C# 設計模式：合成模式完整指南（二）— 安全式合成模式結構與實作

安全式合成模式特點

管理聚集的方法（Add/Remove/GetChild）**僅出現在合成構件類別中**，葉構件完全不包含這些方法。

結構圖

三大角色

完整代碼實作

using System;
using System.Text;
using System.Collections;

abstract class Component
{
    protected string name;
    public Component(string name) => this.name = name;
    public abstract void Display(int depth);
}

class Composite : Component
{
    private ArrayList children = new ArrayList();
    public Composite(string name) : base(name) { }

    public void Add(Component c) => children.Add(c);
    public void Remove(Component c) => children.Remove(c);

    public override void Display(int depth)
    {
        Console.WriteLine(new string('-', depth) + name);
        foreach (Component c in children) c.Display(depth + 2);
    }
}

class Leaf : Component
{
    public Leaf(string name) : base(name) { }
    public override void Display(int depth)
        => Console.WriteLine(new string('-', depth) + name);
}

public class Client
{
    public static void Main()
    {
        Composite root = new Composite("root");
        root.Add(new Leaf("Leaf A"));
        root.Add(new Leaf("Leaf B"));

        Composite comp = new Composite("Composite X");
        comp.Add(new Leaf("Leaf XA"));
        comp.Add(new Leaf("Leaf XB"));
        root.Add(comp);

        root.Add(new Leaf("Leaf C"));

        Leaf l = new Leaf("Leaf D");
        root.Add(l);
        root.Remove(l);

        root.Display(1);
    }
}

執行結果

- root
-- Leaf A
-- Leaf B
-- Composite X
---- Leaf XA
---- Leaf XB
-- Leaf C

安全式優勢

• 編譯時安全：嘗試 leaf.Add(...) 直接編譯錯誤
• 介面純淨：葉節點介面不被管理方法污染
• 意圖明確：代碼清楚表達「只有合成節點可管理子物件」

現代 C# 改良建議

// 介面隔離實現編譯時安全 + 統一介面
public interface IComponent { void Display(int depth); }
public interface IComposite : IComponent 
{ 
    void Add(IComponent c); 
    void Remove(IComponent c); 
}

public class Leaf : IComponent { /* ... */ }
public class Composite : IComposite 
{ 
    private List children = new();
    public void Add(IComponent c) => children.Add(c);
    public void Remove(IComponent c) => children.Remove(c);
    public void Display(int depth) { /* ... */ }
}