If you set Startup Project with Visual Studio, it is saved to .slo file not to a solution file.
You can't change the defaut Startup Project wit Visual Studio.
This plugin makes it possible to change default Startup Project by writing to the solution file.
It also change Startup Project saved at .suo file.After running this plugin, you prompt to reload the solutionbecause the it is changed. This plugin uses https://github.com/michaKFromParis/slnStartupProject
Website (In Japanese)