Does ‘Me’ in VB.NET refer to the instantiated object only

I am not sure I understand why do you want to do this (apart from dealing with an intellectual challenge). This kind of code, where you work directly with some system level API, is very hard to unit test, TDD style or not, and to be frank, I don't find it very valuable to try it in a real project.

Most of the task you describe is calling the right low level API method with the right parameter. And the rest of the code as is may be so trivial, it wouldn't necessarily warrant the introduction of a dedicated interface, mock objects etc. I would be content having an integration test to verify on a higher level that the whole stuff works. But this is just my 2 cents.

Unit testing for me is not about following rulebooks or strict definitions. In real life, I don't really care whether my tests are "real" unit tests or not; as long as my code is being covered by automated and repeatable tests, I am fine. I prefer the pragmatic approach.

No, there's no way to change the instantiated type of an object. A Vehicle is a Vehicle, and will remain such until it goes out of scope.

Your choices here are threefold:

1. Make the type of Vehicle simply a property on the Vehicle object, and default it to Unknown.
   • This works best when there's little functional difference between a Car and a Truck.
   • Functions may return different results depending on the VehicleType, and you'll need to handle the Unknown case, but it's trivial to reassign.
2. Replace the Vehicle with a new object of Car or Truck
   • This would require either a IsActuallyACar() method on the Vehicle which returns a Car, or a Car constructor which takes a Vehicle as an argument (which is the much better choice).
   • Going with this method requires you to manually remove the Vehicle from everywhere that it's stored, and replace it with the newly created object.
3. Rethink your design so that Vehicle is abstract and you don't create the object until you know what type it is.
   • This is probably the best option from an OO perspective.
   • You may need to temporarily store data in a non-Vehicle structure as you gather it, but you don't need to worry about changing the type later.

In your described scenario, I suspect #1 is the best choice. Simply have logic which says "If the object has its type set to Car, then show this, if it's set to Truck show this." It's not great OOP design, but it handles the "Vehicle of unknown type" the best.