Visitor Design Pattern

Represent an operation to be performed on the elements of an object structure. Visitor lets you define a new operation without changing the classes of the elements on which it operates.

using System;
using System.Collections.Generic;
using System.Reflection;

namespace Patterns
{
    
    class MainApp
    {
         
        static void Main()
        {
             // Setup employee collection
            Employees e = new Employees();
             e.Attach(new Clerk());
            e.Attach(new Director());
             e.Attach(new President());

             // Employees are 'visited'
            e.Accept(new IncomeVisitor());
             e.Accept(new VacationVisitor());

             // Wait for user
           Console.ReadKey();
        }
     }

    /// <summary>
    /// The 'Visitor' abstract class
    /// </summary>
    public abstract class Visitor
    {
         // Use reflection to see if the Visitor has a method
        // named Visit with the appropriate parameter type
        // (i.e. a specific Employee). If so, invoke it.
        public void ReflectiveVisit(IElement element)
        {
           Type[] types = new Type[] { element.GetType() };
           MethodInfo mi = this.GetType().GetMethod("Visit", types);

             if (mi != null)
            {
                 mi.Invoke(this, new object[] { element });
             }
        }
     }

    /// <summary>
    /// A 'ConcreteVisitor' class
    /// </summary>
    class IncomeVisitor : Visitor
    {
         // Visit clerk
        public void Visit(Clerk clerk)
        {
             DoVisit(clerk);
        }

         // Visit director
        public void Visit(Director director)
        {
            DoVisit(director);
        }

         private void DoVisit(IElement element)
        {
            Employee employee = element as Employee;

             // Provide 10% pay raise
            employee.Income *= 1.10;
           Console.WriteLine("{0} {1}'s new income: {2:C}",
                 employee.GetType().Name, employee.Name,
                 employee.Income);
         }
    }

     /// <summary>
    /// A 'ConcreteVisitor' class
    /// </summary>
    class VacationVisitor : Visitor
    {
         // Visit director
        public void Visit(Director director)
        {
            DoVisit(director);
        }

         private void DoVisit(IElement element)
        {
            Employee employee = element as Employee;

             // Provide 3 extra vacation days
            employee.VacationDays += 3;
           Console.WriteLine("{0} {1}'s new vacation days: {2}",
                 employee.GetType().Name, employee.Name,
                 employee.VacationDays);
         }
    }

     /// <summary>
    /// The 'Element' interface
    /// </summary>
    public interface IElement
    {
        void Accept(Visitor visitor);
    }

     /// <summary>
    /// The 'ConcreteElement' class
    /// </summary>
    class Employee : IElement
    {
        // Constructor
        public Employee(string name, double income,
             int vacationDays)
        {
             this.Name = name;
             this.Income = income;
             this.VacationDays = vacationDays;
        }

        // Gets or sets name
        public string Name { get; set; }

        // Gets or set income
        public double Income { get; set; }

        // Gets or sets vacation days
        public int VacationDays { get; set; }

         public virtual void Accept(Visitor visitor)
        {
            visitor.ReflectiveVisit(this);
        }
     }

    /// <summary>
    /// The 'ObjectStructure' class
    /// </summary>
    class Employees : List<Employee>
    {
         public void Attach(Employee employee)
        {
           Add(employee);
        }

        public void Detach(Employee employee)
        {
           Remove(employee);
        }

        public void Accept(Visitor visitor)
        {
            // Iterate over all employees and accept visitor
            this.ForEach(employee => employee.Accept(visitor));
            
           Console.WriteLine();
        }
     }

    // Three employee types

    class Clerk : Employee
    {
         // Constructor
        public Clerk()
            : base("Hank", 25000.0, 14)
        {
         }
    }

    class Director : Employee
    {
         // Constructor
        public Director()
            : base("Elly", 35000.0, 16)
        {
         }
    }

    class President : Employee
    {
         // Constructor
        public President()
            : base("Dick", 45000.0, 21)
        {
         }
    }
}