Consider I have got a DLL file which contains some functions and classes that I am not aware of. It might be lack of documentation, or the unwillingness of the programmer to provide the documentation after the release. I want to know, how can I study an unknown DLL file and utilize it in my projects? ( Of course the programmer gives such permission )
I can't post comments for now, since I'm new on this forum
I'm not sure I fully understand your level of understanding here. Do you know some basic stuff in reversing field ?
If so, you should load your DLL in OllyDbg (for example) then click on Debug > Call DLL export to locate the API you're interested in. Then it's classic reversing session.
Otherwise, if you need more specifics, I strongly recommend you to buy "Secrets of reverse engineering - Eldad Eilam" book, in which there is a fully detailed example of a DLL reverse session.