About file name extensions
Applies to: 
Visual Studio 
Visual Studio for Mac
Note
This article applies to Visual Studio 2017. If you're looking for the latest Visual Studio documentation, see Visual Studio documentation. We recommend upgrading to the latest version of Visual Studio. Download it here
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 example shows 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. A versioned ProgID allows 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.