Major or Minor Revision Change when Implementing an Interface

Question

I am using semantic versioning 2.0.0. In my situation I have a concrete implementation of some interface A in one library. And I have the interface that's being implemented (call it B) in another library.

I understand that if I were to add new methods to the interface B then that would be considered a major change for B because any library that implements the interface (such as A) would not compile until implementing whatever methods were added. My reasoning for this being considered a major change is because you've now made an incompatible API change to B (according to semantic versioning). I think that I understand this concept clearly but correct me if I'm wrong.

Here is my question, and the part that I don't understand... What happens to A, the concrete that's actually implementing the interface? Does he get a major or minor revision number change? I guess that from one stand point you could say he gets a minor revision change because adding a public method doesn't make him incompatible like for instance... if you were to change the name of a method... I guess you could also argue that he would get a major change because he needs to implement the interface in order to work. Is either one of these reasonings correct, or neither?

Answer 1

OK. so

you have

public class A : IA {}

and

public interface IA {}

in different libraries

And I have

public class MyA : A {}

for which i am consuming your IA.dll

If you add a method IA.GetAs(), then yes its a major change because A will not compile until you add an implementation of the interface.

It seems like MyA will still compile when I update to both A.dll v2 and IA.dll v2

However, What if I already have MyA.GetAs()? Its possible that my code would end up calling A.GetAs() instead of MyA.GetAs(). So i think you would have to upgrade the major version number.

In general I would consider any change to the exposed methods/properties etc of an object to be a major change.