ChallengeWhat does this code print?
Take a look at the following code, what do you think it will print?
Since it obviously doesn’t print the expected value, why do you think this is the case?
More posts in "Challenge" series:
- (01 Jul 2024) Efficient snapshotable state
- (13 Oct 2023) Fastest node selection metastable error state–answer
- (12 Oct 2023) Fastest node selection metastable error state
- (19 Sep 2023) Spot the bug
- (04 Jan 2023) what does this code print?
- (14 Dec 2022) What does this code print?
- (01 Jul 2022) Find the stack smash bug… – answer
- (30 Jun 2022) Find the stack smash bug…
- (03 Jun 2022) Spot the data corruption
- (06 May 2022) Spot the optimization–solution
- (05 May 2022) Spot the optimization
- (06 Apr 2022) Why is this code broken?
- (16 Dec 2021) Find the slow down–answer
- (15 Dec 2021) Find the slow down
- (03 Nov 2021) The code review bug that gives me nightmares–The fix
- (02 Nov 2021) The code review bug that gives me nightmares–the issue
- (01 Nov 2021) The code review bug that gives me nightmares
- (16 Jun 2021) Detecting livelihood in a distributed cluster
- (21 Apr 2020) Generate matching shard id–answer
- (20 Apr 2020) Generate matching shard id
- (02 Jan 2020) Spot the bug in the stream
- (28 Sep 2018) The loop that leaks–Answer
- (27 Sep 2018) The loop that leaks
- (03 Apr 2018) The invisible concurrency bug–Answer
- (02 Apr 2018) The invisible concurrency bug
- (31 Jan 2018) Find the bug in the fix–answer
- (30 Jan 2018) Find the bug in the fix
- (19 Jan 2017) What does this code do?
- (26 Jul 2016) The race condition in the TCP stack, answer
- (25 Jul 2016) The race condition in the TCP stack
- (28 Apr 2015) What is the meaning of this change?
- (26 Sep 2013) Spot the bug
- (27 May 2013) The problem of locking down tasks…
- (17 Oct 2011) Minimum number of round trips
- (23 Aug 2011) Recent Comments with Future Posts
- (02 Aug 2011) Modifying execution approaches
- (29 Apr 2011) Stop the leaks
- (23 Dec 2010) This code should never hit production
- (17 Dec 2010) Your own ThreadLocal
- (03 Dec 2010) Querying relative information with RavenDB
- (29 Jun 2010) Find the bug
- (23 Jun 2010) Dynamically dynamic
- (28 Apr 2010) What killed the application?
- (19 Mar 2010) What does this code do?
- (04 Mar 2010) Robust enumeration over external code
- (16 Feb 2010) Premature optimization, and all of that…
- (12 Feb 2010) Efficient querying
- (10 Feb 2010) Find the resource leak
- (21 Oct 2009) Can you spot the bug?
- (18 Oct 2009) Why is this wrong?
- (17 Oct 2009) Write the check in comment
- (15 Sep 2009) NH Prof Exporting Reports
- (02 Sep 2009) The lazy loaded inheritance many to one association OR/M conundrum
- (01 Sep 2009) Why isn’t select broken?
- (06 Aug 2009) Find the bug fixes
- (26 May 2009) Find the bug
- (14 May 2009) multi threaded test failure
- (11 May 2009) The regex that doesn’t match
- (24 Mar 2009) probability based selection
- (13 Mar 2009) C# Rewriting
- (18 Feb 2009) write a self extracting program
- (04 Sep 2008) Don't stop with the first DSL abstraction
- (02 Aug 2008) What is the problem?
- (28 Jul 2008) What does this code do?
- (26 Jul 2008) Find the bug fix
- (05 Jul 2008) Find the deadlock
- (03 Jul 2008) Find the bug
- (02 Jul 2008) What is wrong with this code
- (05 Jun 2008) why did the tests fail?
- (27 May 2008) Striving for better syntax
- (13 Apr 2008) calling generics without the generic type
- (12 Apr 2008) The directory tree
- (24 Mar 2008) Find the version
- (21 Jan 2008) Strongly typing weakly typed code
- (28 Jun 2007) Windsor Null Object Dependency Facility
Comments
I'll admit I had to do some digging into the .Net source code to get a few clues, but AsyncLocal is a proxy class for values stored in the ExecutionContext. The Start() method is running in a different ExecutionContext from the Run method, so setting Active.Value = true in Start() has no effect on the observed value in Run(), so you get false both times it prints.
This is the more modern equivalent of something like:
After playing around a bit I figured out that changing the AsyncLocal value does not change the original value from the 'parent' context, it only propagates the new value further down the async 'chain'. You can check that by adding a new async method (Start2) that Start will call and log the results. In the new method the value will be 'true' as it was set within the 'parent' context of the Start method.
However this is not mentioned in the docs (https://learn.microsoft.com/en-us/dotnet/api/system.threading.asynclocal-1?view=net-7.0) and I do think it should be better documented. Sadly the intent of the class is not clear for the doc so this might be expected behavior or a bug :)
@Chris B I would not agree with this is equivalent to ThreadStatic as the value is propagated 'forward' across the async call chain (it is just not being propagated 'backwards' for a lack of a better term). Equivalent for ThreadStatic would be ThreadLocal.
@Dalibor Čarapić: It is somewhat descibed in the docs as "Represents ambient data that is local to a given asynchronous control flow, such as an asynchronous method."
Though, it could be much clearer. The "control flow" refers to the callstack/called methods meaning that the value will only be available or "local to" the method it was set in and all its called methods. "Asynchronous" here means that the value will also be available after an await.
As
Start
doesn't call anything the value gets lost whenStart
is exited.hmm.. I would have expected False then True.
If I remove the
async
keyword and returnTask.CompletedTask
that is what I get.So using the
async
keyword without anawait
is an execution change I didn't expect.Comment preview