When do I use abstract classes versus interfaces with regards to dependency Injection?

Question

I have been reading some articles about the SOLID principles and dependency Inversion. From my point of view, I must use an interface to talk to any class. My classes are chatti

Answer 1

The original definition of an interface is an abstract class with all pure virtual methods (i.e. abstract methods), this was how you describe an interface in C++. If you are not creating virtual functions with default definitions then you really do not need an abstract class at all. If you do have some default functionality that you would like the children of your Message class to inherit (and allow to override or not) then you would use an abstract class. An abstract class can also define protected methods and/or properties and fields, these can be used by classes that inherit from the abstract class but not by classes that use the abstract class. All methods in an interface would be public. In the case you laid out an interface would be fine.