Why do we need interfaces when abstract classes exist? [duplicate]

Question

One interviewer has asked me the below question and I couldn't answer:

Why do we need Interfaces when abstract classes exist?

Whatever the methods we are writing in interface those we can write in Abstract class also. Then why do we need interfaces separately?

Can anybody please tell what is the reason?

Advance thanks...

Answer 1

There are several differences,

• Abstract classes can have only one parent class, whereas a class can implement several interfaces.
• Interfaces cannot contain any implementation, abstract classes can (they can have abstract methods in addition to not abstract methods).

Interfaces are great to focus on a 'view' we can have on a class. This view can be shared by multiple classes implementing the interface.

For instance, DataTable implements IListSource and ISerializable. So, depending on the context, you can view it as a list source to read its data or as a class which instances can be serialized. When you do so, you focus on a specifc view you can have of an instance.

Answer 2

Interface represents a contract, while you can have several implementations of that contract in different (abstract) classes.

public interface IExample
{
    void Do();
}

public abstract class DoFirst : IExample
{
    public void Do()
    {
        Console.WriteLine("Doing it the first way");
    }
}

public abstract class DoSecond : IExample
{
    public void Do()
    {
        Console.WriteLine("Doing it the second way");
    }
}

public class DoFirstConcrete : DoFirst, IExample
{
    public void DoSomethingElse()
    {
        Do();
        Console.WriteLine("Doing something else also with first.");
    }
}

public class DoSecondConcrete : DoSecond, IExample
{
    public void DoSomethingElse()
    {
        Do();
        Console.WriteLine("Doing something else also with second.");
    }
}