Delegates Overview Delegates have the following properties: Delegates are similar to C++ function pointers, but are type safe. Delegates allow methods to be passed as parameters. Delegates can be used to define callback methods. Delegates can be chained together; for example, multiple methods can be called on a single event. Methods don't need to match the delegate signature exactly. For more information, see Covariance and Contra variance. C# version 2.0 introduces the concept of Anonymous Methods, which permit code blocks to be passed as parameters in place of a separately defined method.

委托是对方法的引用。虽然对象可以很容易地作为参数发送到方法、构造函数或其他方法中，但方法有点棘手。但每隔一段时间你可能会觉得需要将一个方法作为参数发送给另一个方法，这时你就需要委托。

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace DelegateApp {

  /// <summary>
  /// A class to define a person
  /// </summary>
  public class Person {
    public string Name { get; set; }
    public int Age { get; set; }
  }

  class Program {
    //Our delegate
    public delegate bool FilterDelegate(Person p);

    static void Main(string[] args) {

      //Create 4 Person objects
      Person p1 = new Person() { Name = "John", Age = 41 };
      Person p2 = new Person() { Name = "Jane", Age = 69 };
      Person p3 = new Person() { Name = "Jake", Age = 12 };
      Person p4 = new Person() { Name = "Jessie", Age = 25 };

      //Create a list of Person objects and fill it
      List<Person> people = new List<Person>() { p1, p2, p3, p4 };

      //Invoke DisplayPeople using appropriate delegate
      DisplayPeople("Children:", people, IsChild);
      DisplayPeople("Adults:", people, IsAdult);
      DisplayPeople("Seniors:", people, IsSenior);

      Console.Read();
    }

    /// <summary>
    /// A method to filter out the people you need
    /// </summary>
    /// <param name="people">A list of people</param>
    /// <param name="filter">A filter</param>
    /// <returns>A filtered list</returns>
    static void DisplayPeople(string title, List<Person> people, FilterDelegate filter) {
      Console.WriteLine(title);

      foreach (Person p in people) {
        if (filter(p)) {
          Console.WriteLine("{0}, {1} years old", p.Name, p.Age);
        }
      }

      Console.Write("\n\n");
    }

    //==========FILTERS===================
    static bool IsChild(Person p) {
      return p.Age < 18;
    }

    static bool IsAdult(Person p) {
      return p.Age >= 18;
    }

    static bool IsSenior(Person p) {
      return p.Age >= 65;
    }
  }
}

输出:

Children:
Jake, 12 years old


Adults:
John, 41 years old
Jane, 69 years old
Jessie, 25 years old


Seniors:
Jane, 69 years old

Delegates Overview Delegates have the following properties: Delegates are similar to C++ function pointers, but are type safe. Delegates allow methods to be passed as parameters. Delegates can be used to define callback methods. Delegates can be chained together; for example, multiple methods can be called on a single event. Methods don't need to match the delegate signature exactly. For more information, see Covariance and Contra variance. C# version 2.0 introduces the concept of Anonymous Methods, which permit code blocks to be passed as parameters in place of a separately defined method.

我同意已经说过的一切，只是试着用另一种说法来解释。

委托可以被视为一个或多个方法的占位符。

通过定义委托，您就在对类的用户说:“请随意将与此签名匹配的任何方法分配给委托，并且每次调用我的委托时都会调用它”。

典型的使用当然是事件。所有的OnEventX委托给用户定义的方法。

委托非常有用，可以为对象的用户提供一些自定义行为的能力。 大多数时候，你可以使用其他方法来达到同样的目的，我不相信你会被强迫创建委托。在某些情况下，这是完成任务最简单的方法。

委托是一个简单的类，用于指向具有特定签名的方法，本质上成为类型安全的函数指针。委托的目的是在一个方法完成后，以结构化的方式方便对另一个(或多个)方法的回调。

虽然可以创建一组大量的代码来执行此功能，但您不需要这样做。你可以使用委托。

创建委托很容易做到。使用"delegate"关键字将类标识为委托。然后指定类型的签名。

当你想要声明你想要传递的代码块时，委托是非常有用的。例如，当使用通用重试机制时。

伪:

function Retry(Delegate func, int numberOfTimes)
    try
    {
       func.Invoke();
    }
    catch { if(numberOfTimes blabla) func.Invoke(); etc. etc. }

或者当你想要做代码块的后期计算，比如你有一些Transform动作的函数，并且想要有一个BeforeTransform和一个AfterTransform动作，你可以在你的Transform函数中计算，而不需要知道BeginTransform是否被填充，或者它必须转换什么。

当然，在创建事件处理程序时也是如此。您不希望现在计算代码，而只希望在需要时计算代码，因此您可以注册一个可以在事件发生时调用的委托。