How to get the full path of file upload control in ASP.NET when we browse file.

You can't. This is a deliberate security feature: it does not reveal anything about the environment on the client to the server. All you get is the file name: the path is meaningless because the server is not in the same environment and can't use it anyway.
The only time it could be of use is if you were restoring a file to it's original location on the client computer - and the browser won't let you do that either!