开闭原则(Open Closed Principle)

开闭原则（Open-Closed Principle，OCP）是指一个软件实体（如类、模块和函数）应该对扩展开放，对修改关闭。如当一个模块需要修改的时，不应该直接修改源代码，这样有可能对现有的工作造成影响。应该通过拓展来实现新需求。

例如，

1）一般的反面设计实现

using System;
namespace ConsoleApplication
{
    /// <summary>
    /// 矩形(Shape.cs)
    /// </summary>
    public class Shape
    {
        private double _width;
        private double _height;
        private double _radius;
        private string _name;
        public Shape(string name, double width, double height)
        {
            this._width = width;
            this._height = height;
            _name = name;
        }
        public double AreaRectangle()
        {
            return _width * _height;
        }
        public void DisplayRectangle()
        {
            Console.WriteLine("{0} 长：{1}，宽：{2}，面积：{3}", _name, _width, _height, this.AreaRectangle());
        }
        public Shape(string name, double radius)
        {
            this._radius = radius;
            this._name = name;
        }
        public double AreaCircle()
        {
            return Math.Round(Math.PI * _radius * _radius);
        }
        public void DisplayCircle()
        {
            Console.WriteLine("{0} 半径：{1}，面积：{2}", _name, _radius, this.AreaCircle());
        }
    }
    class Program
    {
        static void Main(string[] args)
        {
            Shape circle = new Shape("圆", 1);
            circle.DisplayCircle();
            Shape rectangle = new Shape("正方形", 100, 100);
            rectangle.DisplayRectangle();
            Console.ReadKey();
        }
    }
}

2）开闭原则的实现

using System;
namespace ConsoleApplication
{
    //Shape.cs
    public abstract class Shape
    {
        protected string _name;
        public Shape(string name)
        {
            this._name = name;
        }
        /// <summary>
        /// 面积
        /// </summary>
        /// <returns></returns>
        public abstract double Area();
        /// <summary>
        /// 显示
        /// </summary>
        public abstract void Display();
    }
    /// <summary>
    /// 矩形(Rectangle.cs)
    /// </summary>
    public class Rectangle : Shape
    {
        private double _width;
        private double _height;
        public Rectangle(string name, double width, double height)
            : base(name)
        {
            this._width = width;
            this._height = height;
        }
        public override double Area()
        {
            return _width * _height;
        }
        public override void Display()
        {
            Console.WriteLine("{0} 长：{1}，宽：{2}，面积：{3}", _name, _width, _height, this.Area());
        }
    }
    /// <summary>
    /// 圆形(Circle.cs)
    /// </summary>
    public class Circle : Shape
    {
        private double _radius;
        public Circle(string name, double radius)
            : base(name)
        {
            this._radius = radius;
        }
        public override double Area()
        {
            return Math.Round(Math.PI * _radius * _radius);
        }
        public override void Display()
        {
            Console.WriteLine("{0} 半径：{1}，面积：{2}", _name, _radius, this.Area());
        }
    }
    class Program
    {
        static void Main(string[] args)
        {   
            Shape circle = new Circle("圆", 1);
            circle.Display();
            Shape rectangle = new Rectangle("正方形", 100, 100);
            rectangle.Display();
            Console.ReadKey();
        }
    }
}