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ChallengeWhy is this code broken?
time to read 1 min | 37 words
The following code looks straightforward, but it has a really subtle issue.
Can you spot what is going on?
You can ignore the error handling here, by the way, the issue isn’t related to handling unexpected errors.
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Comments
Maybe using the same
HttpClientin parallels request can overuse the socket? TheHttpClientis not disposed? Maybe related with bad handling tasks? OpenRead stream is not seeked and is reading the file from the end, causing sending empty file?I don't thing that will be related with the issue, but why declaring
var file = finstead of doing directly in the foreachforeach (var file in files)?Ivan,
That
flle = fthere is here to avoid variable capture issue, see: https://stackoverflow.com/questions/271440/captured-variable-in-a-loop-in-c-sharp@Oren Eini - on the
file = f- you haven't had to do that in aforeachsince c# 5, where they made a breaking change in order to have the captured loop variable work "as expected".Your code's primary issue is using
Task.Factory.StartNewwith an async delegate. In this case you're waiting on a list ofTask<Task>s, so you're waiting on the wrong thing. I suppose you could unwrap them - but I wouldn't recommend usingStartNewat all. Especially since you're now at its mercy for a task scheduler.Philip R,
You are correct, and I'm shocked. I wasn't aware of this change.
It doesn't check the status of the response. PostAsync won't throw an exception on a non-successful HTTP status code. A call to EnsureSuccessStatusCode() or similar mechanism should be added. Also why does it use Task.Factory.StartNew? Wouldn't just calling an async method be better?
Task.Factory.StartNewdoesn't have an overload that takes aFunc<Task>, so theasync () => ...lambda is actually anasync voidmethod. It won't actually wait until the transfer is complete.Ted,
Those are issues, but actually relatively minor ones. Thomas got it.
Thomas,
Exactly right!
My initial impression was that the code is broken by the use of async/await. Happy to see that it was the case :)
Ok, as Thomas got the error, how is the correct solution?
Carsten, Instead of
Task.Factor.StartNew, useTask.RunOk, thank you. As I never use Task.Factory.StartNew but always Task.Run (just checked all my code), I did not see this difference :-)
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