How to associate a Windows application with a particular file type but share that association with other applications?

Mechanism for opening Office .xml Documents

For Word documents saved in XML and having an .xml extensions Microsoft implemented a special handler to open these files in the corresponding application (This mechanism is not only used for Word documents, but also Excel spreadsheets, InfoPath forms and some other formats).

If you check the Registry you will see that the file type for files with a .xml extension is set to xmlfile:

HKEY_CLASSES_ROOT\.xml (Default) = "xmlfile"

The command that is executed when this file type is opened is specified under

HKEY_CLASSES_ROOT\xmlfile\shell\open\command = ""C:\Program Files\Common Files\Microsoft Shared\OFFICE12\MSOXMLED.EXE" /verb open "%1""

So when an XML file is double-clicked in Explorer, Windows will launch MSOXMLED.EXE. This application is now looking inside the XML file and searches for an XML processing instruction. This processing instruction named mso-application can specify a ProgId:

<?mso-application progid="Word.Document"?>

If this processing instruction is found and the ProgId is one of the supported values MSOXMLED.EXE searches the Registry for the open command specified for that ProgId. For Word.Document there is actually another redirect to Word.Document12 (if Office 2007 is installed) using the CurVer subkey of Word.Document, so we end up with:

HKEY_CLASSES_ROOT\Word.Document.12\shell\Open\command = ""C:\Program Files\Microsoft Office\Office12\WINWORD.EXE" /n /dde"

So finally MSOXMLED.EXE will start the appropriate Office application or launch the default XML application which is specified under

HKEY_CLASSES_ROOT\XEV.GenericApp\shell\open\command

You can actually try this out by calling MSOXMLED.EXE from the command line:

MSOXMLED.EXE /verb OPEN "SampleWordMLDocument.xml"

If you would like to implement the same behavior you would have to implement a handler like MSOXMLED.EXE which looks inside the file for a pre-defined processing instruction and then routes the document to the appropriate application.

Icon handling

Above we looked at the way how document opening and editing is handled. Another mechanism is responsible for displaying a specific icon depending on the processing instruction inside the XML document: an icon handler.

Icon handlers are a type of Explorer Shell extensions which are in-process COM objects which can be associated with certain file types. The one used for XML files is specified in the Registry under

HKEY_CLASSES_ROOT\xmlfile\ShellEx\IconHandler = "{AB968F1E-E20B-403A-9EB8-72EB0EB6797E}"

This GUID is refering to the MSOXEV.dll which will - similarly to MSOXMLEX.EXE inspect the XML file for the ProgId and then provide the correct icon.

As all this is a rather complicated mechanism you should consider carefully if you want to go this way. In my opinion it is far simpler to register a new unique file extension. It is also limited as it will only work with file types that allow you to include some custom information (as the ProgId) in the header of file.

Microsoft does not use this method anymore and uses file extensions instead for their new OpenXML formats (see Why do Office ".xml" files behave differently from other ".xml" files?).

Well, all file associations are stored in the registry. There's an article, "Understanding MS Windows File Associations" that may help.

Beyond that, there are many ways to create file associations programmatically. Though most often this is done in the installer. For example, my favorite installation system NSIS has a macro to handle this: http://nsis.sourceforge.net/FileAssoc

But then there's the challenge of setting a default program to open a certain file type. I don't know what language or install system you're planning on using to do this but it appears that question is answered here for NSIS. Surely Microsoft also has documentation for their way of doing this with Windows Installer. But as you can probably guess, I'm more of an NSIS guy. ;)

In Windows Explorer click on Tools->Folder Options->File Types. By clicking on "New" or "Advanced" you can create/change your own application associations with various file types that will appear when you right click on the file.

EDIT: I don't know if there's a stub that redirects specific files. You can check My Computer\HKEY_CLASSES_ROOT in the registry and see what it's putting in there.