[转载]访问者模式的简洁实现

void Accept(IVisitor visitor);
}

/// 抽象Visitor接口
public interface IVisitor
{
void VisitEmployee(IEmployee employee);
void VisitManager(Manager manager);
}

/// 一个具体的Element
public class Employee : IEmployee
{
private string name;
private double income;
private int vacationDays;

public Employee(string name, double income, int vacationDays)
{
this.name = name;
this.income = income;
this.vacationDays = vacationDays;
}

public string Name
{
get{return name;}
set{name = value;}
}
public double Income
{
get{return income;}
set{income = value;}
}
public int VacationDays
{
get{return vacationDays;}
set{vacationDays = value;}
}

/// 引入Visitor对自身的操作
public virtual void Accept(IVisitor visitor)
{
visitor.VisitEmployee(this);
}
}

/// 另一个具体的Element
public class Manager : Employee
{
private string department;
public string Department { get { return department; } }

public Manager(string name, double income, int vacationDays,
string department)
: base(name, income, vacationDays)
{
this.department = department;
}
/// 引入Visitor对自身的操作
public override void Accept(IVisitor visitor)
{
visitor.VisitManager(this);
}
}

/// 为了便于对HR系统的对象进行批量处理增加的集合类型
public class EmployeeCollection : List<IEmployee>
{
/// 组合起来的批量Accept操作
public virtual void Accept(IVisitor visitor)
{
foreach (IEmployee employee in this)
employee.Accept(visitor);
}
}
Unit Test
private EmployeeCollection employees = new EmployeeCollection();

/// 具体的Visitor, 增加休假天数
class ExtraVacationVisitor : IVisitor
{
public void VisitEmployee(IEmployee employee)
{
employee.VacationDays += 1;
}
public void VisitManager(Manager manager)
{
manager.VacationDays += 2;
}
}

/// 具体的Visitor, 加薪
class RaiseSalaryVisitor : IVisitor
{
public void VisitEmployee(IEmployee employee)
{
employee.Income *= 1.1;
}

public void VisitManager(Manager manager)
{
manager.Income *= 1.2;
}
}

[TestMethod]
public void Test()
{
employees.Add(new Employee(“joe“, 25000, 14));
employees.Add(new Manager(“alice“, 22000, 14, “sales“));
employees.Add(new Employee(“peter“, 15000, 7));

employees.Accept(new ExtraVacationVisitor());
employees.Accept(new RaiseSalaryVisitor());

IEmployee joe = employees[0];
Assert.AreEqual<double>(25000 * 1.1, joe.Income);
IEmployee peter = employees[2];
Assert.AreEqual<int>(7 + 1, peter.VacationDays);

IEmployee alice = employees[1];
Assert.AreEqual<int>(14 + 2, alice.VacationDays);
Assert.AreEqual<double>(22000 * 1.2, alice.Income);
}