C# 運算子多載實戰：一元/二元運算子、成對實作與角色升級範例

為什麼需要運算子多載？

讓自訂型別支援直觀的數學/邏輯運算符號 (如 v1 + v2、++player)，提升程式碼可讀性與領域模型表達力。

一元運算子多載：角色升級系統

public class Player {
    public int Neili { get; private set; } = 10;
    public int Tili { get; private set; } = 50;
    public int Jingyan { get; private set; }
    public int NeiliMax { get; private set; } = 50;
    public int TiliMax { get; private set; } = 50;

    public static Player operator ++(Player p) {
        if (p == null) throw new ArgumentNullException(nameof(p));
        p.Neili += 50;
        p.Tili += 100;
        p.NeiliMax = p.Neili;
        p.TiliMax = p.Tili;
        p.Jingyan = 0;
        return p; // 必須回傳修改後的實例 (或新實例)
    }

    public void Show() {
        Console.WriteLine($"Tili: {Tili}/{TiliMax}, Neili: {Neili}/{NeiliMax}, Exp: {Jingyan}");
    }
}

// 使用
var hero = new Player();
hero.Show();
++hero; // 前綴遞增
hero.Show();

注意：operator ++ 應視為回傳新狀態，不建議直接修改傳入參數再回傳 (易造成副作用混淆)，較佳做法是建立新實例。

二元運算子多載：3D 向量加法

public struct Vector3D {
    public double X, Y, Z;
    public Vector3D(double x, double y, double z) { X = x; Y = y; Z = z; }

    public static Vector3D operator +(Vector3D left, Vector3D right) =>
        new Vector3D(left.X + right.X, left.Y + right.Y, left.Z + right.Z);

    public static Vector3D operator -(Vector3D left, Vector3D right) =>
        new Vector3D(left.X - right.X, left.Y - right.Y, left.Z - right.Z);

    public override string ToString() => $"({X}, {Y}, {Z})";
}

// 使用
var v1 = new Vector3D(3, 2, 1);
var v2 = new Vector3D(0, 6, 5);
var v3 = v1 + v2; // (3, 8, 6)
Console.WriteLine(v3);

成對運算子強制規則

若只實作其中一個，編譯器報錯 CS0216。

等號比較完整實作模式

public class Point : IEquatable {
    public int X { get; }
    public int Y { get; }
    public Point(int x, int y) { X = x; Y = y; }

    public static bool operator ==(Point left, Point right) {
        if (ReferenceEquals(left, right)) return true;
        if (ReferenceEquals(left, null) || ReferenceEquals(right, null)) return false;
        return left.Equals(right);
    }
    public static bool operator !=(Point left, Point right) => !(left == right);

    public bool Equals(Point other) => other != null && X == other.X && Y == other.Y;
    public override bool Equals(object obj) => Equals(obj as Point);
    public override int GetHashCode() => HashCode.Combine(X, Y);
}

最佳實務與陷阱

1. 不可變型別 (struct/readonly class) 最適合運算子多載，回傳新實例。
2. 避免在運算子中拋出例外 (除非引數驗證)，改用 Try 方法模式。
3. true/false 運算子僅用於支援 if (obj) 語法的特殊型別 (如 DBBool)，濫用會造成困惑。
4. 運算子多載不支援繼承多態：Base b = new Derived(); b + b 呼叫 Base.operator+，非 Derived.operator+。