In Visual Studio 2015 for C#, a solution always has a project specified as the startup project.
How can I find out the startup project of a solution?
If you take a look in your solution explorer, you see one project, that is bold:

That is the one, that is setup as start project
You can define also a new one, if you right click on it:

If you have multiple startup projects, you need to go one step further.
Right click your solution on the solution explorer and go to properties. Under Common Properties/Startup Projects you will see an overview about every project and it's auto startup status.

Right click the solution and click on "Set Up StartUp Projects..."
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