8 (Program.cs)

This is physical class filename.

This is the name of a class file that is physically available on our disk drive. Class name and filename could be different, which means if my class name is Program, then my class filename could be Program1.cs. However, it is bad practice to call both class and filename with different names, but you can do that and the compiler won't throw any exception. For more information, refer to https://stackoverflow.com/questions/2224653/c-sharp-cs-file-name-and-class-name-need-to-be-matched.