C# 建構子參數傳遞與基類別初始化：base 關鍵字、多載鏈與車輛繼承實例

建構子參數化：實現靈活初始化

public class Vehicle {
    public int Wheels { get; }
    public float Weight { get; }

    // 無參數建構子：委派給主要建構子
    public Vehicle() : this(4, 1500f) { }

    // 主要建構子：完整初始化
    public Vehicle(int wheels, float weight) {
        Wheels = wheels;
        Weight = weight;
    }
}

• 使用 this(...) 串聯建構子，避免重複初始化邏輯。
• 唯讀屬性 (get only) 僅能在建構子中賦值。

衍生類別呼叫基類別建構子：base 關鍵字

public class Car : Vehicle {
    public int Passengers { get; }

    // 必須顯式呼叫基類別建構子
    public Car(int wheels, float weight, int passengers)
        : base(wheels, weight) { // base 對應 Vehicle(int, float)
        Passengers = passengers;
    }

    // 便利建構子：委派給主要建構子
    public Car(int passengers) : this(4, 1500f, passengers) { }
}

若基類別有無參數建構子，衍生類別可省略 : base() (隱含呼叫)；否則必須顯式指定。

完整車輛繼承體系範例

public abstract class Vehicle {
    public int Wheels { get; protected set; }
    public float Weight { get; protected set; }
    protected Vehicle(int wheels, float weight) { Wheels = wheels; Weight = weight; }
    public abstract void Show();
}

public class Train : Vehicle {
    public int Carriages { get; }
    public int Passengers { get; }

    public Train() : this(0, 0, 0) { }
    public Train(int carriages, int passengers, float weight)
        : base(carriages * 8, weight) { // 假設每節車廂 8 輪
        Carriages = carriages;
        Passengers = passengers;
    }
    public override void Show() => Console.WriteLine($"Train: {Carriages} cars, {Passengers} passengers, {Weight}kg");
}

public class Car : Vehicle {
    public int Passengers { get; }
    public Car(int wheels, float weight, int passengers) : base(wheels, weight) {
        Passengers = passengers;
    }
    public override void Show() => Console.WriteLine($"Car: {Wheels} wheels, {Passengers} passengers, {Weight}kg");
}

// 使用
var vehicles = new Vehicle[] {
    new Vehicle(4, 1500f), // 錯誤：Vehicle 為 abstract
    new Train(10, 500, 200000f),
    new Car(4, 1500f, 5)
};
foreach (var v in vehicles) v.Show();

建構子執行順序詳細時序

1. 衍生類別建構子進入，評估 base(...) 引數。
2. 基類別建構子執行 (遞迴至 object)。
3. 基類別實例欄位初始化運算式 (依宣告順序)。
4. 基類別建構子本體執行。
5. 返回衍生類別，執行衍生類別實例欄位初始化運算式。
6. 執行衍生類別建構子本體 (含 : this(...) 委派鏈)。

常見錯誤與解決方案

現代 C# 建構子簡化語法

• 主要建構子 (C# 12 / .NET 8) - class Point(int X, int Y) { ... } 直接在類別宣告參數。
• record 類別 自動產生建構子、解構子、相等比較。
• required 屬性 (C# 11) 強制物件初始化器提供值，減少建構子多載。