I want to create classes that can be inherited by other classes to use along my project.
i.e. i want to create a separate class for each Mail method, each class will implement differently the Compose method, and more Mail classes will be created in the future.
My question is – Where should I create these classes/interfaces? in the App_Code ?
I would suggest creating a separate assembly and include that assembly as a reference in your Web Project. That will enable you to write independent test cases against your classes / interfaces.