make abstract a method with body for overriding

I have my Beverage class, which has some getters/setters to work with the size of the beverage. This program has to do with the decorator pattern, so I want to combine the behavior of some methods equally named.

My intent is to have a method with body that allows me to get the size of the beverage, but then, I want to be able to override that behavior on child classes.

In sum, I want a method that:

if not overriden, just behaves as the method in the parent class

if overriden, behaves like it is coded

What I did was to create a method called getSizeOfBeverage that behaves like my "old" getSize did, and made the getSize "new" method abstract so I can override it, but I want a solution that does not imply a new method name.

Here is my code:

using System;

namespace Decorator
{
    class Program
    {
        static void Main(string[] args)
        {
            Beverage beverage = new Espresso("small");
            beverage = new Whip(beverage);
            Console.WriteLine(beverage.getDescription() + " $" + beverage.cost());
        }
    }

    abstract class Beverage
    {
      private string description;
      private string size;

      public Beverage()
      {
        setDescription("Unknown beverage");
      }

      public double getCost()
      {
        return cost();
      }

      public abstract double cost();

      public abstract string getDescription();

      public void setDescription(string desc)
      {
        description = desc;
      }

      public string getSizeOfBeverage()
      {
        return size;
      }

      public abstract string getSize();

      public void setSize(string sz)
      {
        size = sz;
      }
    }

    class Espresso : Beverage
    {
      public Espresso(string sz)
      {
        setSize(sz);
        setDescription("Espresso");
      }

      public override double cost()
      {
        return 1.9;
      }

      public override string getDescription()
      {
        return getDescription();
      }

      public override string getSize()
      {
        return getSizeOfBeverage();
      }
    }

    abstract class CondimentDecorator : Beverage
    {
      public abstract override string getSize();
    }

    class Whip : CondimentDecorator
    {
      private Beverage beverage;

      public Whip(Beverage bv)
      {
        beverage = bv;
      }

      public override double cost()
      {
        if (getSize() == "small")
        {
          return 0.1 + beverage.cost();
        }
        else if (getSize() == "medium")
        {
          return 0.15 + beverage.cost();
        }
        else
        {
          return 0.2 + beverage.cost();
        }
      }

      public override string getDescription()
      {
        return beverage.getDescription() + ", whip";
      }

      public override string getSize()
      {
        return beverage.getSizeOfBeverage();
      }
    }
}

---

if not overriden, just behaves as the method in the parent class if

overriden, behaves like it is coded

Every virtual method works like this:

If it is overriden it will behave like it is coded, if not just behaves as the method in the parent class

From the documentation for virtual

The virtual keyword is used to modify a method, property, indexer, or event declaration and allow for it to be overridden in a derived class. For example, this method can be overridden by any class that inherits it:

and abstract

When an instance method declaration includes an abstract modifier, that method is said to be an abstract method. Although an abstract method is implicitly also a virtual method, it cannot have the modifier virtual. An abstract method declaration introduces a new virtual method but does not provide an implementation of that method. Instead, non-abstract derived classes are required to provide their own implementation by overriding that method. Because an abstract method provides no actual implementation, the method-body of an abstract method simply consists of a semicolon.

See the difference between virtual and abstract methods in C#

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