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設計模式實戰：使用泛型改良適配器模式 (C#/C++/VB)

編輯：關於C#

問題背景

經典類別適配器需對每個被適配類別撰寫專屬適配器 (TriangleDrawAdapter, CircleDrawAdapter...)，導致類別爆炸。利用泛型可將「被適配類別」參數化，單一 DrawAdapter<T> 即可處理所有實作 IPaint 的類別。

核心技巧：泛型參數化被適配類別

C++ (Template) - 支援繼承式適配


template <typename T>
class DrawAdapter : public IDrawStrategy, private T 
{
public:
    virtual void draw() const { paint(); } // 直接呼叫 T::paint()
};

// 客戶端動態指定被適配類別
Grapher grapher(&DrawAdapter<Triangle>());
grapher.setShape(&DrawAdapter<Circle>());

C# (Generics) - 採用組合式適配

C# 泛型不支援繼承型別參數 (class Adapter<T> : T 編譯錯誤)，改用組合 + 約束：


public class DrawAdapter<T> : IDrawStrategy where T : IPaint, new()
{
    private readonly T _adaptee = new T(); // 約束要求無參建構子
    
    public void Draw() => _adaptee.Paint();
}

// 使用
var grapher = new Grapher(new DrawAdapter<Triangle>());
grapher.SetShape(new DrawAdapter<Circle>());

C++/CLI (Generics) - 組合式


generic <typename T> where T : IPaint, gcnew()
ref class DrawAdapter : IDrawStrategy
{
    T _adaptee = gcnew T();
    public: virtual void Draw() { _adaptee->Paint(); }
};

VB.NET (Generics) - 組合式


Class DrawAdapter(Of T As {IPaint, New})
    Implements IDrawStrategy
    Private _adaptee As New T()
    Public Sub Draw() Implements IDrawStrategy.Draw
        _adaptee.Paint()
    End Sub
End Class

語言特性對比表

語言	泛型種類	適配方式	關鍵語法
ISO C++	Template	私有繼承	`template<typename T> class Adapter : T`
C++/CLI	Template + Generics	繼承/組合皆可	`template<typename T> ref class` / `generic<typename T>`
C#	Generics	僅組合	`where T : IPaint, new()`
VB.NET	Generics	僅組合	`Of T As {IPaint, New}`

完整 C# 範例


public interface IDrawStrategy { void Draw(); }
public interface IPaint { void Paint(); }

public class Triangle : IPaint { public void Paint() => Console.WriteLine("Draw Triangle"); }
public class Circle : IPaint { public void Paint() => Console.WriteLine("Draw Circle"); }
public class Square : IPaint { public void Paint() => Console.WriteLine("Draw Square"); }

public class Grapher
{
    private IDrawStrategy _strategy;
    public Grapher(IDrawStrategy s = null) => _strategy = s;
    public void DrawShape() => _strategy?.Draw();
    public void SetShape(IDrawStrategy s) => _strategy = s;
}

public class DrawAdapter<T> : IDrawStrategy where T : IPaint, new()
{
    private readonly T _adaptee = new T();
    public void Draw() => _adaptee.Paint();
}

// Client
var g = new Grapher(new DrawAdapter<Triangle>());
g.DrawShape(); // Draw Triangle
g.SetShape(new DrawAdapter<Circle>());
g.DrawShape(); // Draw Circle

優缺點分析

優點	缺點
消除類別爆炸：單一泛型適配器替代 N 個具體適配器	C#/VB 僅支援組合式，無法覆寫被適配類別虛擬方法
編譯期型別安全：約束確保 T 實作 IPaint	要求被適配類別有公用無參建構子 (new() 約束)
客戶端動態組合：執行期決定適配目標	無法適配無法實例化的類別 (抽象類、靜態類)
符合開放封閉原則：新增形狀無需修改適配器代碼	泛型參數化增加認知複雜度