When - and why - should you store data in the Windows Registry?

Question

• Originally (WIN3) configuration was stored in the WIN.INI file in the windows directory.
• Problem: WIN.INI grew too big.
• Solution (Win31): individual INI files in the same directory as the program.
• Problem: That program may be installed on a network and shared by many people.
• Solution(Win311): individual INI files in the user's Window directory.
• Problem: Many people may share a windows folder, and it should be read-only anyway.
• Solution (Win95): Registry with separate sections for each user.
• Problem: Registry grew too big.
• Solution (WinXP): Large blocks of individual data moved to user's own Application Data folder.
• Problem: Good for large amounts of data, but rather complex for small amounts.
• Solution (.NET): small amounts of fixed, read-only data stored in .config (Xml) files in same folder as application, with API to read it. (Read/write or user specific data stays in registry)

Coming at this both from a user perspective and a programmers perspective I would have to say there really isn't a good exceuse to put something in the registry unless it is something like file associations, or machine specific settings.

I come from the school of thought that says that a program should be runnable from wherever it is installed, that the installation should be completely movable within a machine, or even to another machine and not affect the running of it.

Any configurable options, or required dlls etc, if they are not shared should reside in a subdirectory of the installation directory, so that the whole installation is easily moved.

I use a lot of smaller utility like programs, so if it cant be installed on a usb stick and plugged into another machine and just run, then its not for me.

When - You are forced to due to legacy integration or because your customer's sysadmin says "it shall be so" or because you're developing in an older language that makes it more difficult to use XML.

Why - Primarily because the registry is not as portable as copying a config file that is sitting next to the application (and is called almost the same).

If you're using .Net2+ you've got App.Config and User.Config files and you don't need to register DLL's in the registry so stay away from it.

Config files have their own issues (see below), but these can be coded around and you can alter your architecture.

• Problem: Applications needed configurable settings.
• Solution: Store settings in a file (WIN.INI) in the Windows folder - use section headings to group data (Win3.0).
• Problem: WIN.INI file grew too big (and got messy).
• Solution: Store settings in INI files in the same folder as the application (Win3.1).
• Problem: Need user-specific settings.
• Solution: Store user-settings in user-specific INI files in the user's Window directory (Win3.11) or user-specific sections in the application INI file.
• Problem: Security - some application settings need to be read-only.
• Solution: Registry with security as well as user-specific and machine-wide sections (Win95).
• Problem: Registry grew too big.
• Solution: User-specific registry moved to user.dat in the user's own "Application Data" folder and only loaded at login (WinNT).
• Problem: In large corporate environments you log onto multiple machines and have to set EACH ONE up.
• Solution: Differentiate between local (Local Settings) and roaming (Application Data) profiles (WinXP).
• Problem: Cannot xcopy deploy or move applications like the rest of .Net.
• Solution: APP.CONFIG XML file in same folder as application - , easy to read, easy to manipluate, easy to move, can track if changed (.Net1).
• Problem: Still need to store user-specific data in a similar (i.e. xcopy deploy) manner.
• Solution: USER.CONFIG XML file in user's local or roaming folder and strongly-typed (.Net2).
• Problem: CONFIG files are case-sensitive (not intuitive to humans), require very specific open/close "tags", connection strings cannot be set at run-time, setup projects cannot write settings (as easily as registry), cannot easily determine user.config file and user settings are blown with each new revision installed.
• Solution: Use the ITEM member to set connection strings at runtime, write code in an Installer class to change the App.Config during install and use the application settings as defaults if a user setting is not found.

Microsoft policy:

• Before windows 95, we used ini files for application data.
• In the windows 95 - XP era, we used the registry.
• From windows Vista, we use ini files although they are now xml based.

The registry is machine dependent. I have never liked it because its getting to slow and it is almost imposible to find the thing you need. That's why I like simple ini or other setting files. You know where they are (application folder or a user folder) so they are easy portable, and human readable.

Is the world going to end if you store a few window positions and a list of most recently used items in the Windows registry? It's worked okay for me so far.

HKEY-CURRENT-USER is a great place to store trivial user data in small quantities. That's what it's for. It seems silly not to use for its intended purpose just because others have abused it.

Registry reads and writes are threadsafe but files are not. So it depends on whether or not your program is single threaded.

Settings that you want to have available in a user's roaming profile should probably go in the registry, unless you actually want to go to the effort of looking for the user's Application Data folder by hand. :-)