Difference between Interface and Abstract

Difference Virtual and Abstract

When we use Abstract or Sealed or Interface

We would create an abstract class, when want to move the common functionality of 2 or more related classes into a base class and when, we don't want that base class to be instantiated.

Sealed is a C# keyword which is used to seal a class and prevent it to get inherited from any other classes.

Interface help us to develop loosely coupled system, Helps in Dependencies Injection, makes Unit Testing easier.

Can an abstract class have a constructor? If so what is the use?

Yes, an abstract class can have a constructor. In general, a class constructor is used to initialize fields. But it happens before the instantiation of a child-class takes place.

Though you cannot create an instance of an abstract class, we can create instances of the classes that are derived from the abstract class.

Abstract classes can't be directly instantiated. So, it is a good practice to use protected access modifier with abstract class constructor and by using public doesn't make sense.

Questions

Q1) An interface has got four interface methods (m1, m2, m3, m4) and implement these interface methods as follows.

Implement m1() & m2() in base class

Implement m3() & m4() in derived class

A1) Make declare m3, m4 virtual in base class and override it in derived class.

Q2) An two interface has got same method, how can you access differently.

A2)

class Test : A, B {
    void A.Hello() { }
    void B.Hello() { }
}
public class interfacetest {
    public static void Main()  {
        A Obj1 = new Test();
        Obj1.Hello();
        B Obj2 = new Test();
        Obj2.Hello();
    }
}