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?
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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.
Startdoesn't call anything the value gets lost when
hmm.. I would have expected False then True.
If I remove the
asynckeyword and return
Task.CompletedTaskthat is what I get.
So using the
asynckeyword without an
awaitis an execution change I didn't expect.