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;
}
}
}
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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;
}
}
}
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