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About File Extensions
When you register a file extension of a VSPackage, you associate it with a version of Visual Studio. This is important if more than one version of Visual Studio is installed on a computer.
File extensions for VSPackages are registered under HKEY_CLASSES_ROOT key with a default value that points to the associated programmatic identifier (ProgID).
The following is an example of the registration information for the .vcproj file extension:
HKEY_CLASSES_ROOT\ .vcproj\ (default)=" VisualStudio.vcproj.8.0"
Files associated with Visual Studio must have a versioned ProgID, such as VisualStudio.vcproj.8.0, to allow side-by-side installations of the product to maintain file extension associations among product versions. A version-specific ProgID also allows you to use standard verbs, such as open, edit, and so on, without the concern of overwriting or being overwritten by other applications or versions of Visual Studio.
In certain cases, the ProgID associated with a file extension should not be changed. For example, the ProgID for the .htm file extension (progid = htmlfile) is hard coded in a number of places in the operating system, and is widely known and used in association with .htm and .html files.