title: Proper way to create a singleton service in ASP.NET Core - does it have to use the static keyword, or not?
body: <p>This is really confusing for me. Nothing is stopping us from creating a class with instance fields and 
  instance methods, versus creating a class with static fields and static methods. Both can be registered as a singleton.</p>
  
  <p>Is there a preferred approach to this problem, and why?</p>
  
  <p><strong>EDIT:</strong> I'm not asking about creating a class using the singleton pattern. I'm asking about singleton dependency injecting a class versus injecting the same class but with static defined members/fields/methods/properties.</p>