When to use Inheritance in C#?

Question

Hello everyone,

 

Kindly explain "What is the use of Inheritance? and When to use?"  with respect two below programs. 

 

Why I cannot get output from 1st program after using class initialization?

 

Program 1:

 

 

class Program
   {
       class Shape
       {
           public void setWidth(int w)
           {
               width = w;
           }
           public void setHeight(int h)
           {
               height = h;
           }
           public int width;
           public int height;
       }

       // Derived class
       class Rectangle
       {
           Shape objshape = new Shape();
           public int getArea()
           {
               return (objshape.width * objshape.height);
           }
       }
       static void Main(string[] args)
       {
           Shape Rect = new Shape();
           Rectangle objRectangle = new Rectangle();
           Rect.setWidth(5);
           Rect.setHeight(7);

           // Print the area of the object.
           Console.WriteLine("Total area: {0}", objRectangle.getArea());
           Console.ReadKey();
       }
   }

 

 Program 2:

 

class Program
   {
       class Shape
       {
           public void setWidth(int w)
           {
               width = w;
           }
           public void setHeight(int h)
           {
               height = h;
           }
           public int width;
           public int height;
       }

       // Derived class
       class Rectangle : Shape
       {
           public int getArea()
           {
               return (width * height);
           }
       }
       static void Main(string[] args)
       {
           Rectangle Rect = new Rectangle();

           Rect.setWidth(5);
           Rect.setHeight(7);

           // Print the area of the object.
           Console.WriteLine("Total area: {0}", Rect.getArea());
           Console.ReadKey();
       }
   }