The HKEY_CLASSES_ROOT (HKCR) key contains file name extension associations and COM class registration information such as ProgIDs, CLSIDs, and IIDs. It is primarily intended for compatibility with the registry in 16-bit Windows.

Class registration and file name extension information is stored under both the HKEY_LOCAL_MACHINE and HKEY_CURRENT_USER keys. The HKEY_LOCAL_MACHINE\Software\Classes key contains default settings that can apply to all users on the local computer. The HKEY_CURRENT_USER\Software\Classes key contains settings that apply only to the interactive user. The HKEY_CLASSES_ROOT key provides a view of the registry that merges the information from these two sources. HKEY_CLASSES_ROOT also provides this merged view for applications designed for previous versions of Windows.

The user-specific settings have priority over the default settings. For example, the default setting might specify a particular application to handle .doc files. But a user can override this setting by specifying a different application in the registry.

Registry functions such as RegOpenKeyEx or RegQueryValueEx allow you to specify the HKEY_CLASSES_ROOT key. When you call these functions from a process running in the interactive user account, the system merges the default settings in HKEY_LOCAL_MACHINE\Software\Classes with the interactive user's settings at HKEY_CURRENT_USER\Software\Classes. For more information on how these settings are merged, see Merged View of HKEY_CLASSES_ROOT.

To change the settings for the interactive user, store the changes under HKEY_CURRENT_USER\Software\Classes rather than HKEY_CLASSES_ROOT.

To change the default settings, store the changes under HKEY_LOCAL_MACHINE\Software\Classes. If you write keys to a key under HKEY_CLASSES_ROOT, the system stores the information under HKEY_LOCAL_MACHINE\Software\Classes. If you write values to a key under HKEY_CLASSES_ROOT, and the key already exists under HKEY_CURRENT_USER\Software\Classes, the system will store the information there instead of under HKEY_LOCAL_MACHINE\Software\Classes.

Processes running in a security context other than that of the interactive user should not use the HKEY_CLASSES_ROOT key with the registry functions. Instead, such processes can explicitly open the HKEY_LOCAL_MACHINE\Software\Classes key to access the default settings. To open a registry key that merges the contents of HKEY_LOCAL_MACHINE\Software\Classes with the settings for a specified user, these processes can call the RegOpenUserClassesRoot function. For example, a thread that is impersonating a client can call RegOpenUserClassesRoot if it needs to retrieve a merged view for the client being impersonated. Note that RegOpenUserClassesRoot fails if the user profile for the specified user has not been loaded. The system automatically loads the profile for the interactive user when logging on. For other users, you need to call the LoadUserProfile function to explicitly load the user's profile.

If an application is run with administrator rights and User Account Control is disabled, the COM runtime ignores the per-user COM configuration and accesses only the per-machine COM configuration. Applications that require administrator rights should register dependent COM objects during installation to the per-machine COM configuration store (HKEY_LOCAL_MACHINE\Software\Classes). For more information, see AC: UAC: COM Per-User Configuration.

Windows Server 2003 and Windows XP/2000: Applications can register dependent COM objects to either the per-machine or per-user COM configuration store (HKEY_LOCAL_MACHINE\Software\Classes or HKEY_CURRENT_USER\Software\Classes).

HKEY_CLASSES_ROOT (Resource Kit Registry Reference)