Retrieve interface number in Windows driver

In my INF file I write following line to load interface number 01 from my composite USB device:

%DeviceName%=Device_Install, USB\VID_XXXX&PID_XXXX&MI_01

Now how do I retrieve this interface number in my driver? I can access the device instace ID by IWDFDevice::RetrieveDeviceInstanceId which would give me the interface number packed in a string:

"USB\VID_XXXX&PID_XXXX&MI_01\6&336AF67A&0&0002"

I could extract it from this string but there must be a more elegant way?