JTB World Blog: HKEY_CLASSES_ROOT (HKCR) key description

Wednesday, October 17, 2007

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