Microsoft.Windows.Devices.Midi2.Messages
There are many open source and internal libraries that can be used for creating and parsing message data. For a functional MIDI API, however, we did have to create a number of these ourselves, and so surface them in the API so that you may take advantage of them in your own code.
For other open source MIDI libraries, visit the official site midi2.dev, run by members of the MIDI Association. The Windows MIDI Services service and API code uses some of this open source internally.