So I can mock it out for unit tests.

Then you have two implementations…

You don't necessarily need to implement interfaces [in your code] to stub them in unit tests:

    var calculator = Substitute.For<ICalculator>();

If you’re doing that, you may as just mock your concrete class. Mockito supports this in Java. Perhaps this is needed in C#?

Java is virtual by default. C# is not. You could mark every single method virtual and mock it like Java. But it’s easier to define a contract and mock that.

Not one that shows up when I hit "go to implementation".

LOL ok so you’ve never used C#