Abstract Class in C#.NET

 abstract Keyword in C#.NET

Recall, a class in which a method cannot be implemented or a developer cannot provide definition. The object oriented language provide the concept of defining abstract method. The class having an abstract method is to be defined as an abstract class.

C#.NET  is similar to Java. C#.NET provides abstract keyword to defined an abstract method as well as an abstract class. Now in C#.NET, the two-dimensional figure class is implemented as follows:

namespace ConsoleApp1
{
    internal abstract class TwoDFigure
    {
        //Fields to store dimension of the two-dimension figure 
        protected int d1;
        protected int d2;
        //Constructors to initialize the dimensions
        public TwoDFigure (int d1, int d2)
        {
            this.d1 = d1;
            this.d2 = d2;
        }
        //Method to compute the area of the figure
        //no definition, i.e. abstract method
        abstract public void computeArea();
    }
}

The abstract class can only be used by inheriting it. However, its object cannot be created i.e. cannot be instantiated. Recall, Inheritance represents a IS_A relationship. Therefore, a Triangle is a two-dimensional figure, a Rectangle is also a two-dimensional figure. The following code demonstrate how to use the abstract class.

namespace ConsoleApp1
{
    internal class Rectangle : TwoDFigure
    {
        public Rectangle(int d1, int d2) : base(d1, d2)
        {
            
        }
        //abstract method is defined with override keyword in sub class
        public override void computeArea()
        {
            int area;
            area = d1 * d2;
            Console.WriteLine("Area of Rectangle " +area);
        }
    }
    internal class Triangle : TwoDFigure
    {
        public Triangle(int d1, int d2) : base(d1, d2)
        {
        }
       //abstract method is defined with override keyword in sub class
        public override void computeArea()
        {
            double area = 0.0;
            area = (1.0 / 2.0) * d1 * d2;
            Console.WriteLine("Area of Triangle  " + area);
        }
    }
}

Recall, in C#.NET the reference of the super class can refer to the objects of the sub classes and then it calls the method of that class. It is used to achieve runtime polymorphism. In C#.NET, It is called Dynamic Method Dispatch.

The Rectangle and the Triangle class can be tested by defining a class with the main method as follows:

namespace ConsoleApp1
{
    internal class Program
    {
        static void Main(string[] args)
        {
            
            //Create a Reference variable of TwoDFigure class
            TwoDFigure figure;
            //Refers to object of Rectangle class
            figure = new Rectangle(10, 20);
            //Calls computeArea() version of Rectangle class
            figure.computeArea();
            //Refers to object of Triangle class
            figure = new Triangle(10, 20);
            //Calls computeArea() version of Rectangle class
            figure.computeArea();
            Console.ReadKey();
        }
    }
}

The abstract method is a unimplemented method in an abstract class. It is provided implementation in the respective sub classes. It archives runtime Polymorphism. However, in C#.NET, the abstract method is defined in the sub class with the override keyword. 

Imagine the cut-copy and the paste operation in software. Their implementation varies from one software to another software.