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Created attachment 14984 [details]
More detailed explanation and partial traces
When using a CancellationToken to set the timeout on an FtpWebRequest, the TaskCanceledException is successfully raised after the defined timeout when in the foreground (5 minutes in this case), but when the application is in the background the TaskcanceledException is being raised only after 90 minutes plus.
After more investigation I believe it is when the phone Samsung Note 4 goes into Lock/Sleep Mode, rather than just the app being backgrounded.
I don't believe we can call this issue a bug. This is due to the fact that Android doesn't give any guarantees that a background process (defined as one without a foreground activity - that is all activities of the application/process had their OnStop handler called) will ever remain alive (please see this article https://developer.android.com/guide/components/processes-and-threads.html). In such instance Mono, the Xamarin.Android runtime, cannot deliver the promise of cancelling of an operation within the indicated frame of time.
To make your scenario work you should consider creating a *service process* (also described in the above article) which *should not* be killed by the system unless an out-of-memory situation arises.