Talk:Windows Shell Extension

Research

General

• HKEY_CLASSES_ROOT is a merged view of the ...\Software\Classes sub hierarchies in
  • HKLM (HKEY_LOCAL_MACHINE)
  • HKCU (HKEY_CURRENT_USER)
• HKCU is a mirrored view of HKEY_USERS\<User-SID>

• Adding a Key to HKEY_CLASSES_ROOT\Software\Classes will add a key in HKEY_LOCAL_MACHINE\SOFTWARE\Classes and vice versa.

Minimum requirements for new file types

• HKEY_CLASSES_ROOT\.[yourExt]\shell\open\command
• this command must have a Default key that is of type REG_EXPAND_SZ
  • creating this manually gives a REG_SZ type, which will not work and cannot be changed within the regedit GUI
  • REG_EXPAND_SZ type can be created with reg files and programmatically

Mind already existing alternate hierarchies

• HKEY_CLASSES_ROOT\.[yourExt]

If not empty, this key might have data such as extfile which will point to HKEY_CLASSES_ROOT\[extfile]

The second key then looks like this in full length:

• HKEY_CLASSES_ROOT\extfile\shell\open\command

Such structures sometimes get automatically created:

• HKEY_CLASSES_ROOT\ext_auto_file\shell\open\command

User overrides

The following will override HKEY_LOCAL_MACHINE keys as the HKEY_CURRENT_USER has a higher priority.

So, you can also think of HKEY_LOCAL_MACHINE keys as a fallback when HKEY_CURRENT_USER are removed again.

Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\Explorer\FileExts\.yourExt
 OpenWithList
 OpenWithProgids

System overrides (backups ?)

Computer\HKEY_CURRENT_USER\Software\Microsoft\Windows\CurrentVersion\ApplicationAssociationToasts
 Applications\NOTEPAD.EXE_.yourExt (type REG_DWORD)
 other_extension_.yourExt (type REG_DWORD)

Terms

• The H in abbreviations stands for Hive.