Modern operating systems are file oriented, so in Windows you can access documents just by double clicking them without having to worry about opening the program. But I don't know how this is achieved. Is is done entirely by the OS or does the application program itself play a role? So suppose I want to develop a simple text editor that reads .txt files. Then if I specify that a .txt file be opened with that editor, will the latter be able automatically to deal with this request, or do I have to write special code for it?