where in the registry does Windows store, with which program to open certain file types?

Question

After installing Adobe PDF reader: *.PDF will be opened by the same. *.html with web browser. *.txt with notepad. If we want to make notepad++ a default editor of text files. I guess it simply changes the filetype - program association.

Where exactly in the registry are these filetype - program associations stored in the registry? In other word, how does Windows know with which software/program it must open a certain file(type)

Answer 1

The open with associations are all stored in HKEY_CLASSES_ROOT.

This is a special registry hive that combines the local user's associations in HKEY_CURRENT_USER\Software\Classes with the system's associations in HKEY_LOCAL_MACHINE\Software\Classes.

Here's a shot from my registry for the PDF association you mention in your question:

Answer 2

Just to extend Gareth's answer:

From playing with the registry it looks like

• entries from HKEY_CURRENT_USER\Software\Classes override entries from HKEY_LOCAL_MACHINE\Software\Classes in the HKEY_CLASSES_ROOT\Software\Classes
• setting file's extention association from command line using assoc command acts upon HKEY_LOCAL_MACHINE\Software\Classes
• setting file type association from command line using ftype command acts upon HKEY_LOCAL_MACHINE\Software\Classes