App_code and bin folders in sharepoint site

Hi All

In my current sharepoint 2010 site -> Under virtual directory -> Site port -> there are folders (app_code and bin)

Under App_code folder there is one .cs file and under bin folder there are .dlls 

Now i want to compile this .cs file under app_code folder into a dll and place it in gac. How can i achieve this?

I am not sure why the team earlier , had the file placed in app_code folder instead of compiling to dll

Please share if you have any clue why they might have done so?

Also, can i move the dlls that are present in bin folder to GAC? I think i need add as safe assemblies in the webconfig file.

Is here anything else i need to do here,

Please help

Thanks in advance

Usha