If you encounter this situation that green play button, f5 key or Start Debugging option is disabled in visual studio.
Cause:
You have more than one project in Visual Studio solution but none of them is set up as a “Set as Startup Project”.
Solution:
Right click on one of the project and click on the “Set as Startup Project” now the Start green play button, F5 key and Start Debugging option will be enabled.
Abdul Rauf 