Question

An abstract class is the same thing as an interface except it is an actual class, not just a contract.
Why interface is refered to as a contract ?

Answer 1

Both interface and an abstract class are contracts, since they bind you.

But there other differences between an interface and an abstract class.

Like you said, if you need an actual class, with content to the functions or actual data members with default values, or a ctor, then obviously you will need an abstract class.

But many times, the choose between an interface or an abstract class will be technically the same. And sometimes even in the long term, it will be the same.

Your decision then should be based on the nature of what you are looking.

Is it an extra character to your data types? Or does it define what your data type is?

I’ve tried to think of any real world example, but I couldn’t find any as I haven’t done anything with the subject for years now, so I’ll give you a book-like example.

Say we have an abstract class Animal:

public abstract class Animal
{
    abstract string Name;
    abstract bool IsWild;
    abstract bool IsHappy;
}

This could have been easily an interface.

And choosing it to be an interface will actually won’t have any negative effect on your design.

But in the matter of the nature of it, it must be an abstract class, since class Dog is an animal. It must be class Dog : Animal rather than class Dog : IAnimal.

Being an animal is not an extra character of Dog. It what defines it.

And you want the restrict all the animals to inherit only from Animal.

IAnimal let someone inherit something else. like:

public class Dog : Food, IAnimal {}

Interestingly enough, sometimes one might write an abstract class with nothing at all, just for the sake of the same thing.

For example, if there was no common property for any Animal you will still prefer to do:

public abstract class Animal { }

public class Dog : Animal // No actual added value.
{ 
        .........
}