How to deal with usb GUID?

Question

I want to retrieve the VID and PID of usb device,such as mp3 and u-disk.I know how to enum the device infomation by using some of windows API step by step.I just do not know how to define the GUID structure,how to pass the GUID as a parameter to the api,such as SetupDiGetClassDevs(),and especially how to marshal the type of those parameters. Can you give me an detail example in C#?