What is sealed class and sealed method?

In this article I will try to explain sealed class and sealed method in a very simple way.

 

Sealed Class

 

Sealed class is used to define the inheritance level of a class.

 

The sealed modifier is used to prevent derivation from a class. An error occurs if a sealed class is specified as the base class of another class.

 

Some points to remember:  

1.  A class, which restricts inheritance for security reason is declared, sealed class.
2.  Sealed class is the last class in the hierarchy.
3.  Sealed class can be a derived class but can't be a base class.
4.  A sealed class cannot also be an abstract class. Because abstract class has to provide functionality and here we are
     restricting it to inherit.

Practical demonstration of sealed class

 

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

 

namespace sealed_class

{

    class Program

    {

        public sealed class BaseClass

        {

            public void Display()

        {

            Console.WriteLine("This is a sealed class which can;t be further inherited");

        }

    }

 

        public class Derived : BaseClass

        {

            // this Derived class can;t inherit BaseClass because it is sealed

        }

   

        static void Main(string[] args)

        {

            BaseClass obj = new BaseClass();

 

            obj.Display();

 

            Console.ReadLine();

        }

    }

}

 

Sealed Methods

 

Sealed method is used to define the overriding level of a virtual method.

 

Sealed keyword is always used with override keyword.

 

Practical demonstration of sealed method

 

using System;

using System.Collections.Generic;

using System.Linq;

using System.Text;

 

namespace sealed_method

{

    class Program

    {

        public class BaseClass

        {

           

            public virtual void Display()

            {

                Console.WriteLine("Virtual method");

            }

        }

 

       public class DerivedClass : BaseClass

        {

            // Now the display method have been sealed and can;t be overridden

            public override sealed void Display()

            {

                Console.WriteLine("Sealed method");

            }

        }

 

       //public class ThirdClass : DerivedClass

       //{

 

       //    public override void Display()

       //    {

       //        Console.WriteLine("Here we try again to override display method which is not possible and will give error");

       //    }

       //}

 

        static void Main(string[] args)

        {

 

            DerivedClass ob1 = new DerivedClass();

            ob1.Display();

 

            Console.ReadLine();

        }

    }

}