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Here is small repro:
In case of Mono 4.2(+)(after referencesource import of TPL) it will output:
>"SynchronizationContext.Current is null"
But in case of Mono 4.0(-) it will output:
SynchronizationContext.Current should never be null on UI Thread.
Here is also question and answer on MSDN where answer says TPL team is aware of bug, but it's not clear if it was ever fixed(notice this was 2011).
Don't skip this link just because it says WCF, it's not related to WCF :).
Notice that I can't reproduce this on any version of .Net on Windows.
I've looked into this and it looks like the SynchronizationContext is indeed properly "flowed", even if never surfaced in the Current property.
This sample (https://gist.github.com/alexischr/7f0b71abbc452b828d15) demonstrates that TPL uses the correct SyncContext for continuations ( boo() will run within the context even when it's reported null).
The referencesource ExecutionContext code is very complex and I haven't figured out yet how to surface SyncCtx without breaking it.
I tried to reproduce with Mono 18.104.22.168 (2017-06/6425f06) but I didn't get the "SynchronizationContext.Current is null" message, so marking as fixed.