C# 入门教程（四）委托详解

1、什么是委托

在 C#里，委托属于引用类型，其作用是封装和引用一个或多个方法。可以把它想象成一种类型安全的函数指针，不过它比函数指针更强大，因为它支持多播（也就是可以引用多个方法）。委托经常会在事件处理、回调函数以及异步编程中被用到。

• 委托(delegate)是函数指针的"升级版"

  • 实例:C/C++中的函数指针

  #include <stdio.h>
  
  // 函数：加法
  int add(int a, int b) {
      return a + b;
  }
  
  // 函数：减法
  int subtract(int a, int b) {
      return a - b;
  }
  
  int main() {
      // 声明一个函数指针
      int (*operation)(int, int);
  
      // 让函数指针指向add函数
      operation = add;
      printf("加法结果: %d\n", operation(5, 3)); // 输出8
  
      // 让函数指针指向subtract函数
      operation = subtract;
      printf("减法结果: %d\n", operation(5, 3)); // 输出2
  
      return 0;
  }
  

• 一切皆地址

  • 变量(数据)是以某个地址为起点的一段内存中所存储的值
  • 函数(算法)是以某个地址为起点的一段内存中所存储的一组机器语言指令

• 直接调用与间接调用

  • 直接调用:通过函数名来调用函数，CPU通过函数名直接获得函数所在地址并开始执行→返回
  • 间接调用:通过函数指针来调用函数，CPU通过读取函数指针存储的值获得函数所在地址并开始执行→返回

• Java中没有与委托相对应的功能实体

• 委托的简单使用

  • Action委托
  • Func委托

  namespace DelegateExample
  {
      class Program
      {
          static void Main(string[] args)
          {
              Calculatator calculatator = new Calculatator();
  
              //无参数委托
              Action act = new Action(calculatator.Report);
  
              //打印出来的都是一样的
              act();
              act.Invoke();
              calculatator.Report();
  
              //带参数的委托
              Func<int, int, int> func = new Func<int, int, int>(calculatator.Add);
  
              int a = 13;
              int b = 10;
              int z = func.Invoke(a, b);
              Console.WriteLine(z);
  
              Func<int, int, int> func2 = new Func<int, int, int>(calculatator.Sub);
  
              int z1 = func2.Invoke(a, b);
              Console.WriteLine(z1);
          }
      }
  
      class Calculatator
      {
          public void Report()
          {
              Console.WriteLine("I have 3 Methods");
          }
  
          public int Add(int x, int y)
          {
              return x + y;
          }
  
          public int Sub(int x, int y)
          {
              return x - y;
          }
      }
  }
  
  

2、委托的声明（自定义委托）

• 委托是一种类(class)，类是数据类型所以委托也是一种数据类型
• 它的声名方式与一般的类不同，主要是为了照顾可读性和C/C++传统
• 注意声明委托的位置
  • 避免写错地方结果声明成嵌套类型
• 委托与所封装的方法必需类型兼容

• 返回值的数据类型一致
• 参数列表在个数和数据类型上一致(参数名不需要一样)

namespace DelegateExample
{
    public delegate int Calc(int x, int y);

    class Program
    {
        static void Main(string[] args)
        {
            Calculatator calculatator = new Calculatator();

            Calc calc1 = new Calc(calculatator.Add);
            Calc calc2 = new Calc(calculatator.Sub);
            Calc calc3 = new Calc(calculatator.mul);
            Calc calc4 = new Calc(calculatator.Div);

            int a = 10;
            int b = 5;

            int c = calc1.Invoke(a, b);
            Console.WriteLine(c);

            int c1 = calc2.Invoke(a, b);
            Console.WriteLine(c1);

            int c2 = calc3.Invoke(a, b);
            Console.WriteLine(c2);

            int c3 = calc4.Invoke(a, b);
            Console.WriteLine(c3);
        }
    }

    class Calculatator
    {

        public int Add(int x, int y)
        {
            return x + y;
        }

        public int Sub(int x, int y)
        {
            return x - y;
        }

        public int mul(int x, int y)
        {
            return x * y;
        }

        public int Div(int x, int y)
        {
            return x / y;
        }
    }
}

