int result = midiOutGetDevCaps(deviceID, ref outCaps, Marshal.SizeOf(outCaps));

public static extern MMRESULT midiOutGetDevCaps(UIntPtr uDeviceID, ref MIDIOUTCAPS lpMidiOutCaps, uint cbMidiOutCaps);

        midiOutGetDevCaps((UIntPtr) x, out caps, (uint)Marshal.SizeOf(typeof(MIDIOUTCAPS)));