What are abstract classes in C#?

Abstract classes contain abstract methods, which are implemented by the derived class. The derived classes have more specialized functionality.

The following is an example showing the usage of abstract classes in C#.

Example

 Live Demo

using System;
namespace Demo {
   abstract class Shape {
      public abstract int area();
   }
   class Rectangle: Shape {
      private int length;
      private int width;
      public Rectangle( int a = 0, int b = 0) {
         length = a;
         width = b;
         Console.WriteLine("Length of Rectangle: "+length);
         Console.WriteLine("Width of Rectangle: "+width);
      }
      public override int area () {
         return (width * length);
      }
   }
   class RectangleTester {
      static void Main(string[] args) {
         Rectangle r = new Rectangle(14, 8);
         double a = r.area();
         Console.WriteLine("Area: {0}",a);
         Console.ReadKey();
      }
   }
}

Output

Length of Rectangle: 14
Width of Rectangle: 8
Area: 112

Our abstract class above is −

abstract class Shape {
   public abstract int area();
}

The following are the rules about abstract classes.

• You cannot create an instance of an abstract class
• You cannot declare an abstract method outside an abstract class
• When a class is declared sealed, it cannot be inherited, abstract classes cannot be declared sealed.