c# 委托的常见用法

public delegate int Comparison<in T>(T left, T right);//官方给出的定义泛型委托的demo

//在全局定义
public delegate int Comparison<in T>(T left, T right);
namespace Test
{
  //在指定名称空间下定义
  public delegate int Comparison2<in T>(T left, T right);
  public class Student
  {
    //在类内部定义
    public delegate int Comparison3<in T>(T left, T right);
  }
}

//定义委托
public delegate int Comparison<in T>(T left, T right);
public class Test
{
  //定义
  private Comparison<int> comparator;
  public void Show()
  {
    //赋值 请注意，使用的是方法名称，不带括号，将方法附加给委托作为委托的调用方法。
    this.comparator = Compare;
    //调用 
    this.comparator(1, 2);　　　　　//调用方式二    this.comparator.Invoke(1, 2);
  }
  private int Compare(int left, int right) => left.CompareTo(right);
}

Comparison<string> comparer = (left, right) => left.Length.CompareTo(right.Length);

private int Cal(int num) {return num * num;}
private void button1_Click(object sender, EventArgs e)
{
   Func<int, int> action = null;
   //使用+= 或 -= 来增加或移除委托实例
   action += a => { Console.WriteLine($"第{1}次执行" + a); return a + 1; };
   action += Cal;
   action += a => { Console.WriteLine($"第{3}次执行" + a); return a + 3; };
   action -= Cal;
   //如果有返回值的话，返回的是最后一次执行的结果
   int a = action(5);
   Console.WriteLine(a);
}

public delegate void Action();
public delegate void Action<in T>(T arg);
public delegate void Action<in T1, in T2>(T1 arg1, T2 arg2);
// Other variations removed for brevity.

public delegate TResult Func<out TResult>();
public delegate TResult Func<in T1, out TResult>(T1 arg);
public delegate TResult Func<in T1, in T2, out TResult>(T1 arg1, T2 arg2);
// Other variations removed for brevity

public delegate bool Predicate<in T>(T obj);

 Action action = () => Console.WriteLine("委托执行了");
 AsyncCallback asyncCallback = a => Console.WriteLine("回调执行了" + a);
 //System.PlatformNotSupportedException:“Operation is not supported on this platform.”
 IAsyncResult result = action.BeginInvoke(asyncCallback, "asdf"); //委托启动异步调用
 action.EndInvoke(result);

/// <summary>
/// 定义一个委托
/// </summary>
/// <param name="name"></param>
public delegate void ShowInfo(string name);
public class Study
{
  /// <summary>
  /// 声明一个事件
  /// </summary>
  public event ShowInfo ShowInfo;
  public void Show()
  {
    ShowInfo += Study_ShowInfo;
    //只能在“publisher”类中调用
    ShowInfo("asdf");
  }
  private void Study_ShowInfo(string name)
  {
    throw new NotImplementedException();
  }
}