gistfile1.txt

using System;
 // Define an abstract class called 'Shape' 
abstract class Shape { 
// An abstract method for calculating area, to be implemented by derived classes.
 public abstract double CalculateArea(); 
// A non-abstract method with a default implementation.
 public void DisplayArea() { 
       Console.WriteLine('Area: ' + CalculateArea()); }
 } 
// Create a derived class 'Circle' that inherits from 'Shape'
 class Circle : Shape {
 private double radius; 
// Constructor to initialize the radius 
public Circle(double r) {  
      radius = r; }
 // Implement the abstract method to calculate the area of a circle.
 public override double CalculateArea() { 
return Math.PI * radius * radius; } 
} 
class Program { 
static void Main() {
 // Create an instance of the 'Circle' class
 Circle circle = new Circle(5.0);
 // Calculate and display the area of the circle  
   circle.DisplayArea(); } 
}