title: How to extend a sealed class - need simple program explanation and usage in real time
body: <p>Folks</p>
  
  <p>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"</p>
  
  <pre><code>class Sealed A       
  { 
      int a; int b;    
      int Add (int x, int y)    
      {
          return x+y;
      }
  }
  </code></pre>
  
  <p>Thank you....</p>