Odtnhj

I've come across an issue whether it is a good idea to create public methods/properties inside a class

public class MyClass : IMyClass

but not in the interface

public interface IMyClass

mainly for the purpose of unit testing implemented methods from IMyClass inside MyClass. I want to have access from my unit tests to implementation details inside class MyClass to check if some fields meet the expectations. Additionaly, I don't want to expose these details through my interface.

On the other hand, I have a feeling that everything public inside MyClass should be exposed in the interface IMyClass, since this is the case of access modifiers:
public to expose information, private to hide details.

So, my the question is: Is it good to create public methods/properties inside a class MyClass, but not in the interface IMyClass?

A test is supposed to be a client of your code, using your code through its interface.

Writing a test that knows how your code works couples your test to your code, which makes a brittle test - every change to the code will break the test.

Instead, try to exercise your code using only the API. Since it sounds like your class has state in it, your test code will need to query that state in another API call.

If your API doesn't have a querying mechanism, perhaps the test is trying to tell you that such a feature is missing from your code.