Practical Singleton & Dependency Injection question

Question

Say I have a class called PermissionManager which should only exist once for my system and basically fulfills the function of managing various permissions for various actions in

Answer 1

You can indeed start by injecting the PermissionManager. This will make your class more testable.

If this causes problems for the users of that class you can have them use a factory method or an abstract factory. Or you can add a parameterless constructor that for them to call that injects the PermissionManager while your tests use another constructor that you can use to mock the PermissionManager.

Decoupling your classes more makes your classes more flexible but it can also make them harder to use. It depends on the situation what you'll need. If you only have one PermissionManager and have no problem testing the classes that use it then there's no reason to use DI. If you want people to be able to add their own PermissionManager implementation then DI is the way to go.