What is sealed class and sealed method?

-

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();
        }
    }
}

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