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In Abstract class you can create variables and constructors but in interfaces you can't
Abstract class - if your entities are logically related. That is if they follow "is-a" relationship + common behavior that will be shared by all.
Ex: Saving account, Recurring account, Deposit account all derived from abstracted Account class. Interface wont be good choice here.
Interface - If above is not true.
Ex: JSON serializer, XML serializer do not share anything in common except for the fact that they should be able Serialize and Deserialize given input. So ISerializer interface makes sense here than abstract class.
Interface is a contract. Any class that implement a specific interface means that any object of that class have a specific operation that can be used. (i.e. any class can inherit for IComparable so we can sort any collection of that class)
Abstract class specify a family of class that share a common business. (i.e some class can inherit from base class to share some business between them like C# different Streams classes)
