Specifying File Handlers for File Name Extensions
There are a number of ways to determine the application that handles a file which has a particular file extension. The OpenWithList and OpenWithProgids verbs are two ways to specify file handlers under the registry entry for the file extension.
When you right-click a file in Windows Explorer, you see the Open command. If more than one product is associated with an extension, you see an Open With submenu.
You can register different applications to open an extension by setting the OpenWithList key for the file extension in HKEY_CLASSES_ROOT. The applications listed under this key for a file extension appear under the Recommended Programs heading in the Open With dialog box. On Windows 2000 and Windows XP Professional, these applications also appear in the Open With Product Name submenu off the main shortcut menu. The following example shows the applications registered to open the .vcproj file extension.
HKEY_CLASSES_ROOT\ .vcproj\ (default)="VisualStudio.vcproj.10.0" OpenWithList\ devenv.exe
The keys specifying applications are from the list under HKEY_CLASSES_ROOT\Applications.
By adding an OpenWithList key, you declare that your application supports a file extension even if another application takes ownership of the extension. This could be a future version of your application or another application.
Programmatic identifiers (ProgIDs) are friendly versions of ClassIDs that identify a version of an application or COM object. Every co-creatable object should have its own ProgID. For example, VisualStudio.DTE.7.1 starts Visual Studio .NET 2003 while VisualStudio.DTE.10.0 starts Visual Studio. As the owner of a project type or project item type, you must create a version-specific ProgID for your file extension. These ProgIDs may be redundant in that more than one ProgID may start the same application. For more information, see Registering Verbs for File Name Extensions.
Use the following naming convention for versioned file ProgIDs to avoid duplication with registration from other vendors:
You can register different applications that are able to open a particular file extension by adding versioned ProgIDs as values to the HKEY_CLASSES_ROOT\<extension>\OpenWithProgids key. This registry key contains a list of alternate ProgIDs associated with the file extension. The applications associated with the listed ProgIDs appear in the Open With Product Name submenu. If the same application is specified in both the OpenWithList and OpenWithProgids keys, the operating system merges the duplicates.
The OpenWithProgids key is only supported in Windows XP. Because other operating systems ignore this key, do not use it as the only registration for file handlers. Use this key to provide a better user experience in Windows XP.
Add the desired ProgIDs as values of the type REG_NONE. The following code provides an example of registering ProgIDs for a file extension (.ext).
HKEY_CLASSES_ROOT\ .ext\ (default)="MyProduct.ext.10.0" OpenWithProgids progid REG_NONE (zero-length binary value) otherprogid REG_NONE (zero-length binary value)
The ProgID specified as the default value for the file extension is the default file handler. If you modify the ProgID for a file extension that shipped with a previous version of Visual Studio or that can be taken over by other applications, then you must register the OpenWithProgids key for your file extension and specify the new ProgID in the list along with the old ProgIDs you support. For example:
HKEY_CLASSES_ROOT\ .vcproj\ (default)="VisualStudio.vcproj.10.0" OpenWithProgids vcprojfile //old progid VisualStudio.vcproj.8.0 //old progid VisualStudio.vcproj.10.0 //new progid
If the old ProgID has verbs associated with it, then these verbs will also appear under Open With Product Name in the shortcut menu.