When you consume a MidiPacket (because some data was received) you would use the Bytes property to get access to the underlying Midi data. The actual number of valid bytes is stored in the Length property and you should not read beyond that point.

When you produce MidiPackets, you can either create MidiPacket instances by providing both an IntPtr and a Length parameter to your own buffers, or you can provide a byte array as well as a range within the array that determines where the Midi data is stored.