C# 委托與事件詳解（十二）

前例輸出顯示多個訂閱者收到通知。但為何 .NET Framework 事件模式與之不同？為何有 EventArgs 參數？先理解 .NET 編碼規範：

• 委托類型名以 EventHandler 結尾
• 委托原型：void 返回值，兩個參數：Object 與 EventArgs (或其子類)
• 事件名稱 = 委托名去掉 EventHandler
• 繼承 EventArgs 的類型以 EventArgs 結尾

說明：

1. Object sender 代表事件源 (Subject)，如 Heater，回調可透過它訪問觸發者
2. EventArgs 包含觀察者關心的數據，如溫度

這不只是規範，更帶來靈活性。若需在觀察端獲取熱水器型號、產地、價格，傳遞 sender 即可直接訪問，無需修改委托簽名。

重寫範例符合 .NET 規範：

using System;
namespace Delegate {
    public class Heater {
        private int temperature;
        public string type = "RealFire 001";
        public string area = "China Xian";
        public delegate void BoiledEventHandler(Object sender, BoiledEventArgs e);
        public event BoiledEventHandler Boiled;
        public class BoiledEventArgs : EventArgs {
            public readonly int temperature;
            public BoiledEventArgs(int temperature) { this.temperature = temperature; }
        }
        protected virtual void OnBoiled(BoiledEventArgs e) {
            if (Boiled != null) Boiled(this, e);
        }
        public void BoilWater() {
            for (int i = 0; i <= 100; i++) {
                temperature = i;
                if (temperature > 95) {
                    OnBoiled(new BoiledEventArgs(temperature));
                }
            }
        }
    }
    public class Alarm {
        public void MakeAlert(Object sender, Heater.BoiledEventArgs e) {
            Heater heater = (Heater)sender;
            Console.WriteLine("Alarm：{0} - {1}: ", heater.area, heater.type);
            Console.WriteLine("Alarm: 嘀嘀嘀，水已經 {0} 度了：", e.temperature);
        }
    }
    public class Display {
        public static void ShowMsg(Object sender, Heater.BoiledEventArgs e) {
            Heater heater = (Heater)sender;
            Console.WriteLine("Display：{0} - {1}: ", heater.area, heater.type);
            Console.WriteLine("Display：水快燒開了，當前溫度：{0}度。", e.temperature);
        }
    }
    class Program {
        static void Main() {
            Heater heater = new Heater();
            Alarm alarm = new Alarm();
            heater.Boiled += alarm.MakeAlert;
            heater.Boiled += (new Alarm()).MakeAlert;
            heater.Boiled += new Heater.BoiledEventHandler(alarm.MakeAlert);
            heater.Boiled += Display.ShowMsg;
            heater.BoilWater();
        }
    }
}

輸出顯示多個訂閱者（含匿名對象與靜態方法）均正確接收事件。總結：本系列從委托概念切入，引出事件，講解中間代碼，再以熱水器範例實作 Observer 模式，闡述 .NET 委托事件實現方式。