When I discovered and began using project files in 2009 or 2010, I attempted to open a project file from Windows Explorer, assuming that it had an association with UltraEdit, and that it would open the project as if I had selected it from the menu of recent projects in an active UE instance. Silly me, there was no association, quite possibly because Visual Studio 6 had already appropriated that extension for itself.
While it remains true that there is no association per se, I just discovered that one can now be safely created, because my current installation of Visual Studio is 2013, which now sports a collection of long extensions for the multitude of language specific project files that it supports (e. g., vcxproj for Visual C++ and .csproj for C#).
This I accomplished by double-clicking a .prj file in the Windows Explorer, exercising the option to select a program from a list, and pick the only program offered, which happened to be UltraEdit.
While it remains true that there is no association per se, I just discovered that one can now be safely created, because my current installation of Visual Studio is 2013, which now sports a collection of long extensions for the multitude of language specific project files that it supports (e. g., vcxproj for Visual C++ and .csproj for C#).
This I accomplished by double-clicking a .prj file in the Windows Explorer, exercising the option to select a program from a list, and pick the only program offered, which happened to be UltraEdit.