What does HRESULT 0xC00DAFC9 (MF_E_TIMELINECONTROLLER_NOT_ALLOWED) mean?

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the requested MediaPlayer operation is unavailable while a MediaTimelineController is attached.

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MF_E_TIMELINECONTROLLER_UNSUPPORTED_SOURCE_TYPE	MF_E_TIMELINECONTROLLER_CANNOT_ATTACH

MF_E_TIMELINECONTROLLER_NOT_ALLOWED

MF_E_TIMELINECONTROLLER_NOT_ALLOWED means the requested operation conflicts with controller-owned playback. A MediaTimelineController is intended to provide one mechanism for rate, position, and state across multiple MediaPlayer instances; continuing to issue ordinary per-player transport commands can violate that control model.

Find the second transport-control path

Log the exact operation that failed and whether MediaPlayer.TimelineController is non-null at that moment.
Trace direct player play, pause, seek, or rate changes as well as UI and System Media Transport Control command handlers.
Route timeline state changes through MediaTimelineController.Start, Pause, Resume, Position, and ClockRate where the controller owns playback.

Microsoft's synchronization sample explicitly disables each player's CommandManager before assigning the shared controller because normal command routing and timeline control are competing mechanisms. It also disables media transport controls in the UI. Repeating the same rejected operation will not help; diagnose which component is still trying to control an individual player outside the shared timeline.

Microsoft: MediaTimelineController · Microsoft: MediaPlayer.TimelineController · Microsoft sample: synchronizing players

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