I'm developing a program that uses project files to store structured project data and a project manifest. I currently use the .dcproj extension to match the naming scheme of the application, but I found that a few relatively obscure tools already use .dcproj, including Docker-related tooling and another documented file format.
I could still use .dcproj, but I'm wondering whether that would be considered bad practice. File extensions are limited and many short extensions—such as .m—already have several unrelated meanings. How much should I worry about colliding with existing extensions, especially when the other tools are unlikely to be used alongside mine?
4 Answers
Extensions are reused constantly, so an existing use by itself doesn't make your choice invalid. Pick something recognizable, document what the format contains, and consider how likely users are to encounter files from both programs. If the files are mostly internal project files and aren't exchanged widely, reusing a rare extension is generally acceptable.
Another option is to use a conventional format such as JSON or XML and give the project file a descriptive name, or use a compound name like .dcproj.json. That makes the underlying format easier to inspect and reduces ambiguity, though a custom extension is perfectly reasonable if you want the files associated with your application.
Reusing an extension isn't automatically a problem. The important question is whether users might keep files from both applications in the same folders or expect the files to open by double-clicking. If the formats are likely to coexist, a collision can be confusing; if they're used by unrelated audiences and stored in separate project directories, the practical risk is much lower. You could also choose a slightly longer extension to make collisions less likely.
The biggest practical issue is file association. If you register the extension so your application opens it, another application may already be registered for the same extension, and users could see the wrong icon or have to choose which program should open a file. Different applications can share an extension, but it makes the experience less predictable. A small variation such as .dcmproj could preserve your naming scheme while avoiding most of that confusion.
Even when several programs claim an extension, the operating system can let the user choose which application to use. Still, avoiding the conflict is probably cleaner if changing the extension has no downside.
The files will probably live in a dedicated folder under Documents, so overlap seems very unlikely. The existing uses appear to belong to unrelated tools, although I realize that doesn't make a collision impossible.