HKEY_CLASSES_ROOT (HKCR) key description

Here is a tip. Avoid writing, deleting or merging (like .REG files or using regedit.exe) to HKEY_CLASSES_ROOT.

Why?

Basically because HKCR is a merged view based on HKEY_LOCAL_MACHINE\Software\Classes and HKEY_CURRENT_USER\Software\Classes and it can be confusing as to if this will work for the current user, a new user, the administrator user or so.

The registry system uses the following rules to merge information from the two sources:

• The merged view includes all subkeys of the HKEY_CURRENT_USER\Software\Classes key.
• The merged view includes all immediate subkeys of the HKEY_LOCAL_MACHINE\Software\Classes key that do not duplicate the subkeys of HKEY_CURRENT_USER\Software\Classes.
• Below is a list of subkeys that are found in both HKEY_LOCAL_MACHINE\Software\Classes and HKEY_CURRENT_USER\Software\Classes. The immediate subkeys of these keys from the HKEY_LOCAL_MACHINE tree are included in the merged view only if they are not duplicates of immediate subkeys from the HKEY_CURRENT_USER tree. The merged view does not include the HKEY_LOCAL_MACHINE contents of duplicate subkeys.

The following subkeys are found in both HKEY_LOCAL_MACHINE\Software\Classes and HKEY_CURRENT_USER\Software\Classes. From the HKEY_LOCAL_MACHINE tree, the immediate subkeys of these keys are included in the merged view only if they are not duplicates of immediate subkeys from the HKEY_CURRENT_USER tree. The merged view does not include the HKEY_LOCAL_MACHINE contents of duplicate subkeys.

*

*\shellex

*\shellex\ContextMenuHandlers

*\shellex\PropertySheetHandlers

AppID

ClsID

Component Categories

Drive

Drive\shellex

Drive\shellex\ContextMenuHandlers

Drive\shellex\PropertySheetHandlers

FileType

Folder

Folder\shellex

Folder\shellex\ColumnHandler

Folder\shellex\ContextMenuHandlers

Folder\shellex\ExtShellFolderViews

Folder\shellex\PropertySheetHandlers

Installer\Components

Installer\Features

Installer\Products

Interface

Mime

Mime\Database

Mime\Database\Charset

Mime\Database\Codepage

Mime\Database\Content Type

Typelib

The HKCR key contains file extension associations and COM class registration information such as ProgIDs, CLSIDs, and IIDs.

More is found in this MSDN document.