The callback is invoked on UI thread Current Thread UI Thread Post (BeginInvoke)
- 9. Sample code of SynchronizationContext Output ThreadId: 1 Action Called ThreadId: 1
- 10. TaskScheduler • Very Roughly Speaking: Task’s version of SynchronizationContext • Provides “QueueTask” method which enqueues tasks to the Thread pool. • If the LongRunning option is true, it creates a new thread.
- 11. How do SynchronizationContext and TaskScheduler relate to await?
- 12. Write asynchronous code (Before .NET 4.0) • Use TaskScheduler • Use SynchronizationContext Thread 1 Thread 2 ContinueWith GetStringAsync Post back
- 13. Write asynchronous code (Before .NET 4.0) • Use TaskScheduler • Use SynchronizationContext Thread 1 Thread 2 ContinueWith GetStringAsync SynchronizationContext is captured! Post back
- 14. Write asynchronous code (After .NET 4.5) Can use async await keyword ContinueWith GetStringAsync Post back Thread 1 Thread 2
- 16. Problems of Posting callbacks
- 17. Problems of Posting callbacks • Deadlock • If an original thread calls Wait(), Result, or GetResult(), it gets blocked until the thread running the task finishes. • The subsequent thread waits until the original thread becomes available. • Performance • Enqueueing the callback to another thread is an expensive operation Cannot do post back Thread 1 Thread 2 Blocking HeavyAsync() .Result Doing heavy tasks Deadlock!
- 18. Problems of Posting callbacks • Deadlock • If an original thread calls Wait(), Result, or GetResult(), it gets blocked until the thread running the task finishes. • The subsequent thread waits until the original thread becomes available. • Performance • Enqueueing the callback to another thread is an expensive operation Cannot do post back Thread 1 Thread 2 Blocking HeavyAsync() .Result Doing heavy tasks I will restart after Thread 2’s method is done. Deadlock!
- 19. Problems of Posting callbacks • Deadlock • If an original thread calls Wait(), Result, or GetResult(), it gets blocked until the thread running the task finishes. • The subsequent thread waits until the original thread becomes available. • Performance • Enqueueing the callback to another thread is an expensive operation Cannot do post back Thread 1 Thread 2 Blocking HeavyAsync() .Result Doing heavy tasks I will restart after Thread 2’s method is done. I will do post-back when Thread1 becomes available Deadlock!
- 20. Sample code of deadlock Result: 1 : Main Start 1 : ChildSampleAsync Start This block is trying to be executed in Thread1, but Thread 1 is blocked, so the deadlock occurs. Since the “Result” method is called, Thread1 gets blocked
- 22. ConfigureAwait(false) = Prevents capturing SynchronizationContext • Sample code of capturing process • ConfigureAwait(false) bypasses the following block of code.
- 23. If ConfigureAwait(false) is called.. • It will not post the callback. • The code block after the “await” keyword will continue running on the same thread. Thread 1 Thread 2 Blocking HeavyAsync() .Result Doing heavy tasks No post happens and the method is processed in the same thread.
- 24. Sample code of NON-deadlock Result: 1 : Main Start 1 : ChildSampleAsync Start 7 : ChildSampleAsync End 1 : Main End This code block runs on another thread, not in original thread. ConfiureAwait(false) is called.
- 25. Advantages of calling ConfigureAwait(false) • Avoiding deadlocks • Performance • Since no queueing of the callback occurs.
- 26. Guidance of using ConfigureAwait(false) • Should use ConfigureAwait(false) • general-purpose libraries • ASP.NET Core • Console App • Should NOT use ConfigureAwait(false) • Win Forms • ASP.NET (HttpContext.Current might be badly affected) • WPF • Silverlight