How to extend a sealed class - need simple program explanation and usage in real time [closed]

Question

Folks

I have a sealed class as follows. I want to extend this sealed class so as to add a method which will return average of x and y. This is not just extension method using "this" :( Could someone please help me understand the concept of "Extending Sealed class" and its "Real time usage and benefits"

class Sealed A       
{ 
    int a; int b;    
    int Add (int x, int y)    
    {
        return x+y;
    }
}

Thank you....

Answer 1

As @ReedCopsey has already pointed out, the way to extend the functionality of a sealed class is with an Extension Method. Here is one that will do what you are asking:

public sealed class MyClass       
{ 
    int a; int b;    
    int Add (int x, int y)    
    {
        return x + y;
    }
}

public static class MyClassExtensions
{
    public static decimal Average(this MyClass value,  int x, int y)
    {
        return (x + y)/2M;
    }
}

Usage:

    var myClass = new MyClass();

    // returns 15
    var avg = myClass.Average(10, 20);

EDIT As requested, here is the all the code. Create a new Console Application in Visual Studio, replace all the code in the Program.cs file with the code below and run.

using System;

namespace ConsoleApplication1
{
    public sealed class MyClass
    {
        public int X { get; private set; }
        public int Y { get; private set; }

        public MyClass(int x, int y)
        {
            this.X = x;
            this.Y = y;
        }

        int Add()
        {
            return this.X + this.Y;
        }
    }

    public static class MyClassExtensions
    {
        public static decimal Average(this MyClass value)
        {
            return (value.X + value.Y) / 2M;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            var myClass = new MyClass(10, 20);
            var avg = myClass.Average();

            Console.WriteLine(avg);
            Console.ReadLine();
        }
    }
}