3、委托的使用

3.1 实例:把方法当作参数传给另一个方法

• 正确使用1:模板方法，“借用”指定的外部方法来产生结果相当于“填空题”常位于代码中部委托有返回值

namespace DelegateExample
{
    public delegate int Calc(int x, int y);

    class Program
    {
        static void Main(string[] args)
        {
            ProductFactory productFactory = new ProductFactory();
            WrapFactory wrapFactory = new WrapFactory();

            Func<Product> func1 = new Func<Product>(productFactory.MakePizza);
            Func<Product> func2 = new Func<Product>(productFactory.MakeCare);

            Box box1 = wrapFactory.WarpProduct(func1);
            Box box2 = wrapFactory.WarpProduct(func2);

            Console.WriteLine(box1.Product.Name);
            Console.WriteLine(box2.Product.Name);

        }
    }

    class Product
    {
        public string Name { get; set; }
    }

    class Box
    {
        public Product Product { get; set; }
    }


    class WrapFactory()
    {
        public Box WarpProduct(Func<Product> getProduct)
        {
            Box box = new Box();
            Product product = getProduct.Invoke();
            box.Product = product;
            return box;
        }
    }


    class ProductFactory()
    {
        public Product MakePizza()
        {
            Product product = new Product();
            product.Name = "pizza";
            return product;
        }

        public Product MakeCare()
        {
            Product product = new Product();
            product.Name = "care";
            return product;
        }
    }
}

• 正确使用2:回调(callback)方法，调用指定的外部方法相当于”流水线”常位于代码末尾委托无返回值

namespace DelegateExample
{
    public delegate int Calc(int x, int y);

    class Program
    {
        static void Main(string[] args)
        {
            ProductFactory productFactory = new ProductFactory();
            WrapFactory wrapFactory = new WrapFactory();

            Func<Product> func1 = new Func<Product>(productFactory.MakePizza);
            Func<Product> func2 = new Func<Product>(productFactory.MakeCare);

			// 实例化一个日志类，进行价格大于50的时候，打印相关日志
            Logger logger = new Logger();
            Action<Product> Log = new Action<Product>(logger.Log);

            Box box1 = wrapFactory.WarpProduct(func1, Log);
            Box box2 = wrapFactory.WarpProduct(func2, Log);

            Console.WriteLine(box1.Product.Name);
            Console.WriteLine(box2.Product.Name);


        }
    }

    class Logger
    {
        public void Log(Product product)
        {
            Console.WriteLine("产品名：{0}，价格：{1}，创建于：{2}", product.Name, product.Price, DateTime.UtcNow);
        }
    }

    class Product
    {
        public string Name { get; set; }

        public decimal Price { get; set; }
    }

    class Box
    {
        public Product Product { get; set; }
    }


    class WrapFactory()
    {
        public Box WarpProduct(Func<Product> getProduct, Action<Product> LogCallback)
        {
            Box box = new Box();
            Product product = getProduct.Invoke();

            if (product.Price > 50)
            {
                LogCallback.Invoke(product);
            }

            box.Product = product;
            return box;
        }
    }


    class ProductFactory()
    {
        public Product MakePizza()
        {
            Product product = new Product();
            product.Name = "pizza";
            product.Price = 10;
            return product;
        }

        public Product MakeCare()
        {
            Product product = new Product();
            product.Name = "care";
            product.Price = 100;
            return product;
        }
    }

