Abstract Class

Abstraction is "the process of identifying common patterns that have systematic variations; an abstraction represents the common pattern and provides a means for specifying which variation to use" (Richard Gabriel).
An abstract class is a parent class that allows inheritance but can never be instantiated. Abstract classes contain one or more abstract methods that do not have implementation. Abstract classes allow specialization of inherited classes.

Indicates that a class is to be used only as a base class for other classes. This means that you cannot create an instance of the class directly. Any class derived from it must implement all of its abstract methods and accessors. Despite its name, an abstract class can possess non-abstract methods and properties.

public abstract class Shape

{
private float _area;
private System.Drawing.Color _color;
private float _perimeter;
public float Area
{
get
{
return _area;
}
set
{
_area = value;
}
}
public System.Drawing.Color Color
{
get
{
return _color;
}
set
{
_color = value;
}
}
public float Perimeter
{
get
{
return _perimeter;
}
set
{
_perimeter = value;
}
}
public abstract void CalculateArea();
public abstract void CalculatePerimeter();
}

class Rectangle : Shape
{
private float _height;
private float _width;
public Rectangle(float height, float width)
{
_height = height;
_width = width;
}
public float Height
{
get
{
return _height;
}
set
{
_height = value;
}
}
public float Width
{
get
{
return _width;
}
set
{
_width = value;
}
}
public override void CalculateArea()
{
this.Area = _height * _width;
}
public override void CalculatePerimeter()
{
this.Perimeter = (_height * 2) + (_width * 2);
}
public void Display()
{
Console.WriteLine("Perimeter{0}", this.Perimeter);
Console.WriteLine("Area{0}", this.Area);
}
}

static void Main (string[] args)
{
Rectangle objRectangle = new Rectangle(10, 20);
objRectangle.CalculateArea();
objRectangle.CalculatePerimeter();
objRectangle.Display();
}