像类、模块和函数这样的软件实体应该对扩展开放，但对修改关闭。

定义 - 开放关闭原则指出代码的设计和编写应该以这样的方式完成：在现有代码中进行最少的更改来添加新功能。设计的方式应该允许添加新功能作为新类，并尽可能保持现有代码不变。

示例

打开关闭之前的代码原理

using System;
using System.Net.Mail;
namespace SolidPrinciples.Open.Closed.Principle.Before{
   public class Rectangle{
      public int Width { get; set; }
      public int Height { get; set; }
   }
   public class CombinedAreaCalculator{
      public double Area (object[] shapes){
         double area = 0;
         foreach (var shape in shapes){
            if(shape is Rectangle){
               Rectangle rectangle = (Rectangle)shape;
               area += rectangle.Width * rectangle.Height;
            }
         }
         return area;
      }
   }
   public class Circle{
      public double Radius { get; set; }
   }
   public class CombinedAreaCalculatorChange{
      public double Area(object[] shapes){
         double area = 0;
         foreach (var shape in shapes){
            if (shape is Rectangle){
               Rectangle rectangle = (Rectangle)shape;
               area += rectangle.Width * rectangle.Height;
            }
            if (shape is Circle){
               Circle circle = (Circle)shape;
               area += (circle.Radius * circle.Radius) * Math.PI;
            }
         }
         return area;
      }
   }
}

OpenClosed原则之后的代码

namespace SolidPrinciples.Open.Closed.Principle.After{
   public abstract class Shape{
      public abstract double Area();
   }
   public class Rectangle: Shape{
      public int Width { get; set; }
      public int Height { get; set; }
      public override double Area(){
         return Width * Height;
      }
   }
   public class Circle : Shape{
      public double Radius { get; set; }
      public override double Area(){
         return Radius * Radius * Math.PI;
      }
   }
   public class CombinedAreaCalculator{
      public double Area (Shape[] shapes){
         double area = 0;
         foreach (var shape in shapes){
            area += shape.Area();
         }
         return area;
      }
   }
}