What does HRESULT 0xC00D9C42 (MF_E_CLOCK_STATE_ALREADY_SET) mean?

MF_E_CLOCK_STATE_ALREADY_SET

MF_E_CLOCK_STATE_ALREADY_SET indicates a redundant presentation-clock state request. The clock already has the requested state, so repeating the same state change does not represent a new transport transition.

Deduplicate clock control commands

• Track confirmed clock state through IMFClockStateSink notifications.
• Do not issue Pause or Stop from multiple owners such as both a Media Session wrapper and a renderer teardown path.
• When commands are asynchronous at a higher layer, distinguish "requested" state from the state confirmed by the clock callback.

Be careful with Start: Media Foundation documents that calling Start on a running presentation clock can seek it to a new start offset, so not every repeated-looking Start call is semantically redundant. Log the operation and start offset together before collapsing commands.

Microsoft: IMFPresentationClock · Microsoft: IMFPresentationClock::Start · Microsoft: IMFClockStateSink