}

3.2 注意:难精通+易使用+功能强大东西，一旦被滥用则后果非常严重

• 缺点1:这是一种方法级别的紧耦合，现实工作中要慎之又慎
• 缺点2:使可读性下降、debug的难度增加
• 缺点3:把委托回调、异步调用和多线程纠缠在一起，会让代码变得难以阅读和维护
• 缺点4:委托使用不当有可能造成内存泄漏和程序性能下降

4、委托的高级使用

4.1 多播（multicast）委托

多播委托（Multicast Delegate） 是一种特殊的委托类型，它可以同时引用多个方法，当委托被调用时，会依次执行所有被引用的方法。这一特性使得多播委托成为实现事件机制、回调链等场景的核心基础。

多播委托通过将多个方法组合成一个可调用的实体，实现了 “一次调用，多方响应” 的效果，是 C# 中实现事件、回调等功能的核心机制。

namespace DelegateExample
{
    public delegate int Calc(int x, int y);

    class Program
    {
        static void Main(string[] args)
        {
            Student student = new Student() { Id = 1, Color = ConsoleColor.Gray };
            Student student1 = new Student() { Id = 2, Color = ConsoleColor.Red };
            Student student2 = new Student() { Id = 3, Color = ConsoleColor.Green };

            Action action1 = new Action(student.DoHomework);
            Action action2 = new Action(student1.DoHomework);
            Action action3 = new Action(student2.DoHomework);

			Console.WriteLine("====================单播委托======================");
			
            action1.Invoke();
            action2.Invoke();
            action3.Invoke();


            Console.WriteLine("====================多播委托======================");
            //多播委托

            action1 += action2;
            action1 += action3;

            action1.Invoke();
        }
    }

    class Student
    {
        public int Id { get; set; }

        public ConsoleColor Color { get; set; }

        public void DoHomework()
        {
            for (int i = 0; i < 5; i++)
            {
                Console.ForegroundColor = this.Color;
                Console.WriteLine("Student {0} doing work {1} hour", this.Id, i);
            }
        }
    }
}

打印结果，多个单播委托和多播委托打印结果一致；

4.2隐式异步调用

1. 同步与异步的简介

• 中英文的语言差异
• 同步:你做完了我(在你的基础上)接着做
• 异步:咱们两个同时做(相当于汉语中的“同步进行”)

2.同步调用与异步调用的对比

• 每一个运行的程序是一个进程(process)
• 每个进程可以有一个或者多个线程(thread )
• 同步调用是在同一线程内异步调用的底层机理是多线程。
• 串行同步单线程，并行异步多线程

3. 隐式多线程 vs 显式多线程

• 直接同步调用:使用方法名
• 间接同步调用:使用单播/多播委托的Invoke方法

namespace DelegateExample
{
    public delegate int Calc(int x, int y);

    class Program
    {
        static void Main(string[] args)
        {
            Student student = new Student() { Id = 1, Color = ConsoleColor.Gray };
            Student student1 = new Student() { Id = 2, Color = ConsoleColor.Red };
            Student student2 = new Student() { Id = 3, Color = ConsoleColor.Green };
            
            //直接调用
            student.DoHomework();
            student1.DoHomework();
            student2.DoHomework();

            //单播委托间接调用
            Action action1 = new Action(student.DoHomework);
            Action action2 = new Action(student1.DoHomework);
            Action action3 = new Action(student2.DoHomework);
            
            action1.Invoke();
            action2.Invoke();
            action3.Invoke();

            //多播委托间接调用
            action1 += action2;
            action1 += action3;

            action1.Invoke();


            for (int i = 0; i < 10; i++) {
                Console.ForegroundColor = ConsoleColor.Cyan;
                Console.WriteLine("Main thread {0}", i);
            }
        }
    }

    class Student
    {
        public int Id { get; set; }

        public ConsoleColor Color { get; set; }

        public void DoHomework()
        {
            for (int i = 0; i < 5; i++)
            {
                Console.ForegroundColor = this.Color;
                Console.WriteLine("Student {0} doing work {1} hour", this.Id, i);
            }
        }
    }
}

