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

If you are using a dependency-injection framework, then the common way to handle this is to either pass in a PermissionsManager object in the constructor or to have a property of type PermissionsManager that the framework sets for you.

If this is not feasible, then having users get an instance of this class via factory is a good choice. In this case, the factory passes the PermissionManager in to the constructor when it creates the class. In your application start-up, you would create the single PermissionManager first, then create your factory, passing in the PermissionManager.

You are correct that it is normally unwieldy for the clients of a class to know where to find the correct PermissionManager instance and pass it in (or even to care about the fact that your class uses a PermissionManager).

One compromise solution I've seen is to give your class a property of type PermissionManager. If the property has been set (say, in a unit test), you use that instance, otherwise you use the singleton. Something like:

PermissionManager mManager = null;
public PermissionManager Permissions
{
  if (mManager == null)
  {
    return mManager;
  }
  return PermissionManager.getInstance();
}

Of course, strictly speaking, your PermissionManager should implement some kind of IPermissionManager interface, and that's what your other class should reference so a dummy implementation can be substituted more easily during testing.