.NET 進階技巧：型別安全的 EventHandlerList 泛型實作

前言

.NET Framework 提供 EventHandlerList 管理多事件委派，但缺乏型別安全，鍵值為 object 易導致執行期錯誤。本文提供泛型版本 EventHandlerList<T>，編譯期保證鍵值型別正確，消除拼字錯誤風險。

泛型 EventHandlerList<T> 完整實作

public sealed class EventHandlerList : IDisposable
{
    private ListEntry head;

    public EventHandlerList() { }

    // 索引子存取
    public Delegate this[T key]
    {
        get 
        { 
            ListEntry e = Find(key); 
            return e == null ? null : e.m_handler; 
        }
        set 
        { 
            ListEntry e = Find(key); 
            if (e != null) e.m_handler = value; 
            else head = new ListEntry(key, value, head); 
        }
    }

    // 新增處理器 (支援多播)
    public void AddHandler(T key, Delegate value)
    {
        ListEntry e = Find(key);
        if (e != null) e.m_handler = Delegate.Combine(e.m_handler, value);
        else head = new ListEntry(key, value, head);
    }

    // 批次新增
    public void AddHandlers(EventHandlerList listToAddFrom)
    {
        ListEntry current = listToAddFrom.head;
        while (current != null)
        {
            AddHandler(current.m_key, current.m_handler);
            current = current.m_next;
        }
    }

    public void Dispose() { head = null; }

    private ListEntry Find(T key)
    {
        ListEntry found = head;
        while (found != null)
        {
            if (found.m_key.Equals(key)) break;
            found = found.m_next;
        }
        return found;
    }

    public void RemoveHandler(T key, Delegate value)
    {
        ListEntry e = Find(key);
        if (e != null) e.m_handler = Delegate.Remove(e.m_handler, value);
    }

    private sealed class ListEntry
    {
        internal ListEntry m_next;
        internal T m_key;
        internal Delegate m_handler;

        public ListEntry(T key, Delegate handler, ListEntry next)
        {
            m_next = next; m_key = key; m_handler = handler;
        }
    }
}

應用範例：DispatcherCore 事件管理

public class DispatcherCore
{
    readonly EventHandlerList Events = new EventHandlerList();

    public event EventHandler OnReceiveData
    {
        add { Events.AddHandler(EventType.ReceiveData, value); }
        remove { Events.RemoveHandler(EventType.ReceiveData, value); }
    }

    public event EventHandler OnRefreshData
    {
        add { Events.AddHandler(EventType.RefreshData, value); }
        remove { Events.RemoveHandler(EventType.RefreshData, value); }
    }

    private void RaiseEvent(EventType eventType, EventArgs args)
    {
        var handler = Events[eventType] as EventHandler;
        if (handler != null) handler(this, args);
    }

    private enum EventType
    {
        None, ReceiveData, RefreshData, Error, Info
    }
}

優勢比較

適用場景

• 類別擁有大量事件 (10+)，需集中管理
• 事件鍵值適合用列舉型別定義
• 追求程式碼品質、可維護性、重構安全
• 框架/元件庫開發，對外公開事件模型

擴展建議

• 加入 RaiseEvent(T key, params object[] args) 便利方法
• 實作 IEnumerable> 支援列舉
• 提供 EventHandlerList<TKey, TDelegate> 雙泛型版本
• 整合弱引用 (WeakReference) 防止記憶體洩漏

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