命令模式（Command Pattern）
命令模式（Command Pattern）是一种对象行为模式，分离了接受请求的对象与实现处理请求工作的对象，在软件系统中，“行为请求者”与“行为实现者”通常呈现一种“紧耦合”。但在某些场合，比如要对行为进行“记录、撤销/重做、事务”等处理，这种无法抵御变化的紧耦合是不合适的。在这种情况下，将一组行为抽象为对象，实现二者之间的松耦合。这样，已经存在的类可以保持不变，使得增加新类的工作更简单。例如，很多软件的宏命令就提高了系统的自动化程度。命令模式还可以分离用户界面和业务对象，降低系统的耦合度。
using System;using System.Collections;using System.Collections.Generic;namespace ConsoleApplication{ //一般每个接口或类都写在单独的.cs文件中 //本示例为了执行查看方便才写在一起 public interface IGraphCommand { void Draw(); void Undo(); } public struct Point { public Point(int x, int y) { this.X = x; this.Y = y; } public int X { get; set; } public int Y { get; set; } } public class Line : IGraphCommand { private Point startPoint; private Point endPoint; public Line(Point start, Point end) { startPoint = start; endPoint = end; } public void Draw() { Console.WriteLine("Draw Line:{0} To {1}", startPoint.ToString(), endPoint.ToString()); } public void Undo() { Console.WriteLine("Erase Line:{0} To {1}", startPoint.ToString(), endPoint.ToString()); } } public class Rectangle : IGraphCommand { private Point topLeftPoint; private Point bottomRightPoint; public Rectangle(Point topLeft, Point bottomRight) { topLeftPoint = topLeft; bottomRightPoint = bottomRight; } public void Draw() { Console.WriteLine("Draw Rectangle: Top Left Point {0}, Bottom Right Point {1}", topLeftPoint.ToString(), bottomRightPoint.ToString()); } public void Undo() { Console.WriteLine("Erase Rectangle: Top Left Point {0}, Bottom Right Point {1}", topLeftPoint.ToString(), bottomRightPoint.ToString()); } } public class Circle : IGraphCommand { private Point centerPoint; private int radius; public Circle(Point center, int radius) { centerPoint = center; this.radius = radius; } public void Draw() { Console.WriteLine("Draw Circle: Center Point {0}, Radius {1}", centerPoint.ToString(), radius.ToString()); } public void Undo() { Console.WriteLine("Erase Circle: Center Point {0}, Radius {1}", centerPoint.ToString(), radius.ToString()); } } public class Graphics { Stack commands = new Stack(); public void Draw(IGraphCommand command) { command.Draw(); commands.Push(command); } public void Undo() { IGraphCommand command = commands.Pop(); command.Undo(); } } class Program { static void Main(string[] args) { Line line = new Line(new Point(10, 10), new Point(100, 10)); Rectangle rectangle = new Rectangle(new Point(20, 20), new Point(60, 30)); Circle circle = new Circle(new Point(600, 600), 200); Graphics graphics = new Graphics(); graphics.Draw(line); graphics.Draw(rectangle); graphics.Undo(); graphics.Draw(circle); Console.ReadKey(); } }}