What does HRESULT 0x8009804A (WINBIO_E_PRESENCE_MONITOR_ACTIVE) mean?

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the asynchronous biometric session already has an active face or iris presence monitor.

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WINBIO_E_SELECTION_REQUIRED	WINBIO_E_INVALID_SUBFACTOR

WINBIO_E_PRESENCE_MONITOR_ACTIVE

WINBIO_E_PRESENCE_MONITOR_ACTIVE is session state for presence monitoring. Microsoft documents that a single biometric session can have only one active presence monitor at a time, so a second WinBioMonitorPresence request cannot create a parallel monitor on the same handle.

Track presence-monitor lifetime

Associate the monitor with the asynchronous session handle that started it.
Stop notifications with WinBioCancel or close the original session before starting another monitor on that handle.
If separate consumers need presence information, coordinate one monitor or open appropriately designed separate sessions instead of racing duplicate start calls.

Presence monitoring is only supported through an asynchronous session and reports results in WINBIO_ASYNC_RESULT with operation type WINBIO_OPERATION_MONITOR_PRESENCE. This makes duplicate starts easy to detect in a state machine: the error is about an existing monitor, not sensor occupancy or an invalid unit ID.

WinBioMonitorPresence · WinBioAsyncOpenSession

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