If you have multiple Projects in a Visual Studio Solution, you can specify the “Startup Project,” which is the project that runs when you “Start Debugging” or “Start Without Debugging”. The Startup Project appears bold in the Visual Studio Solution Explorer. Step-by-step instructions to set the Startup Project can be found here.
If you use a source repository, perform automated builds, or do any Visual Studio solution file hacking, you may be disappointed to discover that the Startup Project is not saved in the Solution file (.sln), but rather in the Solution User Options file (.suo).
The Solution User Options file contains settings — such as breakpoints, open files and views — that are specific to the current user. Therefore the User Options file is not usually saved in a source repository, so it’s difficult to share the Startup Project across a team.
In addition, while the Solution File is an easy-to-modify XML file, the Solution User Options file is a hard-to-modify binary file, so it’s nearly impossible to programmatically set the Startup Project.
Which means that each developer must manually set the Startup Project for each Visual Studio Solution.