.NET Design Patterns

Visitor


 Definition
 UML diagram
 Participants
 Structural code in C#
 Real-world code in C#
 .NET Optimized code in C#



Definition

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.

Frequency of use:
Low




UML class diagram






Participants


    The classes and objects participating in this pattern are:

  • Visitor  (Visitor)
    • declares a Visit operation for each class of ConcreteElement in the object structure. The operation's name and signature identifies the class that sends the Visit request to the visitor. That lets the visitor determine the concrete class of the element being visited. Then the visitor can access the elements directly through its particular interface
  • ConcreteVisitor  (IncomeVisitor, VacationVisitor)
    • implements each operation declared by Visitor. Each operation implements a fragment of the algorithm defined for the corresponding class or object in the structure. ConcreteVisitor provides the context for the algorithm and stores its local state. This state often accumulates results during the traversal of the structure.
  • Element  (Element)
    • defines an Accept operation that takes a visitor as an argument.
  • ConcreteElement  (Employee)
    • implements an Accept operation that takes a visitor as an argument
  • ObjectStructure  (Employees)
    • can enumerate its elements
    • may provide a high-level interface to allow the visitor to visit its elements
    • may either be a Composite (pattern) or a collection such as a list or a set



Structural code in C#


This structural code demonstrates the Visitor pattern in which an object traverses an object structure and performs the same operation on each node in this structure. Different visitor objects define different operations.

                 

using System;

using System.Collections.Generic;

 

namespace DoFactory.GangOfFour.Visitor.Structural

{

  /// <summary>

  /// MainApp startup class for Structural

  /// Visitor Design Pattern.

  /// </summary>

  class MainApp

  {

    static void Main()

    {

      // Setup structure

      ObjectStructure o = new ObjectStructure();

      o.Attach(new ConcreteElementA());

      o.Attach(new ConcreteElementB());

 

      // Create visitor objects

      ConcreteVisitor1 v1 = new ConcreteVisitor1();

      ConcreteVisitor2 v2 = new ConcreteVisitor2();

 

      // Structure accepting visitors

      o.Accept(v1);

      o.Accept(v2);

 

      // Wait for user

      Console.ReadKey();

    }

  }

 

  /// <summary>

  /// The 'Visitor' abstract class

  /// </summary>

  abstract class Visitor

  {

    public abstract void VisitConcreteElementA(

      ConcreteElementA concreteElementA);

    public abstract void VisitConcreteElementB(

      ConcreteElementB concreteElementB);

  }

 

  /// <summary>

  /// A 'ConcreteVisitor' class

  /// </summary>

  class ConcreteVisitor1 : Visitor

  {

    public override void VisitConcreteElementA(

      ConcreteElementA concreteElementA)

    {

      Console.WriteLine("{0} visited by {1}",

        concreteElementA.GetType().Name, this.GetType().Name);

    }

 

    public override void VisitConcreteElementB(

      ConcreteElementB concreteElementB)

    {

      Console.WriteLine("{0} visited by {1}",

        concreteElementB.GetType().Name, this.GetType().Name);

    }

  }

 

  /// <summary>

  /// A 'ConcreteVisitor' class

  /// </summary>

  class ConcreteVisitor2 : Visitor

  {

    public override void VisitConcreteElementA(

      ConcreteElementA concreteElementA)

    {

      Console.WriteLine("{0} visited by {1}",

        concreteElementA.GetType().Name, this.GetType().Name);

    }

 

    public override void VisitConcreteElementB(

      ConcreteElementB concreteElementB)

    {

      Console.WriteLine("{0} visited by {1}",

        concreteElementB.GetType().Name, this.GetType().Name);

    }

  }

 

  /// <summary>

  /// The 'Element' abstract class

  /// </summary>

  abstract class Element

  {

    public abstract void Accept(Visitor visitor);

  }

 

  /// <summary>

  /// A 'ConcreteElement' class

  /// </summary>

  class ConcreteElementA : Element

  {

    public override void Accept(Visitor visitor)

    {

      visitor.VisitConcreteElementA(this);

    }

 

    public void OperationA()

