What does HRESULT 0xC1220300 (WDSTPC_E_CALLBACKS_NOT_REG) mean?

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required WDS transport callbacks were not registered.

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EVT_WDSMCS_E_NSREG_FAILURE	WDSTPC_E_ALREADY_COMPLETED

WDSTPC_E_CALLBACKS_NOT_REG

WDSTPC_E_CALLBACKS_NOT_REG is a WDS Transport Client lifecycle error. The client API requires callback registration to be complete before WdsTransportClientStartSession; once a session has started, additional callbacks cannot be registered.

Build the callback table before starting the session

Call WdsTransportClientRegisterCallback for every callback required by the consumer before the start call.
Log each TRANSPORTCLIENT_CALLBACK_ID and registration return value; do not infer success from a non-null function pointer in application state.
Keep optional metadata handling separate from required session/data/completion callbacks.

Microsoft explicitly documents the ordering constraint on both the registration and start APIs. The right fix is not to retry StartSession in a loop: first identify the missing callback registration or the earlier registration failure. A useful trace records initialization, session creation, each callback ID, and the start transition in order so race conditions in multi-threaded wrapper code are visible.

RegisterCallback contract · StartSession contract · WDS client API

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