After C# 8.0 how interface are different then Abstract class

An interface is not a class and it is defined by interface. It is an entity.

An abstract class should atleast have one abstract method. without having one abstract

method, there is no point in declaring that class as abstract.

a standard structure is provided by the interface and the derived class must follow it.

a class can only inherit only one abstract class but it can inherit several interfaces.

When to choose between these two.

 

This is a good question and I loved following read on that

 

https://www.codementor.io/vipinc/when-to-use-abstract-classes-and-when-interfaces-c-interview-questions-jtzll1sn7

 

USE AN ABSTRACT CLASS

 

1). When creating a class library which will be widely distributed or reused—especially to clients, use an abstract class in preference to an interface; because, it simplifies versioning. This is the practice used by the Microsoft team which developed the Base Class Library. (COM was designed around interfaces.)

2). Use an abstract class to define a common base class for a family of types.

3). Use an abstract class to provide default behavior.

4). Subclass only a base class in a hierarchy to which the class logically belongs.

USE AN INTERFACE

1). When creating a standalone project which can be changed at will, use an interface in preference to an abstract class; because, it offers more design flexibility.

2). Use interfaces to introduce polymorphic behavior without subclassing and to model multiple inheritance—allowing a specific type to support numerous behaviors.

3). Use an interface to design a polymorphic hierarchy for value types.

4). Use an interface when an immutable contract is really intended.

5). A well-designed interface defines a very specific range of functionality. Split up interfaces that contain unrelated functionality.

But, now that I can declare methods in interface as well how should I decide which one to use, abstract class or interface. While multiple inheritance is a byproduct of what interface can provide us i.e real time impact analysis

Indeed they can be.

That's why my statement about method declaration had "By Default" in it.

 

My point is beside method, there are so many other attributes of interfaces which differ them from abstract classes.

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/proposals/csharp-8.0/default-interface-methods

Interface can have private protected and static members 

Interfaces are still different from Abstract classes.