C# 設計模式：訪問者模式完整指南（三）— 員工薪資與假期管理實戰範例

實戰場景

員工集合接受不同訪問者處理：IncomeVisitor 調薪 10%，VacationVisitor 增加 3 天假期。

完整代碼實作

using System;
using System.Collections;

// 訪問者抽象介面
abstract class Visitor
{
    public abstract void Visit(Element element);
}

// 具體訪問者：調薪
class IncomeVisitor : Visitor
{
    public override void Visit(Element element)
    {
        Employee emp = (Employee)element;
        emp.Income *= 1.10;
        Console.WriteLine($"{emp.Name}'s new income: {emp.Income:C}");
    }
}

// 具體訪問者：加假
class VacationVisitor : Visitor
{
    public override void Visit(Element element)
    {
        Employee emp = (Employee)element;
        emp.VacationDays += 3;
        Console.WriteLine($"{emp.Name}'s new vacation days: {emp.VacationDays}");
    }
}

// 元素抽象介面
abstract class Element
{
    public abstract void Accept(Visitor visitor);
}

// 具體元素：員工
class Employee : Element
{
    public string Name { get; set; }
    public double Income { get; set; }
    public int VacationDays { get; set; }

    public Employee(string name, double income, int vacationDays)
    {
        Name = name; Income = income; VacationDays = vacationDays;
    }

    public override void Accept(Visitor visitor) => visitor.Visit(this);
}

// 物件結構：員工集合
class Employees
{
    private ArrayList employees = new ArrayList();

    public void Attach(Employee emp) => employees.Add(emp);
    public void Detach(Employee emp) => employees.Remove(emp);

    public void Accept(Visitor visitor)
    {
        foreach (Employee e in employees) e.Accept(visitor);
    }
}

// 客戶端測試
public class VisitorApp
{
    public static void Main()
    {
        Employees emps = new Employees();
        emps.Attach(new Employee("Hank", 25000.0, 14));
        emps.Attach(new Employee("Elly", 35000.0, 16));
        emps.Attach(new Employee("Dick", 45000.0, 21));

        IncomeVisitor v1 = new IncomeVisitor();
        VacationVisitor v2 = new VacationVisitor();

        emps.Accept(v1);  // 所有員工調薪 10%
        emps.Accept(v2);  // 所有員工加 3 天假
    }
}

執行結果

Hank's new income: $27,500.00
Elly's new income: $38,500.00
Dick's new income: $49,500.00
Hank's new vacation days: 17
Elly's new vacation days: 19
Dick's new vacation days: 24

核心機制：雙重分派

1. Employees.Accept(visitor) 遍歷集合
2. 調用 employee.Accept(visitor) → 將自己傳給訪問者
3. 訪問者調用 visitor.Visit(employee) → 執行具體操作

**雙重分派本質**：操作取決於**訪問者類型**與**節點類型**雙重決定，而非單一方。

優勢體現

• 新增「年終獎金訪問者」只需新增一個類別，完全不動 Employee/Employees
• 調薪邏輯與加假邏輯完全隔離在各自訪問者中
• 員工類別專注數據持有，不混雜業務操作