打印结果均相同

• 隐式异步调用:使用委托的BeginInvoke

namespace DelegateExample
{
    public delegate int Calc(int x, int y);

    class Program
    {
        static void Main(string[] args)
        {
            Student student = new Student() { Id = 1, Color = ConsoleColor.Gray };
            Student student1 = new Student() { Id = 2, Color = ConsoleColor.Red };
            Student student2 = new Student() { Id = 3, Color = ConsoleColor.Green };

            Action action1 = new Action(student.DoHomework);
            Action action2 = new Action(student1.DoHomework);
            Action action3 = new Action(student2.DoHomework);

            //.NET 5 及更高版本Action.BeginInvoke 受到了限制
            action1.BeginInvoke(null, null);
            action2.BeginInvoke(null, null);
            action3.BeginInvoke(null, null);

            for (int i = 0; i < 10; i++) {
                Console.ForegroundColor = ConsoleColor.Cyan;
                Console.WriteLine("Main thread {0}", i);
            }
        }
    }

    class Student
    {
        public int Id { get; set; }

        public ConsoleColor Color { get; set; }

        public void DoHomework()
        {
            for (int i = 0; i < 5; i++)
            {
                Console.ForegroundColor = this.Color;
                Console.WriteLine("Student {0} doing work {1} hour", this.Id, i);
            }
        }
    }
}

• 显式导步调用:使用Thread

using System.Threading;

namespace DelegateExample
{
    public delegate int Calc(int x, int y);

    class Program
    {
        static void Main(string[] args)
        {
            Student student = new Student() { Id = 1, Color = ConsoleColor.Gray };
            Student student1 = new Student() { Id = 2, Color = ConsoleColor.Red };
            Student student2 = new Student() { Id = 3, Color = ConsoleColor.Green };

            Thread thread1 = new Thread(new ThreadStart(student.DoHomework));
            Thread thread2 = new Thread(new ThreadStart(student1.DoHomework));
            Thread thread3 = new Thread(new ThreadStart(student2.DoHomework));

            thread1.Start();
            thread2.Start();
            thread3.Start();

            for (int i = 0; i < 10; i++)
            {
                Console.ForegroundColor = ConsoleColor.Cyan;
                Console.WriteLine("Main thread {0}", i);
            }
        }
    }

    class Student
    {
        public int Id { get; set; }

        public ConsoleColor Color { get; set; }

        public void DoHomework()
        {
            for (int i = 0; i < 5; i++)
            {
                Console.ForegroundColor = this.Color;
                Console.WriteLine("Student {0} doing work {1} hour", this.Id, i);
            }
        }
    }
}

打印结果：异步调用

• Task 异步调用

using System.Threading;
using System.Threading.Tasks;

namespace DelegateExample
{
    public delegate int Calc(int x, int y);

    class Program
    {
        static void Main(string[] args)
        {
            Student student = new Student() { Id = 1, Color = ConsoleColor.Gray };
            Student student1 = new Student() { Id = 2, Color = ConsoleColor.Red };
            Student student2 = new Student() { Id = 3, Color = ConsoleColor.Green };


            Task task1 = new Task(student.DoHomework);
            Task task2 = new Task(student1.DoHomework);
            Task task3 = new Task(student2.DoHomework);

            task1.Start();
            task2.Start();
            task3.Start();

            for (int i = 0; i < 10; i++)
            {
                Console.ForegroundColor = ConsoleColor.Cyan;
                Console.WriteLine("Main thread {0}", i);
            }
        }
    }

    class Student
    {
        public int Id { get; set; }

        public ConsoleColor Color { get; set; }

        public void DoHomework()
        {
            for (int i = 0; i < 5; i++)
            {
                Console.ForegroundColor = this.Color;
                Console.WriteLine("Student {0} doing work {1} hour", this.Id, i);
            }
        }
    }
}