How to Register a Preview Handler

This topic explains how to register a preview handler associated with a given data type. For the purposes of illustration, examples in this topic use a .xyz file type. Registration of a preview handler is a standard file association-based registration.


Step 1:

First, a file name extension is associated with a ProgID. The following entry associates the xyzfile ProgID subkey with the .xyz file name extension.

      (Default) = [REG_SZ] xyzfile

The xyzfile ProgID subkey is stored with the other ProgIDs as shown here:


Each preview handler ProgID subkey contains a subkey named shellex that contains a subkey always named {8895b1c6-b41f-4c1c-a562-0d564250836f}. The presence of that subkey tells the system that the handler is a preview handler.

The default value of the {8895b1c6-b41f-4c1c-a562-0d564250836f} subkey is the class identifier (CLSID) of your handler. An example of the xyzfile ProgID subkey is shown here, associating a handler of CLSID {ec3a629a-a47c-4245-bc78-b4b63d0e3154}.

            (Default) = [REG_SZ] {ec3a629a-a47c-4245-bc78-b4b63d0e3154}

Step 2:

Next, add the subkey under CLSID for your preview handler. An example is shown here. An explanation of individual entries follows.

         (Default) = [REG_SZ] Fabricam XYZ Preview Handler
         DisplayName = [REG_SZ] @myhandler.dll,-101
         Icon = [REG_SZ] myhandler.dll,201
         AppID = [REG_SZ] {6d2b5079-2f0b-48dd-ab7f-97cec514d30b}
            (Default) = [REG_EXPAND_SZ] %ProgramFiles%\Fabricam\myhandler.dll
            ThreadingModel = [REG_SZ] Apartment
            ProgID = [REG_SZ] xyzfile
            VersionIndependentProgID = [REG_SZ] Version IndependentProgID

The default value for your subkey (here, {ec3a629a-a47c-4245-bc78-b4b63d0e3154}) is not required or used. However, setting it to a nonlocalized string can help you to debug registration issues.

The minus sign (-101) in the .dll resource in the DisplayName entry exists for legacy reasons. The Icon entry, on the other hand, does not require a minus sign.

The AppID value gives a reference to the AppID of the application associated with the file name extension (stored under HKEY_CLASSES_ROOT\APPID. The value used here—{6d2b5079-2f0b-48dd-ab7f-97cec514d30b}—is the ID of the Prevhost.exe surrogate host. 32-bit preview handlers should use AppID {534A1E02-D58F-44f0-B58B-36CBED287C7C} when installed on 64-bit operating systems.

The entries under the InprocServer32 subkey include a reference back to the file name extension's ProgID subkey as well as an entry for a VersionIndependentProgID.

Step 3:

Finally, the preview handler must be added to the list of all preview handlers. This list is used as an optimization by the system to enumerate all registered preview handlers for display purposes. Again, the default value is not required, it simply aids in the debugging process.


In Windows 7, if the application is installed for all users of the computer, use HKEY_LOCAL_MACHINE; if for only one user, use HKEY_CURRENT_USER.


                     (Default) = [REG_SZ] Fabricam XYZ Preview Handler

Preview Handlers and Shell Preview Host

Building Preview Handlers

Preview Handler Guidelines