    {

    }

  }

 

  /// <summary>

  /// A 'ConcreteElement' class

  /// </summary>

  class ConcreteElementB : Element

  {

    public override void Accept(Visitor visitor)

    {

      visitor.VisitConcreteElementB(this);

    }

 

    public void OperationB()

    {

    }

  }

 

  /// <summary>

  /// The 'ObjectStructure' class

  /// </summary>

  class ObjectStructure

  {

    private List<Element> _elements = new List<Element>();

 

    public void Attach(Element element)

    {

      _elements.Add(element);

    }

 

    public void Detach(Element element)

    {

      _elements.Remove(element);

    }

 

    public void Accept(Visitor visitor)

    {

      foreach (Element element in _elements)

      {

        element.Accept(visitor);

      }

    }

  }

}


Output
ConcreteElementA visited by ConcreteVisitor1
ConcreteElementB visited by ConcreteVisitor1
ConcreteElementA visited by ConcreteVisitor2
ConcreteElementB visited by ConcreteVisitor2




Real-world code in C#


This real-world code demonstrates the Visitor pattern in which two objects traverse a list of Employees and performs the same operation on each Employee. The two visitor objects define different operations -- one adjusts vacation days and the other income.

                 

using System;

using System.Collections.Generic;

 

namespace DoFactory.GangOfFour.Visitor.RealWorld

{

  /// <summary>

  /// MainApp startup class for Real-World

  /// Visitor Design Pattern.

  /// </summary>

  class MainApp

  {

    /// <summary>

    /// Entry point into console application.

    /// </summary>

    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' interface

  /// </summary>

  interface IVisitor

  {

    void Visit(Element element);

  }

 

  /// <summary>

  /// A 'ConcreteVisitor' class

  /// </summary>

  class IncomeVisitor : IVisitor

  {

    public void Visit(Element 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 : IVisitor

  {

    public void Visit(Element element)

    {

      Employee employee = element as Employee;

 

      // Provide 3 extra vacation days

      Console.WriteLine("{0} {1}'s new vacation days: {2}",

        employee.GetType().Name, employee.Name,

        employee.VacationDays);

    }

  }

 

  /// <summary>

  /// The 'Element' abstract class

  /// </summary>

  abstract class Element

  {

    public abstract void Accept(IVisitor visitor);

  }

 

  /// <summary>

  /// The 'ConcreteElement' class

  /// </summary>

  class Employee : Element

  {

    private string _name;

    private double _income;

    private int _vacationDays;

 

    // Constructor

    public Employee(string name, double income,

      int vacationDays)

    {

      this._name = name;

      this._income = income;

      this._vacationDays = vacationDays;

    }

 

    // Gets or sets the name

    public string Name

    {

      get { return _name; }

      set { _name = value; }

    }

 

    // Gets or sets income

    public double Income

    {

      get { return _income; }

      set { _income = value; }

    }

 

    // Gets or sets number of vacation days

    public int VacationDays

    {

      get { return _vacationDays; }

      set { _vacationDays = value; }

    }

 

    public override void Accept(IVisitor visitor)

    {

      visitor.Visit(this);

    }

  }

 

  /// <summary>

  /// The 'ObjectStructure' class

  /// </summary>

  class Employees

  {

    private List<Employee> _employees = new List<Employee>();

 

    public void Attach(Employee employee)

    {

      _employees.Add(employee);

    }

 

    public void Detach(Employee employee)

    {

      _employees.Remove(employee);

    }

 

    public void Accept(IVisitor visitor)

    {

      foreach (Employee e in _employees)

      {

        e.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)

    {

    }

  }

}


Output
Clerk Hank's new income: $27,500.00
Director Elly's new income: $38,500.00
President Dick's new income: $49,500.00

Clerk Hank's new vacation days: 14
Director Elly's new vacation days: 16
President Dick's new vacation days: 21




.NET Optimized code in C#


The .NET optimized code demonstrates the same real-world situation as above but uses modern, built-in .NET features, such as, generics, reflection, object initializers, automatic properties, etc. You can find an example on our Singleton pattern page.

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