DRVM_ADD_THRU

Sent to a MIDI input device driver to add a given device to the list of output devices that receive MIDI throughput messages. If the MIDI input driver returns MMSYSERR_NOTSUPPORTED, the system will simulate throughput to one MIDI output or MIDI throughput device. This message is not intended to be sent directly by an application; instead, it is sent implicitly when an application calls the midiConnect function.

The return value is an error code, or zero (MMSYSERR_NOERROR) if the operation is successful. Possible error codes are:

MMSYSERR_NOTSUPPORTED	The driver does not support MIDI throughput. Returning this value causes the system to simulate throughput on behalf of the driver.
MMSYSERR_NOTENABLED	The driver failed to load or initialize.
MIDIERR_NOTREADY	The MIDI hardware is busy with other data.

DWORD dwParam1: Specifies the handle of the MIDI output device to be added to the throughput list.
DWORD dwParam2: Address of user-instance data. This parameter can be NULL.