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.
Is there a preferred approach to this problem, and why?
EDIT: 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.
You need to understand the different lifetimes for DI and your needs, in order to choose the proper life span.
If in doubt, make it transient.
EDIT
I guess its worth to mention that if you create a services as Transient and they have static properties which you constantly change, then this will affect every other created instance, since static affects the type itself, rather than an instance.
I'm asking about [...] injecting the same class but with static defined members/fields/methods/properties
That is possible but useless. If a controller requires such a singleton object, then you still have no instance method to call on this object. Your controller have to call static methods on the class.
[...] creating a class with instance fields and instance methods
That is a better way to do because it will allow you to mock the object for unit testing.
If you want concrete example, I've coded a class with instance methods to store data about users, which is registered as a singleton object, and used in controller.
Hope it makes things clearer for you.
static class fields can be modified by other instances of the same class. by registering a class as singleton with static fields you will be relying on the fact that never in any other parts of the app any other instances of the same class will be created.
i realize that you will focus on obtaining an instance from DI - however, if anywhere in the app there is a point anohter instance of the class is instantiated directly and not via DI - the static property will be exposed to potential change of it state (value).
working with singleton instance properties/fields will ensure you only work with the properties/fields controlled by the instance you get from DI.
