C# 命令模式讲解和代码示例
命令是一种行为设计模式， 它可将请求或简单操作转换为一个对象。
此类转换让你能够延迟进行或远程执行请求， 还可将其放入队列中。
复杂度：
流行度：
使用示例： 命令模式在 C# 代码中很常见。 大部分情况下， 它被用于代替包含行为的参数化 UI 元素的回调函数， 此外还被用于对任务进行排序和记录操作历史记录等。
识别方法： 命令模式可以通过抽象或接口类型 （发送者） 中的行为方法来识别， 该类型调用另一个不同的抽象或接口类型 （接收者） 实现中的方法， 该实现则是在创建时由命令模式的实现封装。 命令类通常仅限于一些特殊行为。
概念示例
本例说明了命令设计模式的结构并重点回答了下面的问题：
- 它由哪些类组成？
- 这些类扮演了哪些角色？
- 模式中的各个元素会以何种方式相互关联？
Program.cs: 概念示例
using System; namespace RefactoringGuru.DesignPatterns.Command.Conceptual { // The Command interface declares a method for executing a command. public interface ICommand { void Execute(); } // Some commands can implement simple operations on their own. class SimpleCommand : ICommand { private string _payload = string.Empty; public SimpleCommand(string payload) { this._payload = payload; } public void Execute() { Console.WriteLine($"SimpleCommand: See, I can do simple things like printing ({this._payload})"); } } // However, some commands can delegate more complex operations to other // objects, called "receivers." class ComplexCommand : ICommand { private Receiver _receiver; // Context data, required for launching the receiver's methods. private string _a; private string _b; // Complex commands can accept one or several receiver objects along // with any context data via the constructor. public ComplexCommand(Receiver receiver, string a, string b) { this._receiver = receiver; this._a = a; this._b = b; } // Commands can delegate to any methods of a receiver. public void Execute() { Console.WriteLine("ComplexCommand: Complex stuff should be done by a receiver object."); this._receiver.DoSomething(this._a); this._receiver.DoSomethingElse(this._b); } } // The Receiver classes contain some important business logic. They know how // to perform all kinds of operations, associated with carrying out a // request. In fact, any class may serve as a Receiver. class Receiver { public void DoSomething(string a) { Console.WriteLine($"Receiver: Working on ({a}.)"); } public void DoSomethingElse(string b) { Console.WriteLine($"Receiver: Also working on ({b}.)"); } } // The Invoker is associated with one or several commands. It sends a // request to the command. class Invoker { private ICommand _onStart; private ICommand _onFinish; // Initialize commands. public void SetOnStart(ICommand command) { this._onStart = command; } public void SetOnFinish(ICommand command) { this._onFinish = command; } // The Invoker does not depend on concrete command or receiver classes. // The Invoker passes a request to a receiver indirectly, by executing a // command. public void DoSomethingImportant() { Console.WriteLine("Invoker: Does anybody want something done before I begin?"); if (this._onStart is ICommand) { this._onStart.Execute(); } Console.WriteLine("Invoker: ...doing something really important..."); Console.WriteLine("Invoker: Does anybody want something done after I finish?"); if (this._onFinish is ICommand) { this._onFinish.Execute(); } } } class Program { static void Main(string[] args) { // The client code can parameterize an invoker with any commands. Invoker invoker = new Invoker(); invoker.SetOnStart(new SimpleCommand("Say Hi!")); Receiver receiver = new Receiver(); invoker.SetOnFinish(new ComplexCommand(receiver, "Send email", "Save report")); invoker.DoSomethingImportant(); } } } Output.txt: 执行结果
Invoker: Does anybody want something done before I begin? SimpleCommand: See, I can do simple things like printing (Say Hi!) Invoker: ...doing something really important... Invoker: Does anybody want something done after I finish? ComplexCommand: Complex stuff should be done by a receiver object. Receiver: Working on (Send email.) Receiver: Also working on (Save report.) 
