HKEYCLASSESROOT

HKEY_CLASSES_ROOT, commonly abbreviated as HKCR, is a registry hive in Windows that serves as a centralized store for information about registered file types, ProgIDs, and COM components. It provides the mapping between file extensions, the programs that can open them, and the commands used to launch those programs.

HKCR represents a merged view of two separate sources: HKEY_LOCAL_MACHINE\Software\Classes (machine-wide registrations) and HKEY_CURRENT_USER\Software\Classes (per-user registrations). When both levels define the same extension or ProgID, the per-user entry takes precedence in the merged view, allowing user-specific associations without changing system-wide settings.

The data in HKCR is organized into subkeys by file extension (for example, .txt) and by ProgID (for example, txtfile). A typical file extension key has a default value that references a ProgID, and may include other values such as Content Type. The ProgID key contains subkeys like shell, open, and command, which define the actions and the exact command line used to open a file. HKCR also contains the CLSID branch with information about COM components, including InprocServer32 and other registration details.

HKCR is essential for Windows to determine how to handle files, which applications launch for specific file types, and how COM objects are activated. Because it influences system behavior, edits to HKCR should be performed with caution, preferably through official settings dialogs or regedit by experienced users.

Changes in HKCR affect all applications that rely on file associations and COM registrations, and the hive is a core part of the Windows registry’s application and file-type infrastructure.