How to find IP address

I am a .NET programmer and I am across an issue related to finding the IP address of the client computer. It is not tough to find that but the challenge comes if the client is hiding behind any proxy server or any gateway. What am I supposed to do if I want to find the IP of the client in such case.
I have searched the net regarding the matter but haven’t come across anything helpful yet. or anything else? Any input in the matter will be helpful, be it any tool or new technology that I have to follow or any book/journal I have to refer or any URL that shall I visit. I don’t know where and how to go ahead in the matter. Please help