11.6 HKCR
HKCR is the backbone of the OLE/ActiveX
subsystem. The shell, the Explorer, and many applications depend on
the data stored here for prosaic tasks such as deciding which icon to
display with a file or what to do when the user double-clicks a file.
OLE applications need this data to determine what servers to start
when a user embeds or links a foreign object into a document, and the
Distributed OLE and DCOM systems need it too.
In Windows NT 4.0 and previous versions of NT, this root key got its
data solely from the software classes subkey under HKLM, that is
HKLM\Software\Classes. This worked fine, although the system could
register program classes only on a per-machine basis. In Windows
2000, the software classes under HKCU\Software\Classes are merged
with the HKLM software classes to make up HKCR. Defined as per-user
class registration, this allows program classes to be registered
independently for each user.
11.6.1 HKCR\ext
The original Win3.x scheme for linking a
file with the application that created it was to
associate the file's extension
with the name of an executable. With the introduction of OLE, though,
it became necessary to associate file contents with DLLs, since an
OLE server might be a DLL instead of a standalone application. The
Win 3.x File and Program Managers were primitive at best, and to
improve them Microsoft needed a lot more information about files,
their creators, and their types.
The starting point for these improvements is the set of
filename-extension keys that live under HKCR.
These keys are named after file extensions: HKCR\.txt, HKCR\.html,
and so on. The default value of each of these keys contains a string
called the application identifier such as "textfile" or
"htmlfile." This value is used to look up an HKCR subkey
of the same name.
While it's possible to add other values to file association
keys, no part of the system will use them, and they're subject
to being overwritten, so don't count on your values being
available if you keep them here.
11.6.2 HKCR\ fileType
For
each file association key, there is
usually a single key whose name matches the application identifier.
These keys are called class-definition keys.
Continuing the previous example, let's say there's a key
named HKCR\htmlfile. To figure out what to do with a file when the
user double-clicks it, the shell follows three steps:
Strip the extension from the file and use it to find a file
association key such as HKCR\.doc or HKCR\.pl. Open the file association key and get its default value, then use
that value to look for a key with the same name. For example, if
HKCR\.pl's default value is "Perl script," the
shell looks for HKCR\Perl script and tries to open it. This subkey is
called the application identifier key. The application identifier key contains values the shell parses to
figure out how to open or edit a file, or to create a new copy of a
particular file type or object.
Each application identifier key can have a
number of values and subkeys. Which ones a particular
application's key has varies. Here are the most common subkeys:
- CLSID
This subkey's default value contains the class
ID, or CLSID, assigned to an OLE object. OLE applications
(including Explorer and the shell) can use this CLSID to keep track
of a file or object's type. - DefaultIcon
The default value of this subkey contains the path to an executable
or DLL and a resource ID. When it needs the icon for a file, the
Explorer looks up the application's class-definition key, gets
the DefaultIcon value if it exists, and loads that icon. By changing
this value, you can alter the icons displayed for a particular file
type. - Shell\
Edit, Shell\Open, and Shell\Print
These subkeys each have a further subkey: Command. When the system
sees one of the Shell\XXX keys, it knows that this class type can be
opened, printed, or edited. The value of the Command subkey gives the
actual command that performs the requested action. - shellex
The shellex subkey makes it possible for clever programmers to add
items to the Properties dialog for a particular file type. The value
of the shellex\PropertySheetHandlers can specify a CLSID; when the
shell looks up the CLSID's key, it can figure out which
property sheet or dialog to open for that item.
11.6.3 HKCR\CLSID
This subkey contains all the CLSIDs for
classes installed on the system. Each CLSID key contains a value that
provides a human-readable name for the class (which appears in the
Insert Object dialog of most OLE-compatible applications). There are
a variety of other subkeys that can be attached to a particular entry
under HKCR\CLSID\clsid. The most important ones are InprocServer32
and InprocServer. These specify which DLL implements the code to
create or edit objects of this CLSID's type.
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