C# Delegate

多播委托（Multicast Delegates）

// 定义委托
public delegate void MyDelegate(string message);

// 方法A
public static void MethodA(string message)
{
    Console.WriteLine($"Method A: {message}");
}

// 方法B
public static void MethodB(string message)
{
    Console.WriteLine($"Method B: {message}");
}

static void Main()
{
    MyDelegate combinedDelegate = new MyDelegate(MethodA);
    combinedDelegate += MethodB;
    combinedDelegate.Invoke("Hello, World!");
    // 输出：Method A: Hello, World!
    // Method B: Hello, World!

    combinedDelegate -= MethodB;
    combinedDelegate.Invoke("Hello, World!");
    // 输出：Method A: Hello, World!

    Console.ReadKey();
}

例如还可以复杂一点

// 定义委托
public delegate void SayHello(string userName);

// 英文方法
private static void EnglishSay(string userName)
{
    Console.WriteLine("Hello!" + userName);
}

// 中文方法
private static void ChineseSay(string userName)
{
    Console.WriteLine("你好!" + userName);
}

// 将委托当参数传入另一个方法
private static void Say(string userName, SayHello say)
{
    say(userName);
}

static void Main()
{
    Say("小明", EnglishSay);
    Say("小明", ChineseSay);

    Console.ReadKey();
}

匿名方法

// 定义委托
public delegate void AnonymousDelegate(int number);

static void Main()
{
    AnonymousDelegate anonymous = delegate (int num)
    {
        Console.WriteLine($"Square of {num}: {num * num}");
    };

    anonymous(5); // 输出：Square of 5: 25

    Console.ReadKey();
}

Action用法

static void Main()
{
    Action<string> lambda = message => Console.WriteLine($"From Lambda: {message}");

    lambda("Hello from a lambda!"); // 输出：From Lambda: Hello from a lambda!

    Console.ReadKey();
}

当然Action也可以传方法

// 定义一个方法给Action
private static void GetString(string name)
{
    Console.WriteLine(name);
}
static void Main()
{
    // Action有参无返回值（需定义方法并调用）
    Action<string> stringAction = new Action<string>(GetString);
    stringAction("小明");

    // Action有参无返回值（匿名方法直接调用）
    Action<string> string2Action = new Action<string>(ac =>
    {
        Console.WriteLine(ac);// ac等于调用传参的小萌
    });
    string2Action("小萌");
}

Func用法

static void Main()
{
    Func<int, string> squareAndFormat = num => $"The square of {num} is {num * num}";
    string result = squareAndFormat(7);
    Console.WriteLine(result);// 输出：The square of 7 is 49
    Console.ReadKey();
}

同样Func也可以传方法

// 定义一个方法给Func
public static bool IsNull(string text)
{
    return string.IsNullOrEmpty(text) || string.IsNullOrWhiteSpace(text);
}
static void Main()
{
    // Func有参有返回值（需定义方法并调用）
    Func<string, bool> isNullFunc = new Func<string, bool>(IsNull);
    Console.WriteLine(isNullFunc(""));

    // Func有参有返回值（匿名方法直接调用）
    Func<string, bool> isNull2Func = new Func<string, bool>(ac =>
    {
        return string.IsNullOrEmpty(ac) || string.IsNullOrWhiteSpace(ac);// ac等于调用传参的空字符串
    });
    Console.WriteLine(isNull2Func(""));

    Console.ReadKey();
}

异步委托调用（BeginInvoke/EndInvoke）

// 定义委托
public delegate int LongRunningTaskDelegate();
static void Main()
{
    // 休眠两秒返回固定值
    LongRunningTaskDelegate task = () =>
    {
        Thread.Sleep(2000);
        return 66;
    };

    Console.WriteLine("开始");
    //IAsyncResult result = task.BeginInvoke(null, null);
    var waroTask = Task.Run(()=> task.Invoke());

    // 在这里执行其他任务...
    Console.WriteLine("在这里执行其他任务");

    //int returnValue = task.EndInvoke(result);
    int returnValue = waroTask.Result;
    Console.WriteLine($"Task returned: {returnValue}");

    Console.ReadKey();
}

注释的代码会出现错误：System.PlatformNotSupportedException:“Operation is not supported on this platform.”

详细请看这里：迁移 .NET Core 的 Delegate.BeginInvoke 调用

事件（Event）

// 定义事件和委托
public class StringEventArgs : EventArgs
{
    public string Message { get; }

    public StringEventArgs(string message)
    {
        Message = message;
    }
}

public delegate void MessageEventHandler(object sender, StringEventArgs e);

// 引发事件
public class EventPublisher
{
    // 定义事件
    public event MessageEventHandler MessageReceived;

    protected virtual void OnMessageReceived(StringEventArgs e)
    {
        MessageReceived?.Invoke(this, e);
    }

    public void SendMessage(string message)
    {
        var args = new StringEventArgs(message);
        OnMessageReceived(args);
    }
}

// 订阅和取消订阅事件
public class EventSubscriber
{
    public void Subscribe(EventPublisher publisher)
    {
        publisher.MessageReceived += OnMessageReceived;
    }

    public void Unsubscribe(EventPublisher publisher)
    {
        publisher.MessageReceived -= OnMessageReceived;
    }

    private void OnMessageReceived(object sender, StringEventArgs e)
    {
        Console.WriteLine($"Subscriber received message: {e.Message}");
    }
}

// 调用
static void Main()
{
    var publisher = new EventPublisher();
    var subscriber = new EventSubscriber();

    subscriber.Subscribe(publisher);
    publisher.SendMessage("Hello, world!");

    subscriber.Unsubscribe(publisher);
    publisher.SendMessage("Hello, world!");

    Console.ReadKey();
}

这里调用了两次SendMessage，但是只有第一次会触发OnMessageReceived，第二次不会触发。