Protected abstract or public abstract method in ab

问题:

Hi I have an abstract class in which I have some public methods and some abstract ones. I have the public so that they implement the common methods for the derived classes.

What is confusing me is why I will want to define a public abstract method instead of protected abstract. That makes no sense to me to define a public abstract method in abstract class.... because if is an abstract will be overridden, in the derived classes, but the same is if is defined as public but somehow it makes more sense to define it as protected as we know that we will override that in the derived classes.

Is it wrong to define the method as public abstract in an abstract class? Which is better and why?

回答1:

For the same reason you want a public method in an object :) You just don't know the particular implementation at this stage. It is common in classes with very high level of abstraction, middlewares for example.

Edit: It is 100% legal. You just need to be sure that it is functionality that you want to expose to the rest of the world in every concrete implementation. Entry point methods (ex: start, execute, parse..) are usually of this kind.

回答2:

It depends on what you want to achieve. For example, you have a Television class that has 3 methods, TurnOn, TurnOff, and Draw.

You only want clients to TurnOn or TurnOff the TV but only its subclass should know what and how to Draw on the screen. So, Television will look more or less like below.

public abstract class Television
{
    public abstract void TurnOn();

    public abstract void TurnOff();

    protected abstract void Draw();
}

Then each company has its own implementation.

public sealed class MyTelevision
    : Television
{
    public override void TurnOn()
    {
        Console.WriteLine("Turn on my tv");
    }

    public override void TurnOff()
    {
        Console.WriteLine("Turn off my tv");
    }

    protected override void Draw()
    {
        // code here.
    }
}

Clients can TurnOn or TurnOff a TV but cannot Draw anything on the screen.

回答3:

The Abstract Class itself has to be as accessible as the Classes, which inherit from it. So if the inherited Classes are Public, the Abstract Class has to be public too.

Public Abstract has the same Idea like other Public Methods: If you have a Abstract Class, you will pass this arround. So if this Method should get called from outside, it's public. If the method is just for communication between Child and Parent, protected is the way to go. Easy example, see the Main-Method as the user of the abstract class:

    static void Main(string[] args)
    {
        Animal cat = new Cat();
        Animal dog = new Dog();

        cat.Eat();
        dog.Eat();

        cat.Move();
        dog.Move();
    }


    public abstract class Animal
    {
        public abstract void Eat();
        protected abstract void ComplexMoving();

        public void Move()
        {
            ComplexMoving();
        }

    }

    public class Dog : Animal
    {
        public override void Eat()
        {
            Debug.WriteLine("Dog says Namnam");
        }

        protected override void ComplexMoving()
        {
            Debug.WriteLine("Dog no stupid");
        }
    }

    public class Cat: Animal
    {
        public override void Eat()
        {
            Debug.WriteLine("Cat says namnam");
        }

        protected override void ComplexMoving()
        {
            Debug.WriteLine("Cat does a slalom");
        }
    }